designing and detailing post tensioned bridges to accommodate

ATLSS is a National Center for Engineering Research on Advanced Technology for Large Structural Systems

117 ATLSS Drive Bethlehem, PA 18015-4729

Phone: (610)758-3525 www.atlss.lehigh.edu

Fax: (610)758-5902 Email: [email protected]

Federal Highway Administration Project

DTFH61-11-H-0027

Advancing Steel and Concrete Bridge Technology to Improve

Infrastructure Performance

Task No. 11

Designing and Detailing Post Tensioned Bridges to Accommodate Non-

Destructive Evaluation

Subtask 11.1

Literature Review

January 2015

By

Christina Cercone

Clay Naito

John Corven

Stephen Pessiki

Wesley Keller

Shamim Pakzad

ATLSS REPORT NO. 14-01

http://www.atlss.lehigh.edu/

ATLSS Report 14-01 FHWA Subtask 11.1 Page i

Abstract

Post-tensioned concrete bridges represent a major component of the American bridge inventory. Many

new post-tensioned concrete bridges will be constructed to meet our infrastructure needs. Post-tensioning

(PT) tendons are comprised of prestressing strand, ducts, anchorages, grout, and corrosion protection

equipment. Current details for the construction of post-tensioning tendons do not facilitate the inspection

of the various tendon components. Recent cases of unexpected corrosion of post-tensioning tendons have

illustrated the importance of developing detailing changes that would allow improved inspection and

integration of nondestructive evaluation (NDE) methods.

The overall research program aims to develop guidance for the design and detailing of post-tensioned

bridges and tendon components to facilitate integration of NDE methods. The effort is focused on

evaluation of grouted internal and external post tensioning systems in precast I-girder, precast U-girder,

precast segmental box girder, and cast in-place box girder superstructures. Substructure elements are

outside the scope of this effort.

This report represents a brief overview of post-tensioned bridge construction and issues associated with

them. The current methods available for inspection are summarized and their benefits and limitations are

discussed. A comprehensive literature review on currently available NDE methods for post-tensioned

bridge systems is provided. The literature review focuses on NDE techniques that can be used to detect

the following issues in post–tensioned bridge girder systems: (1) Grout Voids/Condition, (2)

Strand/Anchorage Corrosion, and (3) Remaining Prestress Force. Within each of these categorizes NDE

techniques that can be used to identify issues with existing post-tensioned systems and NDE techniques

that can be integrated into new construction are presented. The literature review indicates that Electrically

Isolated Tendons, Embedded Half-Cell Methods, Time Domain Reflectometry and Ultrasonic Testing

Methods are promising detection tools that can be integrated into new construction. For existing

construction, other NDE methods may be useful. This includes: Impact Echo for grout void detection,

Magnetic Flux Leakage for corrosion, Radiography for corrosion and grout voids, Ground Penetrating

Radar for location of tendons, and Acoustic Emission for strand breakage. In all cases, visual techniques

should not be overlooked as they provide an effective tool for confirmation of in-situ damage during field

inspection.

ATLSS Report 14-01 FHWA Subtask 11.1 Page ii

Table of Contents

1 Introduction ........................................................................................................................................... 1

2 Brief Overview on Post-tensioned Bridge Construction and Issues ..................................................... 2

2.1 Post-Tensioned Bridge Construction ............................................................................................ 2

2.1.1 Cast-in-Place Bridges on Falsework ..................................................................................... 2

2.1.2 Post-Tensioned AASHTO, Bulb-T, and Spliced Girders ..................................................... 3

2.1.3 Segmental Box Girder Bridges ............................................................................................. 3

2.1.4 Transverse Top Slab Post-Tensioning .................................................................................. 5

2.1.5 Post-Tensioning of Substructures ......................................................................................... 6

2.2 Post-Tensioned Components......................................................................................................... 7

2.2.1 Anchorages ........................................................................................................................... 7

2.2.2 Ducts ..................................................................................................................................... 9

2.2.3 Permanent Grout Caps ........................................................................................................ 10

2.2.4 Prestressing Strand .............................................................................................................. 11

2.2.5 Post-Tensioning Bars .......................................................................................................... 11

2.3 Damage Conditions Observed in PT Tendons ............................................................................ 12

2.3.1 Corrosion Along the Length of Internal Tendons ............................................................... 12

2.3.2 Cantilever Tendon Corrosion through Segment Joints ....................................................... 13

3.3.3 External Tendon Failure by Corrosion at the Anchorages .................................................. 13

2.3.3 Cases of Post-Tensioned Tendon Corrosion ....................................................................... 14

3 Overview of Applicable Inspection Methods ..................................................................................... 16

3.1 Acoustic Emission ...................................................................................................................... 16

3.1.1 Applications ........................................................................................................................ 16

3.1.2 Methodology ....................................................................................................................... 16

3.1.3 Limitations .......................................................................................................................... 18

3.1.4 Viability in Post-Tensioned Applications ........................................................................... 18

3.1.5 References ........................................................................................................................... 19

3.2 Electrically Isolated Tendons ...................................................................................................... 19

3.2.1 Applications ........................................................................................................................ 19

3.2.2 Methodology ....................................................................................................................... 20

3.2.3 Limitations .......................................................................................................................... 23


3.2.5 References ........................................................................................................................... 24

3.3 Ground Penetrating Radar ........................................................................................................... 25

3.3.1 Applications ........................................................................................................................ 25

3.3.2 Methodology ....................................................................................................................... 25

ATLSS Report 14-01 FHWA Subtask 11.1 Page iii

3.3.3 Limitations .......................................................................................................................... 28


3.3.5 References ........................................................................................................................... 30

3.4 Half-Cell Potential ...................................................................................................................... 31

3.4.1 Applications ........................................................................................................................ 31

3.4.2 Methodology ....................................................................................................................... 31

3.4.3 Limitations .......................................................................................................................... 34


3.4.5 References ........................................................................................................................... 35

3.5 Impact Echo ................................................................................................................................ 35

3.5.1 Applications ........................................................................................................................ 35

3.5.2 Methodology ....................................................................................................................... 36

3.5.3 Limitations .......................................................................................................................... 37


3.5.5 References ........................................................................................................................... 37

3.6 Infrared Thermography ............................................................................................................... 39

3.6.1 Applications ........................................................................................................................ 39

3.6.2 Methodology ....................................................................................................................... 39

3.6.3 Limitations .......................................................................................................................... 40


3.6.5 References ........................................................................................................................... 40

3.7 Magnetic Flux Leakage ............................................................................................................... 41

3.7.1 Applications ........................................................................................................................ 41

3.7.2 Methodology ....................................................................................................................... 41

3.7.3 Limitations .......................................................................................................................... 44


3.7.5 References ........................................................................................................................... 44

3.8 Radiography ................................................................................................................................ 45

3.8.1 Applications ........................................................................................................................ 45

3.8.2 Methodology ....................................................................................................................... 45

3.8.3 Limitations .......................................................................................................................... 47


3.8.5 References ........................................................................................................................... 48

3.9 Signal Processing for Defect Detection ...................................................................................... 48

3.9.1 Applications ........................................................................................................................ 48

3.9.2 Methodology and Applications ........................................................................................... 49

ATLSS Report 14-01 FHWA Subtask 11.1 Page iv

3.9.3 References ........................................................................................................................... 51

3.10 Time Domain Reflectometry ...................................................................................................... 52

3.10.1 Applications ........................................................................................................................ 52

3.10.2 Methodology ....................................................................................................................... 52

3.10.3 Limitations .......................................................................................................................... 55


3.10.5 References ........................................................................................................................... 56

3.11 Ultrasonic Testing ....................................................................................................................... 56

3.11.1 Applications ........................................................................................................................ 56

3.11.2 Methodology ....................................................................................................................... 57

3.11.3 Limitations .......................................................................................................................... 58


3.11.5 References ........................................................................................................................... 59

3.12 Visual Inspection ........................................................................................................................ 60

3.12.1 Applications ........................................................................................................................ 60

3.12.2 Methodology ....................................................................................................................... 60

3.12.3 Limitations .......................................................................................................................... 61


3.12.5 References ........................................................................................................................... 61

4 Summary of NDE methods and tools ................................................................................................. 63

4.1 Future Work ................................................................................................................................ 65

5 Agency Sponsored Studies.................................................................................................................. 67

5.1 Department of Transportation Reports ....................................................................................... 67

5.1.1 Improved Inspection Techniques for Steel Prestressing/Post-tensioning Strand – Final

Report –Volume 1 ............................................................................................................................... 67

5.1.2 FDOT Protocol for Condition Assessment of Steel Strands in Post-tensioned Segmental

Concrete Bridges- Final Report – Volume II ...................................................................................... 67

5.1.3 Inspection Methods and Techniques to Determine Non Visible Corrosion of Prestressing

Strands in Concrete Bridge Components Task 2 – Assessment of Candidate NDT Methods ............ 68


Strands in Concrete Bridge Components Task 3 - Forensic Evaluation and Rating Methodology .... 69

5.1.5 Nondestructive Method to Detect Corrosion of Steel Elements in Concrete ...................... 69

5.1.6 Effect of Voids in Grouted, Post-Tensioned Concrete Bridge Construction: Volume 1 –

Electrochemical Testing and Reliability Assessment ......................................................................... 70

5.1.7 Detection of Voids in Prestressed Concrete Bridges using Thermal Imaging and Ground-

Penetrating Radar ................................................................................................................................ 71

5.1.8 New Directions for Florida Post-Tensioned Bridges – Load Rating Post-Tensioned

Concrete Segmental Bridges (Volume 10A)....................................................................................... 71

ATLSS Report 14-01 FHWA Subtask 11.1 Page v


Concrete Beam Bridges (Volume 10B) .............................................................................................. 72

5.1.10 Evaluating Nondestructive Testing Techniques to Detect Voids in Bonded Post-Tensioned

Ducts – Final Report ........................................................................................................................... 72

5.1.11 Test and Assessment of NDT Methods for Post-Tensioning Systems in Segmental

Balanced Cantilever Concrete Bridges ............................................................................................... 72

5.1.12 New Directions for Florida Post-Tensioned Bridges – Post-Tensioning in Florida Bridges

(Volume 1) .......................................................................................................................................... 75

5.1.13 New Directions for Florida Post-Tensioned Bridges – Design and Construction Inspection

of Precast Segmental Balanced Cantilever Bridges (Volume 2) ........................................................ 75

5.1.14 New Directions for Florida Post-Tensioned Bridges –Design and Construction Inspection

of Precast Segmental Span-By-Span Bridges (Volume 3) .................................................................. 75

5.1.15 New Directions for Florida Post-Tensioned Bridges- Design and Construction Inspection

of Precast Concrete Spliced I-Grider Bridges (Volume 4) ................................................................. 76

5.1.16 New Directions for Florida Post-Tensioned Bridges - Design and Construction Inspection

of Cast-In-Place Segmental Balanced Cantilever Bridges (Volume 5) .............................................. 76

5.1.17 New Directions for Florida Post-Tensioned Bridges - Design and Construction Inspection

of Bridges Cast-In-Place on Falsework (Volume 6) ........................................................................... 76

5.1.18 New Directions for Florida Post-Tensioned Bridges – Design and Construction of Post-

Tensioned Substructures ..................................................................................................................... 76

5.1.19 New Directions for Florida Post-Tensioned Bridges – Design and Construction of

Transverse Post-Tensioning of Superstructures (Volume 8) .............................................................. 77

5.1.20 New Directions for Florida Post-Tensioned Bridges – Condition Inspection and

Maintenance of Florida Post-Tensioned Bridges (Volume 9) ............................................................ 77

5.1.21 Mid-Bay Bridge Post-Tensioning Evaluation ..................................................................... 77

5.1.22 Initial Development of Methods for Assessing Condition of Post-Tensioned Tendons of

Segmental Bridges .............................................................................................................................. 78

5.1.23 Tensile Test Results of Post Tensioning Cables from the Midbay Bridge ......................... 79

5.1.24 Corrosion Evaluation of Post-Tensioned Tendons in Florida Bridges ............................... 79

5.2 ACI Reports ................................................................................................................................ 80

5.2.1 Corrosion of Prestressing Steels (ACI 222.2R-01) ............................................................. 80

5.3 NCHRP Reports .......................................................................................................................... 80

5.3.1 Non-Destructive Evaluation Method for Determination of Internal Grout Conditions inside

Bridge Post-Tensioning Ducts using Rolling Stress Waves for Continuous Scanning ...................... 80

5.3.2 Nondestructive Methods for Condition Evaluation of Prestressing Steel Strands in

Concrete Bridges, Final Report Phase I: Technology Review ............................................................ 81

5.4 Federal Highway Administration (FHWA) ................................................................................ 82

5.4.1 Conclusions, Recommendations and Design Guidelines for Corrosion Protection of Post-

Tensioned Bridges .............................................................................................................................. 82

5.4.2 Improving Bridge Inspections ............................................................................................. 82

ATLSS Report 14-01 FHWA Subtask 11.1 Page vi

5.4.3 Magnetic-Based NDE of Prestressed and Post-Tensioned Concrete Members – The MFL

System 82

5.4.4 Demonstration of Dual-Band Infrared Thermal Imaging for Bridge Inspection ................ 83

5.5 ASTM Standards ......................................................................................................................... 84

5.5.1 ASTM A 36 -12: Standard Specification for Carbon Structural Steel ................................ 84

5.5.2 ASTM A 53 -12: Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-

Coated, Welded and Seamless ............................................................................................................ 84

5.5.3 ASTM A 240 -13: Standard Specification for Chromium and Chromium-Nickel Stainless

Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications................................ 84

5.5.4 ASTM A 416 -12: Standard Specification for Steel Strand, Uncoated Seven-Wire for

Prestressed Concrete ........................................................................................................................... 85

5.5.5 ASTM A 653 -13: Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or

Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process ....................................................... 85

5.5.6 ASTM A 722 -12: Standard Specification for Uncoated High-Strength Steel Bars for

Prestressing Concrete .......................................................................................................................... 85

5.5.7 ASTM C876 – 09: Standard Test Method for Corrosion Potentials of Uncoated

Reinforcing Steel in Concrete ............................................................................................................. 85

5.5.8 ASTM D 1693 -13: Standard Test Method for Environmental Stress-Cracking of Ethylene

Plastics 85

5.5.9 ASTM D4101-14: Standard Specification for Polypropylene Injection and Extrusion

Materials 85

5.5.10 ASTM F 405 – 13: Standard Specification for Corrugated Polyethylene (PE) Pipe and

Fittings 85

5.6 Other ........................................................................................................................................... 86

5.6.1 Post-tensioned Multistrand Anchorage Capacity Deterioration Due to Corrosion: John Day

Lock Project ........................................................................................................................................ 86

5.6.2 Guide Specification for Grouted Post-Tensioning .............................................................. 86

5.6.3 Quality Control and Monitoring of Electrically Isolated Post-Tensioning Tendons in

Bridges 86

5.6.4 Swiss Guideline Measures to Ensure Durability of Post-Tensioning Tendons in Structures

87

6 Grout Condition .................................................................................................................................. 89

6.1 Application of Gamma Ray Scattering Technique for Non-Destructive Evaluation of Voids in

Concrete .................................................................................................................................................. 89

6.2 Quantitative Evaluation of Contactless Impact Echo for Non-Destructive Assessment of Void

Detection within Tendon Ducts .............................................................................................................. 89

6.3 Non-Destructive Testing Methods to Identify Voids in External Post-Tensioned Tendons ....... 89

6.4 Detecting Voids in Grouted Tendon Ducts of Post-Tensioned Concrete Structures Using Three

Different Methods ................................................................................................................................... 90

6.5 Concrete Bridge Condition Assessment with Impact Echo Scanning ........................................ 90

6.6 Inspection of Voids in External Tendons of Posttensioned Bridges ........................................... 90

ATLSS Report 14-01 FHWA Subtask 11.1 Page vii

6.7 Modified SIBIE Procedure for Ungrouted Tendon Ducts Applied to Scanning Impact-Echo ... 91

6.8 On-Site Measurement of Delamination and Surface Crack in Concrete Structure by Visualized

NDT 91

6.9 Identification of Ungrouted Tendon Duct in Prestressed Concrete by SIBIE ............................ 91

6.10 Estimation of Surface-Crack Depth in Concrete by Scanning SIBIE Procedure ........................ 92

6.11 Imaging of Internal Cracks in Concrete Structures Using the Surface Rendering Technique .... 92

6.12 Ultrasonic Imaging Methods for Investigation of Post-tensioned Concrete Structures: A Study

of Interfaces at Artificial Grouting and Its Verification .......................................................................... 93

6.13 Imaging Concrete Structures Using Air-Coupled Impact-Echo ................................................. 93

6.14 Impact-Echo Scanning Evaluation of Grout/Void Conditions Inside Bridge Post-Tensioning

Ducts for Tendon Corrosion Mitigation .................................................................................................. 93

6.15 Impact-Echo Scanning for Grout Void Detection in Post-tensioned Bridge Ducts - Findings

from a Research Project and a Case History ........................................................................................... 94

6.16 Sensitivity Studies of Grout Defects in Posttensioned Bridge Ducts Using Impact Echo

Scanning Method .................................................................................................................................... 94

6.17 Imaging of Ungrouted Tendon Ducts in Prestressed Concrete by Improved SIBIE................... 95

6.18 Automated NDE of Post-Tensioned Concrete Bridges Using Imaging Echo Methods .............. 95

6.19 Impact Echo Scanning for Discontinuity Detection and Imaging in Posttensioned Concrete

Bridges and Other Structures .................................................................................................................. 96

6.20 Complementary Application of Radar, Impact-Echo and Ultrasonics for Testing Concrete

Structures and Metallic Tendon Ducts .................................................................................................... 96

6.21 Contribution of Capacitance Probes for Nondestructive Inspection of External Post-Tensioned

Ducts 97

6.22 Ultrasonic Imaging of Concrete Elements Using Reconstruction by Synthetic Aperture

Focusing Technique ................................................................................................................................ 97

6.23 Ultrasonic Guided Waves for Inspection of Grouted Tendons and Bolts ................................... 97

6.24 Guidance on the use of NDE on Voided Post-Tensioned Concrete Bridge Beams using Impact

Echo 98

6.25 Use of the MegascanTM

Imaging Process in Inspection Systems for Post-Tensioned Bridges and

Other Major Structures ........................................................................................................................... 98

6.26 Comparison of NDT Techniques on a Post-Tensioned Beam Before its Autopsy ..................... 98

6.27 Stack Imaging of Spectral Amplitudes Based on Impact-Echo for Flaw Detection ................... 99

6.28 Applications of Impact-Echo for Flaw Detection ....................................................................... 99

6.29 Ultrasonic Tomography of Grouted Duct Post-Tensioned Reinforced Concrete Bridge Beams 99

6.30 : Nondestructive Evaluation of Concrete and Masonry ............................................................ 100

6.31 Detecting Voids in Grouted Tendon Ducts of Post-Tensioned Concrete Structures using the

Impact Echo Method ............................................................................................................................. 100

7 Strand Corrosion ............................................................................................................................... 101

7.1 Acoustic Emission Monitoring of Reinforced Concrete under Accelerated Corrosion ............ 101

7.2 Corrosion Damage Quantification of Prestressing Strands using Acoustic Emission .............. 101

ATLSS Report 14-01 FHWA Subtask 11.1 Page viii

7.3 Detection of Corrosion of Post-Tensioned Strands in Grouted Assemblies ............................. 101

7.4 Monitoring of Electrically Isolated Post-Tensioning Tendons ................................................. 102

7.5 Evaluation of NDT Methods for Detection of Prestressing Steel Damage at Post-Tensioned

Concrete Structures ............................................................................................................................... 102

7.6 Enhanced Durability, Quality Control and Monitoring of Electrically Isolated Tendons ........ 103

7.7 Corrosion of the Strand-Anchorage System in Post-Tensioned Grouted Assemblies .............. 103

7.8 Long-Term Monitoring of Electrically Isolated Post-Tensioning Tendons .............................. 103

7.9 Electrical Isolation as Enhanced Protection for Posttensioning Tendons in Concrete Structures

(PL3) 104

7.10 Experience with Electrically Isolated Tendons in Switzerland ................................................. 104

7.11 Protection Against Corrosion and Monitoring of Posttensioning Tendons in Prestressed

Concrete Railway Bridges in Italy ........................................................................................................ 104

7.12 Corrosion Protection and Monitoring of Electrically Isolated Post-Tensioning Tendons ........ 105

7.13 Mechanism of Corrosion of Steel Strands in Post Tensioned Grouted Assemblies ................. 105

7.14 Location of Prestressing Steel Fractures in Concrete ............................................................... 105

7.15 Half-Cell Potential Measurements – Potential Mapping on Reinforced Concrete Structures .. 106

7.16 Ultrasonic Imaging – A Novel Way to Investigate Corrosion Status in Post-Tensioned Concrete

Members ............................................................................................................................................... 106

7.17 Comparison of NDT Techniques on a Post-Tensioned Beam Before its Autopsy ................... 106

7.18 Recent Developments in SQUID NDE ..................................................................................... 107

7.19 SQUID Array for Magnetic Inspection of Prestressed Concrete Bridges ................................. 107

7.20 Continuous Acoustic Monitoring of Grouted Post-Tensioned Concrete Bridges ..................... 107

8 Strand Location ................................................................................................................................. 109

8.1 Application of Ground Penetrating Radar (GPR) as a Diagnostic Technique in Concrete Bridge

109

8.2 Rebar Detection Using GPR: An Emerging Non Destructive QC Approach ........................... 109

8.3 Results of Reconstructed and Fused NDT Data Measured in the Laboratory and On-Site

Bridges .................................................................................................................................................. 110

8.4 Ground Penetrating Radar for Concrete Evaluation Studies ..................................................... 110


Structures and Metallic Tendon Ducts .................................................................................................. 110

8.6 Nondestructive Evaluation of Concrete Infrastructure with Ground Penetrating Radar ........... 111

8.7 Comparison of NDT Techniques on a Post-Tensioned Beam Before its Autopsy ................... 111

8.8 Condition Assessment of Transportation Infrastructure Using Ground-Penetrating Radar ...... 111

8.9 Automated NDE of PT Concrete Structures ............................................................................. 112

9 Remaining Prestress .......................................................................................................................... 113

9.1 Detection of Initial Yield and Onset of Failure in Bonded Post-Tensioned Concrete Beams .. 113

9.2 Estimation of Existing Prestress Level on Bonded Strand Using Impact-Echo Test ................ 113

ATLSS Report 14-01 FHWA Subtask 11.1 Page ix

9.3 Determination of the Residual Prestress Force of In-Service Girders using Non-Destructive

Testing113

9.4 Non-Destructive Evaluation of the Stress Levels in Prestressed Steel Strands using

Acoustoelastic Effect ............................................................................................................................ 114

9.5 Health Monitoring to Detect Failure of Prestressing (PS) Cables in Segmental Box-Girder

Bridges .................................................................................................................................................. 114

9.6 A Smart Steel Strand for the Evaluation of Prestress Loss Distribution in Post-Tensioned

Concrete Structures ............................................................................................................................... 115

9.7 Comparison of Prestress Losses for a Prestress Concrete Bridge Made with High-Performance

Concrete ................................................................................................................................................ 115

9.8 Ultrasonic Wave Propagation in Progressively Loaded Multi-Wire Strands ........................... 115

9.9 Application of a New Nondestructive Evaluation Technique to a 25-Year-Old Prestressed

Concrete Girder ..................................................................................................................................... 116

10 NDE Methods with Multiple Applications ................................................................................... 117

10.1 Guidelines for the Thermographic Inspection of Concrete Bridge Components in Shaded

Conditions ............................................................................................................................................. 117

10.2 Comparison of NDT Methods for Assessment of a Concrete Bridge Deck ............................. 117

10.3 Use of Neutron Radiography and Tomography to Visualize the Autonomous Crack Sealing

Efficiency in Cementitious Materials ................................................................................................... 118

10.4 Commissioning of Portable 950 keV/3.95 MeV X-band Linac X-Ray Sources for On-Site

Transmission Testing ............................................................................................................................ 118

10.5 Non-Destructive Radiographic Evaluation and Repairs to Pre-Stressed Structure Following

Partial Collapse ..................................................................................................................................... 119

10.6 Application of Thermal IR Imagery for Concrete Bridge Inspection ....................................... 119

10.7 Gamma-Ray Inspection of Post Tensioning Cables in a Concrete Bridge ............................... 119

10.8 Environmental Effects on Subsurface Defect Detection in Concrete Structures Using Infrared

Thermography ....................................................................................................................................... 120

10.9 Gamma-Ray Imaging for Void and Corrosion Assessment in PT Girders ............................... 120

10.10 Evaluation of Radar and Complementary Echo Methods for NDT of Concrete Elements... 120

10.11 Thermographic Crack Detection by Eddy Current Excitation .............................................. 121


Bridges 121

10.13 Time-Domain Reflectometry to Detect Voids in Posttensioning Ducts ............................... 121

10.14 Ultrasonic C-Scan Imaging of Post-Tensioned Concrete Bridge Structures for Detection of

Corrosion and Voids ............................................................................................................................. 122

10.15 Progress in Ultrasonic Imaging of Concrete ......................................................................... 122

10.16 Recent Research in Nondestructive Evaluation of Civil Infrastructures ............................... 123

10.17 Detecting Corrosion in Existing Structures Using Time Domain Reflectometry ................. 123

10.18 Ultrasonic C-scan Imaging: Preliminary Evaluation for Corrosion and Void Detection in

Posttensioned Tendons .......................................................................................................................... 124

ATLSS Report 14-01 FHWA Subtask 11.1 Page x

10.19 Time Domain Reflectometry for Void Detection in Grouted Posttensioned Bridges ........... 124

10.20 Use of the Megascan Imaging Process in Inspection Systems for Post Tensioned Bridges and

Other Major Structures ......................................................................................................................... 125

10.21 Non-Contact Ultrasonic Imagining for Post-Tensioned Bridges to Investigate Corrosion and

Void Status ............................................................................................................................................ 125

10.22 Experiments to Relate Acoustic Emission Energy to Fracture Energy of Concrete ............. 125

10.23 Corrosion Detection of Steel Cables Using Time Domain Reflectometry ........................... 126

10.24 The Impact-Echo Method: An Overview ............................................................................. 126

10.25 Accuracy of NDE in Bridge Assessment .............................................................................. 126

10.26 Detecting Faults in Posttensioning Ducts by Electrical Time-Domain Reflectometry ......... 127

10.27 Non-Destructive Examination of Corroded Concrete Structures using Radiography .......... 127

10.28 Using Emissivity-Corrected Thermal Maps to Locate Deep Structural Defects in Concrete

Bridge Decks ......................................................................................................................................... 128

10.29 Imaging of Reinforced Concrete: State-of-the-Art Review ................................................. 128

10.30 Principles of Thermography and Radar for Bridge Deck Assessment .................................. 128

10.31 Automated NDE of PT Concrete Structures ......................................................................... 129

11 Sensor Networks and Damage Detection ...................................................................................... 130

11.1 Automatic Delamination Detection of Concrete Bridge Decks Using Impact Signals ............. 130

11.2 Autoregressive Statistical Pattern Recognition Algorithms for Damage Detection in Civil

Structures .............................................................................................................................................. 130

11.3 Procedures for Fatigue Crack Growth Monitoring and Fatigue Life Prediction Using Acoustic

Emission Data and Neural Networks .................................................................................................... 130

11.4 Time Series: Theory and Methods (2nd

Edition) ....................................................................... 131

11.5 Discrete Wavelet Transform to Improve Guided-Wave-Based Health Monitoring of Tendons

and Cables ............................................................................................................................................. 131

11.6 Pattern Recognition Techniques for the Emerging Field of Bioinformatics: A Review ........... 131

11.7 Introduction to Time Series and Forecasting (2nd

Edition) ....................................................... 132

11.8 Resampling Methods: A Practical Guide to Data Analysis ...................................................... 132

11.9 Parameter Estimation and Hypothesis Testing in Linear Models ............................................. 132

11.10 Large-Scale Simulation Studies in Image Pattern Recognition ............................................ 132

11.11 The Jackknife and Bootstrap ................................................................................................. 132

11.12 Document Analysis- From Pixels to Contents ...................................................................... 133

11.13 Continuous Speech Recognition by Statistical Methods....................................................... 133

ATLSS Report 14-01 FHWA Subtask 11.1 Page xi

Table of Figures

Figure 2.1: Cast-in-place post-tensioned construction in California............................................................ 2 Figure 2.2: Post-tensioned bulb-T girder fabrication and installation ......................................................... 3 Figure 2.3: Precast segmental balanced cantilever construction .................................................................. 4 Figure 2.4: Cast-in-Place segmental construction using form travelers....................................................... 4 Figure 2.5: Span-by-span construction ........................................................................................................ 5 Figure 2.6: Typical span post-tensioning for span-by-span construction .................................................... 5 Figure 2.7: Transverse post-tensioning in the top slab of box girder ............................................................ 6 Figure 2.8: Precast segmental piers ............................................................................................................... 7 Figure 2.9: Basic bearing plate anchorage system ....................................................................................... 7 Figure 2.10: Multi-plane anchorage system and confinement reinforcement .............................................. 8 Figure 2.11: Anchorage system for flat duct tendon (Courtesy of DSI) ....................................................... 8 Figure 2.12: Post-tensioning bar anchorage system ..................................................................................... 9 Figure 2.13: Corrugated metal duct .............................................................................................................. 9 Figure 2.14: Corrugated plastic duct .......................................................................................................... 10 Figure 2.15: Permanent plastic grout caps ................................................................................................. 11 Figure 2.16 7-Wire Prestressing Strand (American Spring Wire Corp) ..................................................... 11 Figure 2.17: Post-Tensioning Bar (VSL) .................................................................................................... 12 Figure 2.18: Box girder damage due to improper drainage ....................................................................... 13 Figure 2.19: Epoxy joint leaking in early precast segmental bridges with internal tendons ....................... 13 Figure 2.20: Corrosion of strands just behind the wedge plate .................................................................. 14 Figure 2.21: External tendon corrosion along the free length of the tendon .............................................. 14 Figure 3.1: Acoustic emission monitoring of concrete beam flexure tests: (a) test assembly, .................. 17 Figure 3.2: Laboratory tests of acoustically monitored post-tensioned concrete beams under accelerated

corrosion (Figure adapted from Mangual et al. 2012) ................................................................................ 18 Figure 3.3: Electrically isolated tendon schematic (Figure adapted from Della Vedova and Elsener, 2006)

.................................................................................................................................................................... 20 Figure 3.4: Connection box for impedance measurements (Elsener, 2004b) ............................................ 21 Figure 3.5: Principle of measuring the electrical impedance of a tendon with the LCR meter (Elsener and

Buchler, 2011)............................................................................................................................................. 21 Figure 3.6: Electrical equivalent circuit for an electrically isolated tendon with small defect (Elsener and

Buchler, 2011)............................................................................................................................................. 22 Figure 3.7: Limit Values (28 days after injection) (ASTRA, 2007) ........................................................... 22 Figure 3.8: Evolution of electrical resistance over time (Della Vedova and Elsener, 2006) ..................... 23 Figure 3.9: Schematic of GPR Process (Maierhofer, 2003) ....................................................................... 25 Figure 3.10: Example of GPR components (Jones et al., 2010) ................................................................ 26 Figure 3.11: Mobile system with GPR antennas and data acquisition system (Iyer, Sinha and Schokker,

2005) ........................................................................................................................................................... 27 Figure 3.12: 2D Radargram of Sample 1 revealing in the position of the embedded plastic ducts

(Cheilakou et al., 2012) ............................................................................................................................... 27 Figure 3.13: Transverse scan showing reduced refelection amplitude indication corrosion (Jones et al.,

2010) ........................................................................................................................................................... 28 Figure 3.14: Reference Electrode Circuitry (ASTM C 876, 2009) ............................................................ 32 Figure 3.15: Half-cell electrode and multimeter (Naito, Jones and Hodgson, 2010) ................................ 33 Figure 3.16: Half-cell potential map (Naito, Jones and Hodgson, 2010)................................................... 34 Figure 3.17: Schematic illustration of the impact echo method (Carino, 2001) ......................................... 36 Figure 3.18: Finite element simulation of stress wave propagation in a linear elastic medium due to

impact loading (Carino, 2001) .................................................................................................................... 36 Figure 3.19: Frequency spectra from impact echo tests: (a) solid slab and (b) slab with a subsurface void

(Carino, 2001) ............................................................................................................................................. 37

ATLSS Report 14-01 FHWA Subtask 11.1 Page xii

Figure 3.20: Thermal image of a concrete block with subsurface voids (void depth shown) (Washer,

Fenwick and Nelson, 2013) ........................................................................................................................ 40 Figure 3.21: Magnetic flux leakage due to defect (DaSilva et al., 2009).................................................... 42 Figure 3.22: MFL system for post-tensioned bridge inspection (Azizinamini and Gull, 2012a) ............... 43 Figure 3.23: MFL graph indicating no corrosion (Jones et al., 2010)......................................................... 43 Figure 3.24: MFL graph indicating corrosion (Jones et al., 2010) ............................................................. 43 Figure 3.25: Portable 7.5MeV X-ray Betatron (Sentinel, 2014) ................................................................. 46 Figure 3.26: Schematic illustration of radiographic testing (Rao, 2007) .................................................... 46 Figure 3.27: Radiographic images of post-tensioned concrete: (a) fully grouted post-tensioning steel duct,

(b) voided post-tensioning steel duct (Brown and St. Leger, 2003) ........................................................... 47 Figure 3.28: Functional block diagram for typical time domain reflectometer (Liu et al., 2002) ............. 52 Figure 3.29: Twin-conductor transmission line (Liu et al., 2002) ............................................................. 53 Figure 3.30: Time domain reflectometry return of 3m rebar sample with 50% pitting corrosion in middle

(Liu et al., 2002).......................................................................................................................................... 53 Figure 3.31: Reflected TDR voltage wave signal (Li et al., 2005) ............................................................. 54 Figure 3.32: Pulse-echo test configuration for the inspection of post-tensioning steel using guided

ultrasonic waves (Beard, Lowe and Cawley, 2003) .................................................................................... 57 Figure 3.33: Illustration of ultrasonic imaging: equipment (top) and test configuration (bottom) for a shear

wave array (Azizinamini and Gull, 2012a) ................................................................................................. 58 Figure 3.34: Illustration of a conventional SAFT image (top) and a phase modified SAFT image (bottom)

collected from a post-tensioned concrete beam. The color in the phase modified image indicates the

phase of the reflected wave (Azizinamini and Gull, 2012a) ....................................................................... 58 Figure 3.35: Photos a and b show duct partially grouted with strand exposure. Photo c ............................ 61

ATLSS Report 14-01 FHWA Subtask 11.1 Page 1

1 Introduction

Post-tensioned concrete bridges represent a major component of the American bridge inventory and due

to the benefits provided by this construction technique it is likely that many new post-tensioned concrete

bridges will be built in the future to meet our infrastructure needs. Post-tensioning tendons are comprised

of prestressing strand, ducts, anchorages, grout, and corrosion protection equipment. Current details for

the construction of post-tensioning tendons do not facilitate the long term inspection of the various tendon

components. Recent cases of unexpected corrosion of post-tensioning tendons have illustrated the

importance of developing detailing changes that would allow improved inspection and integration of

nondestructive evaluation (NDE) methods.

This research program aims to develop guidance for the design and detailing of post-tensioned bridges

and tendon components to facilitate integration of NDE methods. This guidance includes evaluation of

both internal and external post tensioning systems with emphasis on precast I-girder, precast U-girder,

precast segmental box girder, and cast in-place box girder superstructures. This study focuses only on

grouted PT systems and bridge superstructure elements.

This report represents a comprehensive literature review on topics associated with the goals of the

research effort. Included in section 2 is an overview of PT bridge systems. The details on the components

of these systems, the materials used, and the problems observed are presented and discussed. Section 3

details the methods used for NDE inspection of concrete systems. The discussion is tailored toward the

application of these methods for PT systems. This section is written to provide the background on the

methods. Section 4 discusses the merits and limitations of the NDE methods and provides a

recommendation on the use of the methods in PT systems.

A summary of published literature relating to deterioration of post-tensioned bridge systems is included in

sections 5 through 11 of the report. The summary includes published reports from reputable

organizations, refereed journal papers and conference proceedings. Within each of these sections the title,

author, and brief abstract of the publication is provided. The literature review focuses on NDE techniques

that can be used to detect the following issues in post–tensioned bridge girder systems: (1) Grout

Voids/Condition, (2) Strand/Anchorage Corrosion, and (3) Remaining Prestress Force. Within each of

these categorizes NDE techniques that can be used to identify issues with existing PT systems and NDE

techniques that can be integrated into new construction will be presented.

Section 5 contains related literature published by agencies and organizations including state departments

of transportation, Federal Highway Administration (FHWA), and National Cooperative Highway

Research Program (NCHRP). Many of these reports are comprehensive and address the inspection of the

PT system as a whole including NDE techniques for the detection of grout voids/condition,

strand/anchorage corrosion and remaining prestress force. Since a number of the reports in this section

review many different NDE techniques they also provide guidance and recommendations on their

findings. This section is subdivided based on the agency producing the report.

Sections 6 (grout condition), 7 (strand corrosion), 8 (strand location), and 9 (remaining prestress force)

identify journal articles and conference papers that contain information on NDE techniques that can be

used to specifically address these issues. Section 10 contains literature on NDE techniques that can be

used to identify one or more of the issues addressed in sections 6 through 8. Section 11 summarizes

literature related to how data can be collected using sensor networks to identify damage.


2 Brief Overview on Post-tensioned Bridge Construction and Issues

To understand the technology needed for improved inspection methods for post-tensioned bridge systems

it is necessary to have a basic understanding of the systems and the damage that has been observed. This

topic has been covered in depth by Corven (2002) (See sections 5.1.12 through 5.1.20). For completeness

of this report a short overview of post-tensioned bridge construction and observed damage conditions are

summarized in this section. A detailed effort on this topic will be provided in follow-on reports.

2.1 Post-Tensioned Bridge Construction

Prestressing can be applied to concrete bridge members in two ways, by pretensioning or post-tensioning.

In pretensioned members the prestressing strands are tensioned against restraining bulkheads before the

concrete is cast. After the concrete has been placed, allowed to harden and attained sufficient strength, the

strands are released and their force is transferred to the concrete member. Prestressing by post-tensioning

involves installing and stressing prestressing strand or bar tendons after the concrete has been placed,

hardened and attained a minimum compressive strength for that transfer. This section presents the major

types of post-tensioned concrete bridges.

2.1.1 Cast-in-Place Bridges on Falsework

Bridges of this type have a superstructure cross-section of solid or cellular construction. They are built

on-site using formwork supported by temporary falsework (Figure 2.1). Formwork creates the shape of

the concrete section and any internal voids or diaphragms. Reinforcement and post-tensioning ducts are

installed in the forms and then the concrete is placed, consolidated and cured. When the concrete attains

sufficient strength, post-tensioning strands are installed and stressed to predetermined forces.

Figure 2.1: Cast-in-place post-tensioned construction in California

Longitudinal post-tensioning is typically comprised of multi-strand tendons draped along the length of the

girder to a designed profile. In continuous spans, the tendon profile lies in the bottom of the girder in the

mid-span region and rises to the top of the section over interior supports. In simple spans and at the

expansion ends of continuous spans, post-tensioning anchors are arranged vertically so that the resultant

of the tendon anchor force passes close to the centroid of the section.


2.1.2 Post-Tensioned AASHTO, Bulb-T, and Spliced Girders

Precast, post-tensioned AASHTO and bulb-T girders may be cast in segments and later post-tensioned

together to make simple spans or a series of continuous spans. The individual girder segments are

typically pre-tensioned at the precast plant to carry their own self weight during transportation to the site

and erection. Figure 2.2 show the fabrication and stressing operations for a simple span bulb-T girder.

Post-tensioning ducts that are cast into the webs are spliced at the cast-in-place joints. The ducts follow a

smoothly curved, draped profile along each girder line. For continuous spans, the profiles rise to the top

of the girders over the interior piers and drape to the bottom flange in mid-span regions. Longer spans

can be built using similar techniques. A variable depth girder section cantilevering over a pier can be

spliced to a typical precast girder in the main and side-spans.

Figure 2.2: Post-tensioned bulb-T girder fabrication and installation

2.1.3 Segmental Box Girder Bridges

2.1.3.1 Precast Segmental Balanced Cantilever Bridges

Precast segmental balanced cantilever construction, shown in Figure 2.3, involves the symmetrical

erection of segments about a supporting pier. When a segment is lifted into position, adjoining match-

cast faces are coated with epoxy and temporary post-tensioning bars are installed and stressed to attach

the segment to the cantilever. Typically, after a new, balancing segment, is in place on each end of the

cantilever, post-tensioning tendons are installed and stressed from one segment on one end of the

cantilever to its counter-part on the other.


Figure 2.3: Precast segmental balanced cantilever construction

2.1.3.2 Cast-in-Place Segmental Balanced Cantilever Bridges

Cast-in-place balanced cantilever construction, depicted in Figure 2.4, uses form travelers to cast

segments at the end of the cantilevers. Form travelers support the concrete of the newly cast segment

until it has reached a satisfactory strength for post-tensioning. The types of longitudinal post-tensioning

tendons used in cast-in-place balanced cantilever construction are the same as for precast segment

balanced cantilever.

Figure 2.4: Cast-in-Place segmental construction using form travelers


2.1.3.3 Precast Segmental Balanced Cantilever Bridges

Span-by-span construction involves the erection of all segments of a span on a temporary support system

with small closure joints cast at one or both ends next to the segments over the pier. Figure 2.5 shows

span-by-span construction using an overhead gantry to temporarily support the precast segments.

Figure 2.5: Span-by-span construction

Span-by-span bridges typically use tendons that are external to the concrete box girder. Figure 2.6 shows

a typical layout of external tendons in a span-by-span bridge. The tendons anchor high at each end of the

span in diaphragms cast with the end segments. Within the span, the tendons deviate through deviators to

provide the needed vertical tendon profile.

Figure 2.6: Typical span post-tensioning for span-by-span construction

2.1.4 Transverse Top Slab Post-Tensioning

Top slabs of precast and cast-in-place segmental, and similar boxes cast-in-place on falsework are often

transversely post-tensioned. Transverse post-tensioning typically comprises internal, multi-strand

tendons grouted after stressing. Tendons are spaced at regular, frequent intervals, approximately 2 to 3

feet along the length of the structure. Tendons anchor in the block-outs in the edges of the top slab

cantilever wings. Block-outs are subsequently filled with concrete and are usually covered with a traffic


barrier. Figure 2.7 shows the positioning of transverse post-tensioning tendons in a cast-in-place balanced

cantilever box girder bridge.

Figure 2.7: Transverse post-tensioning in the top slab of box girder

2.1.5 Post-Tensioning of Substructures

Substructures for most bridges are typically built using reinforced concrete construction. Special bents,

required to miss underlying obstacles, or to accommodate wide bridges, may also contain post tensioning

tendons. Post-tensioning tendons are sometimes used in hammerheads of t-piers, straddle bents, and

cantilever piers.

Precast concrete segmental piers, similar to that shown in Figure 2.8, with vertical post-tensioning have

been used on many projects. The piers are vertically post-tensioned with a combination of PT bars and

strand tendons. The post-tensioning bars are used to temporarily secure precast segments and compress

epoxy in the joints as they are erected prior to installing permanent strand tendons. Strand tendons are

typically full height, being anchored in the pier cap at the top and in the footing at the bottom.


Figure 2.8: Precast segmental piers

2.2 Post-Tensioned Components

This section presents the most commonly used types of post-tensioning components used in bridge

construction.

2.2.1 Anchorages

2.2.1.1 Basic Bearing Plate Systems

A basic bearing plate is a flat plate bearing directly against concrete. This includes square, rectangular, or

round plates, sheared or torch cut from readily available steel plate, normally ASTM A36. Basic bearing

plates are used in conjunction with galvanized sheet metal or plastic trumpets to transition from the strand

spacing in the wedge plate to the duct (Figure 2.9).

Figure 2.9: Basic bearing plate anchorage system


2.2.1.2 Special Bearing Plate Systems

A special bearing plate or anchorage device is any anchorage hardware that transfers tendon force into the

concrete but does not meet normal analytical design requirements for basic bearing plates. Covered by

this definition are devices having single or multiple plane bearing surfaces, and devices combining

bearing and wedge plate in once piece. These anchorages typically require increased confinement

reinforcement and should be accepted on the basis of physical tests. Figure 2.10 shows a cut-away view

of a multi-plane anchorage system. These systems are commonly confined with spiral reinforcement

around the anchor. Figure 2.11 shows the components of the anchorage system for a four strand tendon in

flat duct, commonly used in slabs.

Figure 2.10: Multi-plane anchorage system and confinement reinforcement

Figure 2.11: Anchorage system for flat duct tendon (Courtesy of DSI)

2.2.1.3 Post-Tensioning Bar Anchor Systems

Anchorage systems for post-tensioning bars are comprised of bearing plates and anchor nuts similar to the

components shown in Figure 2.12.


Figure 2.12: Post-tensioning bar anchorage system

2.2.2 Ducts

2.2.2.1 Corrugated Steel

Corrugated ducts and connectors should be fabricated from galvanized sheet steel that meets the

requirements of ASTM A653, with coating designation G90 (PTI/ASBI M50.3-12, 2012). The ducts are

spirally wound to the necessary diameter from strip steel with a minimum wall thickness of 0.45mm (26-

gauge) for ducts less than 66mm (2-5/8 in) diameter or 0.6mm (24-gauge) for ducts of greater diameter.

These ducts are manufactured with welded or interlocking seams with sufficient rigidity to maintain the

correct profile between supports during concrete placement (Figure 2.13). Ducts should also be able to

flex without crimping or flattening. Joints between sections of duct and between ducts and anchor

components should be made with positive, metallic connections that provide a smooth interior alignment

with no lips or abrupt angle changes.

Figure 2.13: Corrugated metal duct


2.2.2.2 Smooth, Rigid, Steel Pipe

Rigid steel ducts are typically used in those portions of external tendons deviating though segmental

bridge pier segment diaphragms or deviators. In these areas of curved tendon alignments, the steel pipe

should be pre-fabricated to the required radius. Smooth steel pipes should conform to ASTM A53/A53M

“Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc Coated, Welded and Seamless,”

Grade B Schedule 40.

2.2.2.3 Corrugated Plastic

Corrugated plastic ducts, as shown in Figure 2.14, are also used for tendons internal to the concrete.

These ducts should be seamless and fabricated from polyethylene or polypropylene (ASTM F405 and

D4101) meeting the requirements of Section 4.3.5.2 of “Guide Specification for Grouted Post-

Tensioning” (PTI/ASBI M50.3-12, 2012).

Figure 2.14: Corrugated plastic duct

2.2.3 Permanent Grout Caps

Permanent grout caps similar to those shown in Figure 2.15 are provided to protect the tendons at the

anchorages. Permanent grout caps are made of a non-corrosive material such as fiber reinforced plastic,

stainless steel, or galvanized ferrous metal with a minimum thickness of zinc of 120 μm. To ensure an

enduring, maintenance-free, life of 75 years fiber reinforced plastic caps should contain an anti-oxidant

additive with an environmental stress cracking endurance of 192 hours per ASTM D1693; stainless steel

caps should meet the requirements of ASTM A240 Type 316.


Figure 2.15: Permanent plastic grout caps

2.2.4 Prestressing Strand

The strand used in post-tensioned bridge systems must meet the requirements of ASTM A416,“Standard

Specification for Steel Strand Uncoated Seven-Wire for Prestressed Concrete,” and consist of wires

having a center wire enclosed tightly by six helically placed outer wires with uniform pitch of not less

than 12 and not more than 16 times the nominal diameter of the strand. Unless otherwise noted on the

contract documents uncoated Grade 270, low –relation, 7- wire strand (Figure 2.16) should be used

(PTI/ASBI M50.3-12, 2012).

Figure 2.16 7-Wire Prestressing Strand (www.amspringwire.com)

2.2.5 Post-Tensioning Bars

Bars used in post-tensioned tendons shall conform to ASTM A722, “Standard Specification for Uncoated

High-Strength Steel Bar for Prestressing Concrete.” Bars have a minimum ultimate tensile strength of

150,000 psi (1035 MPa). Unless otherwise noted in the contract documents, Grade 150, uncoated, high

strength, thread bar shall be used (PTI/ASBI M50.3-12, 2012).


Figure 2.17: Post-Tensioning Bar (www.vsl.net)

2.3 Damage Conditions Observed in PT Tendons

This section provides a brief overview of damage found in post-tensioned bridges related to post-

tensioning corrosion.

2.3.1 Corrosion Along the Length of Internal Tendons

Salt laden water that permeates into sound concrete over time can eventually reach tendons embedded in

concrete elements. The damage shown in Figure 2.18 is an extreme case where a box girder bridge’s

drainage system leaked into the core of the box over the course of 25 years. Both the surrounding duct

and enclosed strands were highly corroded.


Figure 2.18: Box girder damage due to improper drainage

2.3.2 Cantilever Tendon Corrosion through Segment Joints

Some precast segmental bridges have reported water leaking through the epoxied joints (Figure 2.19) and

efflorescence coming from the top slab continuity tendons. These bridges were of a vintage where plastic

duct and duct couplers were not used. As a result, there is concern that the leaking joints could lead to

corrosion in the internal cantilever tendons.

Figure 2.19: Epoxy joint leaking in early precast segmental bridges with internal tendons

3.3.3 External Tendon Failure by Corrosion at the Anchorages

Two span-by-span bridges have experienced failure of an external post-tensioning tendon as a result of

corrosion at the end anchorages. Examination of the removed tendon indicated a void in the grout and

heavy pitting of the prestressing strands inside the anchor head (Figure 2.20). Initially, the corrosion was

attributed to excessive bleed water at grout/void interface. Further investigation indicated that cyclical

recharge of the void in the anchor head by water contaminated by wind-born ocean salt spray was a


primary cause of tendon corrosion. The contaminated water leaked through the expansion joints and ran

down the inside faces of the segment diaphragms onto the anchorages.

Figure 2.20: Corrosion of strands just behind the wedge plate

3.3.4 External Tendon Corrosion along the Free Length of Tendon

A few tendons in early span-by-span post-tensioned bridges have experienced corrosion along the free

length of the tendons, between the diaphragms and deviators (Figure 2.21). This type of damage was

attributed to a breach in the surrounding polyethylene duct, allowing access of moisture and development

of isolated corrosion.

Figure 2.21: External tendon corrosion along the free length of the tendon

2.3.3 Cases of Post-Tensioned Tendon Corrosion

Table 2.1 presents a number of post-tensioned bridges that experienced tendon corrosion throughout their

service life. Included in the table is the bridge name, bridge type, location of the bridge and observed

damage to the tendons. The degree of damage observed varied included significant tendon corrosion,

complete failure of tendons, and in some extreme cases complete collapse of the bridge. In cases where


the tendons did not reach the point of failure the tendon corrosion observed was significant enough to

cause bridge closures and/or require repairs to the system.

Bridge Name Bridge Type Location Year Observed

Damage Causes of Corrosion

Niles Channel

Bridge*

Precast

Segmental PT

Box Girder

Florida 1999 Tendon

Failure

Water infiltration into tendon

anchorage with voids

Mid Bay

Bridge*

Precast

Segmental PT

Box Girder

Florida 2000 Tendon

Failure

Cracked PT ducts and exposed strand

along water bleed trails

Sunshine

Skyway**

Precast

Segmental PT

Box Girder /

Cable Stay

Florida 2000 Tendon

Failure

Poor grout quality/practices, voids

near anchorage, cracked HDPE

ducts, water infiltration through

segmental joints#

Ringling

Causeway***

Precast

Segmental PT

Box Girder

Florida 2011 Tendon

Failure

Deficient Grout (Grout having high

moisture content, high pore water

pH, low total chloride concentrations

and high sulfate concentrations)

Varina-Enon

Bridge++

Cable Stay with

PT Box Girder Virginia 2001

Tendon

Failure

Voids in tendons and absence of

grout#

Bickton

Meadows

Footbridge

Precast

Segmental UK 1967

Bridge

Collapse

Mortar joints allowed moisture,

chlorides and oxygen transport at

joints#

Ynys-Y-

Gwas+++

Segmental PT Whales 1985

Bridge

Collapse

Transverse joints between segments

filled with dry mortar caulking

allowing water infiltration#

Malle

Bridge+++

Precast

Segmental Belgium 1992

Bridge

Collapse

Voids in duct and ingress of water

and chlorides

*(Powers et al., 1999), **(Garcia, 2006), ***(Goldsberry, 2013),+ ++

(Sprinkel and Napier, 2008), +++

(Youn and Kim, 2006), #(Trejo et al., 2009)

Table 2.1: Cases of Post-Tension Tendon Corrosion


3 Overview of Applicable Inspection Methods

To address the issues with current PT bridge construction, as discussed in section 2, an overview of the

applicable inspection methods are provided. Each overview includes a summary on the use of the

method, a description of the method, accuracy as reported in the literature, and recommended use with

respect to post-tensioned grouted bridge systems. The methods examined in this section in, alphabetical

order, are summarized below.

Acoustic Emission

Electrically Isolated Tendons

Ground Penetrating Radar

Half-Cell Potential

Impact Echo

Infrared Thermography

Radiography

Time Domain Reflectometry

Ultrasonic Testing

Visual Inspection

3.1 Acoustic Emission

3.1.1 Applications

Acoustic emission (AE) monitoring techniques have applications in various fields. In the chemical

industry AE is used to detect stress corrosion cracking, pitting and crevice corrosion in stainless steel. In

the field of civil engineering this method has been used to monitor railroad, highway bridges, load-

bearing structures, pipes and storage tanks (Chang and Liu, 2003). AE has been applied for the detection

and localization of corrosion-induced cracking in steel reinforcing bars (Di Benedetti et al., 2012), for

detecting the onset of corrosion of bonded prestressing tendons (Azizinamini and Gull, 2012a), measuring

mass loss in prestressing steel (Mangual et al. 2012), for identifying initial yield in bonded post-

tensioning tendons (Salamone et al., 2012), and for the identification of wire fracture in bonded, partially

bonded and unbonded prestressing strands (Cullington et al., 2001).

3.1.2 Methodology

Acoustic emission monitoring is a nondestructive method that can be used for both global and local

monitoring. AE can refer to both the monitoring technique itself and the phenomenon on which this

technique is based. The AE phenomenon refers to the transient elastic waves that are generated by the

rapid release of energy from localized material or bond failures, per ASTM E1316. The AE technique

refers to the use of one or more sensors to capture these events that may take place in a material (Di

Benedetti et al., 2012). Acoustic emission monitoring is considered to be a “passive” monitoring

technique where the detection system must wait for an occurrence, usually due to corrosion, wire fracture

or cracking, to capture stress waves (Ciolko and Tabatabai, 1999). Effective implementation of this

method in the field requires continuous monitoring and a permanent data acquisition system.

AE monitoring provides information regarding plastic changes in a material or structure by recording the

transient stress waves generated by the rapid release of energy from localized material or interfacial bond

failures. These stress waves propagate through the material and are recorded by surface-mounted and/or

embedded sensors. By utilizing a network of spatially distributed sensors, the energy source can then be

located by using triangulation techniques. The recorded waveforms can also be compared to experimental

and/or numerical simulation data in order to identify causality (i.e. the damage mechanism that provided


the source of energy release). Material failures are typically characterized by short burst emissions (short

rise time) with intensities that are larger than the continuous background noise in the recording.

An application of AE monitoring is illustrated in Figure 3.1 for a concrete beam flexure test. The typical

shape of the resulting waveform and time history recording of cumulative AE energy are presented in

Figure 3.1 (b) and (c), respectively. The energy released by the damage mechanism is measured as the

area under the amplitude-time curve, while rise time and signal duration (defined in Figure 3.1 (b)) can

provide clues regarding the type of damage mechanism. By utilizing a network of sensors along the

surface of the structure, variations in AE events at different transducer positions can be calculated. The

location of the AE event can then be determined by using triangulation techniques.

Figure 3.1: Acoustic emission monitoring of concrete beam flexure tests: (a) test assembly,

(b) typical acoustic emission signal, (c) load and acoustic emission energy histories

(Figure adapted from Landis and Baillon, 2002)

A schematic illustration of the test assembly utilized by Mangual et al. (2012) for studying mass loss in

prestressing strands under accelerated corrosion is presented in Figure 3.2(a). The study suggests that AE

monitoring, using the approach outlined previously, can provide estimates of corrosion-induced mass loss

that are comparable with the half-cell potential method. Figure 3.2(b) shows an increase in cumulative

AE energy refered to as cumulative signal strength (CSS) that occurs near the onset of corrosion in the

post-tensioning steel. Note that this sharp increase was only observed in one of the nine cases illustrated.

(a) (b)

(c)


(b)

Figure 3.2: Laboratory tests of acoustically monitored post-tensioned concrete beams under accelerated

corrosion (Figure adapted from Mangual et al. 2012)

An examination of AE monitoring techniques for corrosion detection in post-tensioning tendons was

conducted by Cullington et al., 2001. They utilized a SoundPrint® acoustic monitoring system to examine

a 10 m long post-tensioned concrete beam. The study found that wire fractures in unbonded tendons

produced very large events which could be easily detected by the monitoring system. Fractures in

grouted tendons were found to be more difficult to detect due to the fact that the magnitude of the event

caused by the wire fracture was much smaller (Cullington et al., 2001). The system examined was later

installed for monitoring on the Railway Viaduct in Huntingdon U.K.

3.1.3 Limitations

One of the major limitations of AE monitoring is that this method is not able to detect existing damage,

i.e. damage occurring prior to the installation of the structural monitoring system. AE application in the

field can also be expensive and can generate large volumes of data that can be difficult to interpret

(Azizinamini and Gull, 2012a). When this method is applied in the field expertise is required to

differentiate signals created by damage events from those generated by ambient noise. This method also

requires supporting experimental and/or numerical data for calibration and validation of signal processing

and data interpretation algorithms. A priori knowledge of the baseline condition of the structure is

necessary since the procedure relies on an estimation of cumulative damage. The accuracy of AE

monitoring is influenced by signal attenuation (distance between the AE event and surface transducers),

specimen geometry (multiple reflective surface and orientations can complicate data interpretation), and

material composition. Finally, for post-tensioning applications, the AE method requires a high number of

closely spaced sensors due to the leakage of the ultrasonic waves into the concrete (Bartoli et al., 2009).

3.1.4 Viability in Post-Tensioned Applications

AE is currently best suited for continuous monitoring of fracture sensitive details, e.g. post-tensioning

steel anchorage regions and tie down locations, and where environmental corrosion poses a significant

(a) (b)


threat. The use of sensor networks can enhance damage detection and location capabilities. Acoustic

emission can be successfully applied in practice for the detection of corrosion induced failures of

prestressing wire for unbonded tendons and stay cables (Iyer, Schokker and Sinha, 2002). Although

recent laboratory experiments have shown promising results for detecting the onset of corrosion in

bonded prestressing tendons (Azizinamini and Gull, 2012a) this technology has not been used validated in

realistic PT bridge structures and would require additional evaluation to ensure that it is capable of

measuring damage associated with breakage of individual wires or strands. Additional research is needed

to validate the approach for realistic field applications.

3.1.5 References

Further information on acoustic emission monitoring can be found in the following references included in

this report:


Report –Volume 1 (Azizinamini and Gull, 2012a)


Concrete Bridges- Final Report – Volume II (Azizinamini and Gull, 2012b)

5.3.2 Nondestructive Methods for Condition Evaluation of Prestressing Steel Strands in Concrete

Bridges, Final Report Phase I: Technology Review (Ciolko and Tabatabai, 1999)

7.1 Acoustic Emission Monitoring of Reinforced Concrete under Accelerated Corrosion (Di

Benedetti et al., 2012)

7.2 Corrosion Damage Quantification of Prestressing Strands using Acoustic Emission (Mangual

et al., 2012)

7.16 Ultrasonic Imaging – A Novel Way to Investigate Corrosion Status in Post-Tensioned

Concrete Members (Iyer, Schokker and Sinha, 2002)

7.20 Continuous Acoustic Monitoring of Grouted Post-Tensioned Concrete Bridges (Cullington

et al., 2001)

9.1 Detection of Initial Yield and Onset of Failure in Bonded Post-Tensioned Concrete Beams

(Salamone et al., 2012)


Bridges (Bartoli et al., 2009)

10.16 Recent Research in Nondestructive Evaluation of Civil Infrastructures (Chang and Liu,

2003)

10.22 Experiments to Relate Acoustic Emission Energy to Fracture Energy of Concrete (Landis

and Bailon, 2002)

10.29 Imaging of Reinforced Concrete: State-of-the-Art Review (Pla-Rucki and Eberhard, 1995)

11.1 Automatic Delamination Detection of Concrete Bridge Decks Using Impact Signals

(Zhang, Harichandran, and Ramuhalli, 2012)

11.3 Procedures for Fatigue Crack Growth Monitoring and Fatigue Life Prediction Using

Acoustic Emission Data and Neural Networks (Barsoum et al., 2011)

3.2 Electrically Isolated Tendons

3.2.1 Applications

Electrically isolated tendons (EIT) are an enhanced tendon/anchorage detailing system used in bonded

post-tensioned systems. One of the main advantages in comparison to traditional tendon/anchorage

systems is that an EIT system provides enhanced corrosion protection of the tendons and allows for

quality control during construction and monitoring during the service life of the system (Della Vedova

and Elsener, 2006). EIT systems are typically used in situations where corrosion due to stray current is a


potential issue or in systems where a high level of protection of the strands is desired. In addition to

quality control monitoring the EIT system also has the potential to identify breeches in the corrosion

protection system of the tendon throughout the service life of the structure by monitoring for the ingress

of water into the grouted duct.

3.2.2 Methodology

The electrically isolated tendon system for internal grouted post-tensioning applications was developed to

provide an increased level of protection and provide monitoring capability of the tendons. Electrically

isolated tendons differ from typical post-tensioned system details in that they provide full electrical

isolation of the PT tendons from the normal rebar network. This requires the use of electrically isolated

anchor heads, corrugated plastic ducts (polyethylene or polypropylene), and special care at grout vents.

Detailing of EITs is critical to the effective implementation of the system, particularly near the

anchorages in order to ensure complete encapsulation and electrical isolation. An overview of the design

of an electrically isolated system is provided by Della Vedova and Elsener, 2006. The following provides

a summary of the detailing required at the anchorage region to ensure electrical isolation of the system.

In order to achieve electrical isolation a mechanically resistant insulation plate is placed between the steel

anchor head with wedges that block the strand and cast iron bearing plate. This electrically isolates the

tendon from the non-prestressed reinforcement network. In addition a plastic trumpet is tightly connected

to the duct inside the anchorage to isolate the strand from the cast-iron bearing anchorage. An electric

terminal must attached to the anchorage head and appropriately routed to allow access for impedance

measurements. It is important that an access box be used to collect the electrical terminals from the

tendons and that it be positioned to provide easy access for future inspection and maintenance personnel

(Della Vedova and Elsener, 2006). A schematic of a typically electrically isolated tendon anchorage is

presented in Figure 3.3.

Figure 3.3: Electrically isolated tendon schematic (Figure adapted from Della Vedova and Elsener, 2006)

One of the major advantages of using electrically isolated tendons is that the corrosion protection of the

steel strands can be monitored during construction and throughout the service life of the structure using

AC impedance measurements (electrical resistance measurements). The impedance measurements require

a sound electrical connection to each tendon and an additional connection to the non-prestressed

reinforcement in the component (Elsener, 2004b). Typically, all connection wires are routed to an

accessible connection box, which will be used by inspectors to take measurements (Figure 3.4). The

impedance measurements are taken with a portable LCR meter, which can measure the inductance (L),

the capacitance (C), and resistance (R) of a component. For the chosen measurement frequency the LCR


meter then calculates and displays the ohmic resistance (R), the capacitance (C), and the loss factor (D),

which are then recorded by the inspector (Elsener, 2004b).

Figure 3.4: Connection box for impedance measurements (Elsener, 2004b)

The impedance measurements taken are performed between the steels stand in the duct and the normal

(non-prestressed) reinforcement network in the concrete (Figure 3.5). Therefore, the measuring system

includes the grout within the duct, the duct itself (including any defects) and the concrete surrounding the

duct (Elsener and Buchler, 2011). The concrete and grout are pure resistances (in the range of measuring

frequencies between 100 and 1000 Hz), in contrast the plastic duct is in essence a capacitance in parallel

with a very high resistance (Figure 3.6), as a result system defects and/or imperfections are represented by

ohmic resistance in parallel (Elsener, 2008). The ohmic resistance, capacitance, and loss factor

measurements can determine the degree of electrical isolation at any time after grouting, which is used for

quality control and long term monitoring of the corrosion protection of the tendons (Della Vedova,

Elsener, Evangelista, 2004).

Figure 3.5: Principle of measuring the electrical impedance of a tendon with the LCR meter (Elsener and

Buchler, 2011)


Figure 3.6: Electrical equivalent circuit for an electrically isolated tendon with small defect (Elsener and

Buchler, 2011)

It is recommended by Elsener and Buchler, 2011, that impedance measurements be taken after the

tendons have been stressed but before introducing the grouting material. These resistance measurements

can be used to identify short circuits or unexpected low impedance values between the prestressing

strands and the normal reinforcement, allowing the possibility for repairs to be made. For quality control

purposes in order for electrically isolated tendons to meet the acceptance criteria resistance measurements

must be taken after the tendon is grouted and compared to the published limiting values (Figure 3.7).

Based on the 2007 revision to the Swiss guideline “Measures to Ensure Durability of Post-Tensioning

Tendons in Structures” (ASTRA, 2007) the electrical resistance measurements may be performed anytime

between 7 and 56 days and normalizing the measured values to 28 days. In addition, the acceptance

criteria have been adjusted to reflect the various applications for which EIT can specified; preventing

fretting between the normal reinforcement and prestressing strand (fatigue), use for the purpose of long

term monitoring (monitoring) and for protection against stray current (stray current).

Figure 3.7: Limit Values (28 days after injection) (ASTRA, 2007)

For long term monitoring the resistance values can be analyzed over time to provide information on the

condition of the corrosion protection system. A continuous increase in the resistance measurements over

time is expected due to the hydration of the grout and concrete surrounding the tendon. Figure 3.8 shows

an example of the increasing trend of electrical resistance measurements that is expected with time.

Based on this principle that the resistance measurements should increase over time early warning signs of

a breech in the corrosion protection system can potentially be identified by deviations from the trend line

(decreasing resistance measurements). It should be noted that it is important to monitor the temperature at

the time of inspection because temperature variations can cause some variation in the expected

measurements.


As previously mentioned a deviation in the trend of the resistance measurements over time can indicate

that a defect in the corrosion protection system is present. An increase in moisture level of the grout or

concrete is typically the cause of the decrease in measured resistance, so the ingress of water potentially

containing chlorides can be identified at an early stage. If inspections are conducted a regular intervals

the electrical impedance measurements can provide early warnings that the corrosion protection systems

has been breached before corrosion begins (Elsener, 2005).

Figure 3.8: Evolution of electrical resistance over time (Della Vedova and Elsener, 2006)

The use of electrically isolated tendons for monitoring purposes have been tested in laboratory

experiments (Elsener and Buchler, 2011; Elsener, 2008; Elsener 2004a) and have more recently been

implemented in a number of post-tensioned bridges in Switzerland and Italy (Elsener, 2008; Della

Vedova and Elsener, 2006; Elsener 2004b; Della Vedova and Evangelista, 2004; Della Vedova, Elsener

and Evangelista, 2004) where data measurements for quality control monitoring have been collected and

published. In addition guidelines for implementing electrically isolated tendons into post-tensioned

construction have been developed and published in a Swiss guideline “Measures to Ensure Durability of

Post-Tensioning Tendons in Structures” (ASTRA, 2007). Example of bridges where electrically isolated

tendons are current implemented include Piaceza Viaduct and Marchiazza Viaduct in Italy and P.S. du

Milieu, Wiesebrucke Basel and Glattal Viaduct in Switzerland.

3.2.3 Limitations

One of the limitations that arise with respect to NDE using electrically isolated tendons is that fact that

this type of evaluation cannot be used on existing systems with conventional non-isolated

tendon/anchorage detailing. When implementing EITs into new construction extreme care must be taken

during the construction phase to insure that electrical isolation of the tendon is achieved. If a short circuit

is present then future NDE is not possible, making the system ineffective. Currently the acceptance

criteria for EITs is strict but can be achieved if there is adequate planning and detailing in the design stage

assuring there is room for the tendon between normal rebar, careful workmanship during the construction

phase and the control over the ducts for leaks at joints, couplers, welding or defects is taken (Elsener,

2005).

Another limitation of the EIT system is that it although it can potentially identify breeches in the

corrosion protection system the method cannot currently identify the location of defect along length of the


tendon. Being able to determine the location of the damage along the length tendon would be an area

worth investigating for improvement of the EIT system. The information gained by being able to

determine defect location would make this a more complete system by allowing the area of damage to

quickly be investigated with respect to durability and allow for repairs to be made. Some recent efforts

have been made to determine if applying the magnetic flux method using the electrical connections

between the prestressing strand and normal reinforcing can be used to successfully identify defect

location along the strand (Elsener and Buchler, 2011). It should also be noted that although these systems

have been implemented there have been no known cases where damage has been identified based on long

term monitoring techniques. This may be attributed to the fact that these systems have not been in place

long enough (15-20 years) where corrosion of the tendons is expected, but this should continue to be

investigated over time to establish the capability for long term monitoring.


Electrically isolated tendons are applicable where internal grouted tendons require the highest level of

protection and/or for monitoring of the tendons for quality control and throughout the life of the structure.

Currently the main advantage of this system is the ability to provide quality control measurements leading

to an overall higher level of protection for the strands. The condition of the corrosion protection system,

particularly the ability to the ingress of water can potentially be detected through long term monitoring of

the impedance measurements. This method can be used to identify early warning signs of deterioration to

the corrosion protection system identifying possible tendon corrosion. NDE using EITs cannot be applied

to as built systems that do not already have electrically isolated tendons. NDE of the EITs using

impedance measurements appears to be viable tool to monitor the corrosion protection system of the

tendon based on the presented literature. It is recommended that the EIT system be examined further to

verify its ability to detect breeches in the corrosion protection system as well as potential to identify

location of defects along the length of the tendon.

3.2.5 References

Further information on electrically isolated tendons can be found in the following references included in

this report:


Bridges (Elsener and Buchler, 2011)


(ASTRA, 2007)

7.4 Monitoring of Electrically Isolated Post-Tensioning Tendons (Elsener, 2008)

7.6 Enhanced Durability, Quality Control and Monitoring of Electrically Isolated Tendons (Della

Vedova and Elsener, 2006)

7.8 Long-Term Monitoring of Electrically Isolated Post-Tensioning Tendons (Elsener, 2005)

7.9 Electrical Isolation as Enhanced Protection for Posttensioning Tendons in Concrete Structures

(PL3) (Elsener, 2004a)

7.10 Experience with Electrically Isolated Tendons in Switzerland (Elsener, 2004b)

7.11 Protection Against Corrosion and Monitoring of Posttensioning Tendons in Prestressed

Concrete Railway Bridges in Italy (Della Vedova and Evangelista, 2004)

7.12 Corrosion Protection and Monitoring of Electrically Isolated Post-Tensioning Tendons

(Della Vedova, Elsener and Evangelista, 2004)


3.3 Ground Penetrating Radar

3.3.1 Applications

Ground penetrating radar (GPR) has a large variety of uses as an NDE technique in many different fields

including geotechnical, structural, environmental and mining applications. In the field of structural

engineering GPR is typically used in concrete structures to detect the location of reinforcing bars, tension

cables, conduits, grade beams, voids in the concrete, and slab thickness (Gehrig, Morris and Bryant,

2004). GPR has also been investigated for applications of strand corrosion detection (Jones et al., 2010)

and to assess grout conditions in plastic ducts (Zhou, Wang & Zhang, 2012), which is an extension of this

method that requires continued research efforts. For post tensioned systems GPR is primarily used to

identify the location of metallic ducts and has more recently been investigated for locating plastic ducts

(Cheilakou et al., 2012).

3.3.2 Methodology

Ground penetrating radar, for the application of locating embedded metallic components in concrete

structures, is a commonly used NDE method with commercially available test equipment. GPR

techniques for civil engineering applications are based on the propagation of high frequency

electromagnetic waves, typically between 20MHz and 2.5 GHz, through the specimen under

investigation. When an impulse encounters an interface between material layers with different dielectric

properties a portion of the wave is reflected back to the receiving point (Kohl and Streicher, 2006). Some

examples of what constitutes as an interface between layers include in a concrete system can include;

reinforcing bars, metal or plastic ducts, voids and the end of a specimen. Figure 3.9 shows the principle of

GPR reflections at inhomogeneities in the material. The velocity at which the pulses propagates through

the specimen and the intensity of the reflections are a function of the dielectric properties of the material,

therefore if the permittivity of a material is known then the depth at which the reflection occurred can be

determined using the propagation time (Maierhofer, 2003). Knowing the depth at which the reflection

occurred means the location of items of interest such as metallic ducts in PT systems can be identified.

Figure 3.9: Schematic of GPR Process (Maierhofer, 2003)

A typical ground penetration radar system used for concrete infrastructure applications generally requires

the following components; a control unit (computer), pulse generator, antennae cable of receiving and

transmitting, and a video monitor. Ground penetrating radar has a wide variety of NDE applications

therefore there are many antenna manufactures, antenna types, signal setting options, operating

frequencies and software packages to choose from. It is important that each GPR system be designed to

meet the NDE needs of the system being evaluated (Gehrig, Morris and Bryant, 2004). An example of a


GPR system used for the purpose of identifying corrosion in prestressing beam can be seen in Figure

3.10. When taking measurements the GPR system is placed and moved in a known pattern on the surface

of the material under investigation generating an image in real time. Systematically surveying an area in

a rectangular grid pattern can allow for a radar image of the ground to be built up. These images can then

be displayed as two-dimensional representations or as a three-dimensional reconstruction (Zhou, Wang

and Zhang, 2012).

Figure 3.10: Example of GPR components (Jones et al., 2010)

Typically there are two classifications of antennae technology used for evaluating concrete structures; (1)

ground coupled and (2) air or horn coupled. Ground coupled equipment typically maintains direct contact

with the specimen surface and generally provides more qualitative then quantitative information and is

able to penetrate deeper into the material (Maser, 1996). Examples of suitable applications of ground

coupled antenna evaluation of concrete structures are bridge decks, streets, highways, parking lots,

retaining walls and foundations systems. Air coupled technology provides more quantitative results at

higher resolutions but for shallower penetration depths and is a noncontact method that can operated 20-

50 cm above the surface from a moving vehicle traveling a normal highway speed (Figure 3.11) (Maser,

1996). Air coupled technology is typically used for road condition evaluation because of its ability to

efficiently collect data. In addition to the antenna technology there are also two different types of

antennas that can be used in a radar system; (1) bistatic and (2) monostatic antennae. A bistatic antenna

uses two antennae, one to transmit and the other to receive while a monostatic antennas uses one antenna

that is capable of transmitting and receiving. In the case of concrete evaluation, monostatic antennae tend

to perform better due to their higher data collection and processing efficiency (Gehrig, Morris and Bryant,

2004). Typical inspections can be carried out using 500 MHz, 900 MHz, 1 GHz and 1.5 GHz bowtie

antennas as well as 1 and 2.5 GHz horn antennas (Maierhofer, 2003).


Figure 3.11: Mobile system with GPR antennas and data acquisition system

(Iyer, Sinha and Schokker, 2005)

The most common application for GPR in concrete structures is for locating embedded metallic

components. In post-tensioned systems this method is best suited for the locate tendon ducts and

determine the depth of concrete cover (Maierhofer et al., 2004). Detecting metallic ducts utilizes the

same principles as detecting rebar; the reflections caused by the metallic tendon ducts and rebar can be

observed and recorded in order to determine their location. The ability of the GPR method to determine

the location of both tendon ducts and reinforcing bars has been validated in a number of different

publications (Bala, Garg and Jain, 2011; Kohl and Streicher, 2006; Gehrig, Morris and Bryant, 2004;

Maierhofer et al., 2004; Maierhofer, 2003; Derobert, Aubagnac and Abraham, 2002). In addition to using

GPR to locate metallic ducts laboratory experiments have explored the capability of this method to locate

embedded plastic ducts. A research effort by Cheilakou et al., 2012 focused on the inspection of different

concrete blocks with embedded steel reinforcement bars and plastic ducts. It was concluded that the GPR

system used for this investigation was able to provide accurate depth and position of the top rebar and

plastic ducts (Cheilakou et al., 2012). An example of a 2D radargram produced during this study showing

the position of an embedded plastic duct can be seen in Figure 3.12.

Figure 3.12: 2D Radargram of Sample 1 revealing in the position of the embedded plastic ducts

(Cheilakou et al., 2012)


GPR has also been investigated for identifying grout condition and voids within post tensioned ducts. It

was determined that GPR can only be applied in this capacity when plastic ducts are used due to the fact

that electromagnetic waves cannot penetrate through metal (Wiggenhauser, Streicher and Friese, 2011).

In particular GPR is likely sensitive to the occurrence of soft, non-setting and chalky grouts or cases or

water intrusion since these are likely to exhibit higher levels of moisture and electrolytes with in the grout

(Azizinamini and Gull, 2012a). The application of GPR technology as a method to determine grout

condition within post tensioned ducts has been explored in various research studies (Zhou, Wang and

Zhang, 2012; Pollock et al., 2008; Maierhofer et al., 2004; Derobert, Aubagnac and Abraham, 2002).

Results from Pollock et al., 2008 concluded that PT strands and simulated air voids within the grouted

steel ducts were not detectable; however, simulated voids within plastic ducts were generally detectable in

GPR images. This application of GPR shows promise but needs to be investigated further to overcome

issues such as void orientation which can affect the ability of the method to successfully identify grout

voids within the plastic ducts (Pollock et al., 2008).

In addition to the previous applications GPR has been investigated as a potential method to identify

corrosion. When using GPR as a NDE technique for corrosion detection there are two main principles

that are applied: (1) corroded reinforcement will produce a GPR reflection and (2) the moisture condition

in the concrete that produced the corrosion will alter the GPR reflection (Jones et al., 2010). Laboratory

experiments were conducted by Jones et al., 2010 to investigate the ability of the GPR method to identify

strand corrosion in prestressed beams. For the application of prestressing steel a high frequency antenna is

needed due to the face that prestressing steel is more closely spaced and has a smaller diameter than

conventional steel reinforcement. Scans of the each beam were made in both the transverse and

longitudinal direction. An example of a transverse scan where corrosion is indicated can be seen in Figure

3.13. Upon the completion of the study the following conclusions were made by the authors. The method

appears to only pick up heavy corrosion damage in the strand: i.e. heavy pitting and wire loss and did not

allow for localized identification of corrosion (Jones et al., 2010).

Figure 3.13: Transverse scan showing reduced refelection amplitude indication corrosion (Jones et al.,

2010)

3.3.3 Limitations

One of the main issues affecting the accuracy and implementation of ground penetrating radar is the fact

that metal interfaces produce 100% reflection of the wave. This means that identifying duct location in


areas with larger amounts of reinforcing (typically where bars are spaced less than 7 cm apart

(Mairerhofer, 2003)) is not possible. An additional result of the complete reflection of the wave at metal

interfaces is that this method cannot be used to inspect grout condition within metal ducts because it is

physically infeasible to test inside a metal duct with the type of waves used in GPR (Derobert, Aubagnac

and Abraham, 2002).

GPR scans can also be difficult to interpret and the estimation of depth of reflectors (bars or voids)

depends on an assumed velocity of wave propagation (Ciolko and Tabatabai, 1999). The accuracy of the

ground penetrating radar method is also currently limited by penetration depth, depth resolution and

horizontal resolution. The effect of these parameters on the accuracy as presented by Maierhofer, 2003 is

briefly summarized. Penetration depth depends on the damping of the electromagnetic waves, which is

dependent on the absorption in the material, loss due to the effective angle of the antenna and loss due to

scattering and reflection. Typically the depth of penetration decrease as the frequency of the antenna

increases. The depth of resolution is affected by the duration of the electromagnetic impulses and

therefore on the bandwidth of the antenna used. Antennas with higher frequencies results in increased

depth resolution. The horizontal resolution, which is the distance between two adjacent reflections

centers, depends on the damping of the electromagnetic waves in the material, the antenna aperture and

the frequency (Maierhofer, 2003).

Ground penetrating radar is also affected by electric properties of the system being evaluated. The GPR

method requires that the permittivity of the material be known. The permittivity of the materials,

however, is influenced by outside parameters such as; material temperature, moisture content, salt

content, pore structure and pulse frequency. Consequently, in order to have accurate results using GPR,

calibration measurements of the material need to be taken, which in many cases may require taking core

samples of the specimen

The application of ground penetrating radar as a method for detecting strand corrosion and grout

condition within plastic tendon ducts still has many limitations. In the case of prestressing strand

corrosion GPR was found to only be able to accurately detect heavy pitting and wire loss (Jones et al.,

2010). This limits the ability to detect corrosion at early stages and requires significant levels of corrosion

to be present in the field. When investigating grout condition this method has not proved to be

consistently accurate. Testing of GPR on plastic ducts with simulated voids has shown that voids in the

tendon duct were detectable if the width (larger dimension) of the void was oriented facing the emitted

microwaves, while voids with the thickness (smallest dimension) oriented facing the microwaves tended

to be masked by the presence of the tendon (Pollock et al., 2008). In addition there is little published

literature currently available on GPRs sensitivity to other grout conditions issues such as soft, non-setting

and chalky grout or water intrusion. Further investigation and technological improvements to allow for

increased sensitivity to corrosion levels and grout voids in plastic should be explored for this method

before it is recommended for use in the field for these applications.

.


Ground penetrating radar can be quickly and easily implemented on bridge decks due to the fact that it

can be performed from a moving vehicle. Even in the case of post tensioned beams and girders, which

normally utilizes hand help devises, GPR can still easily be implemented since access to only one side of

the structural component is required. This method is generally not expensive to use in comparison to

other NDE techniques and does not pose a significant safety or heath risk to the user or the public. The

electromagnetic emissions of GPR are typically below 0.001 W/m2 which is significantly less than the

OSHA regulation of 100 W/m2, this means that a bridge can remain open to the public during inspections

conducted using GPR.


Typically this method has been found to be successful in the field for identifying the location metallic

ducts, assuming that there is no interference due to congested rebar layouts. Recent advances in

technology have also made GPR useful in situations where subsurface imaging is desired as it can

produce one of the highest resolutions of any subsurface imaging technique. This method has also shown

promise in the area of grout void detection of plastic tendon ducts and corrosion detection but is not

currently recommended for this application in the field given the current state of technology. Continued

research on this method to expand its NDE capabilities for inspection PT bridge systems is recommended

due to its low cost and ease and speed of application.

3.3.5 References

Further information on GPR methods can be found in the following references included in this report:






Strands in Concrete Bridge Components Task 2 – Assessment of Candidate NDT Methods (Jones

et al., 2010)


Penetrating Radar (Pollock et al., 2008)



6.4 Detecting Voids in Grouted Tendon Ducts of Post-Tensioned Concrete Structures Using

Three Different Methods (Zhou, Wang and Zhang, 2012)


Structures and Metallic Tendon Ducts (Maierhofer et al., 2004)

6.26 Comparison of NDT Techniques on a Post-Tensioned Beam Before its Autopsy (Derobert,

Aubagnac and Abraham, 2002)

7.17 Comparison of NDT Techniques on a Post-Tensioned Beam Before its Autopsy (Cullington

et al., 2001)

8.1 Application of Ground Penetrating Radar (GPR) as a Diagnostic Technique in Concrete

Bridge (Cheilakou et al., 2012)

8.2 Rebar Detection Using GPR: An Emerging Non Destructive QC Approach (Bala, Garg and

Jain, 2011)

8.4 Ground Penetrating Radar for Concrete Evaluation Studies (Gehrig, Morris and Bryant,

2004)

8.6 Nondestructive Evaluation of Concrete Infrastructure with Ground Penetrating Radar

(Maierhofer, 2003)

8.8 Condition Assessment of Transportation Infrastructure Using Ground-Penetrating Radar

(Maser, 1996)

10.10 Evaluation of Radar and Complementary Echo Methods for NDT of Concrete Elements

(Maierhofer et al., 2008)


Bridges (Kohl and Streicher, 2006)


Corrosion and Voids (Iyer, Sinha and Schokker, 2005)


2003)

10.25 Accuracy of NDE in Bridge Assessment (Martin et al., 1998)


10.31 Automated NDE of PT Concrete Structures (Wiggenhauser, Streicher and Friese, 2011)

3.4 Half-Cell Potential

3.4.1 Applications

The half-cell potential method can be applied to concrete structures to detect corrosion in standard steel

reinforcing bars and prestressing strands. This method is used to produce data related to active corrosion

of the steel reinforcing. Half-cell potential was developed and primarily used to evaluate concrete

systems with standard rebar (Naito, Jones and Hodgson, 2010). This NDE method has also been explored

in many applications for corrosion detection in post-tensioned and prestressed applications (Mangual et

al., 2012; Naito, Jones and Hodgson, 2010; Salas et al., 2004; DMJM Harris, 2003; Powers, Sagues and

Virmani, 1999)

3.4.2 Methodology

The half-cell potential method is an established method for corrosion evaluation of mild steel

reinforcement in concrete structures. This method has been standardized as ASTM C876: “Standard Test

Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete.” When the embedded steel

undergoes corrosion there is a flow of electrical current between the areas of different voltage potential

along the steel. The current travels from the region of corrosion (anode) to the regions of passive steel

(cathode), though the concrete or grout (the electrolyte). The evaluation of steel corrosion in concrete

systems using the half-cell potential method is based on the measurement of a high voltage differential

between the external reference electrode (half-cell) and the embedded steel. The reference electrode

consists of a metal rod immersed in a solution. For common applications in concrete systems the half-cell

electrode is composed of copper rod in a copper sulfate solution (CSE), a silver rod in a silver chloride

solution (SCE), or Mercury/Mercury Chloride (Naito, Jones and Hodgson, 2010). The structural

component of concern is investigated by using the reference electrode to take voltage differential

measurements at numerous predetermined points over the surface. The recorded data can then be used to

identify areas of corrosion based on the measured potential.


Figure 3.14: Reference Electrode Circuitry (ASTM C 876, 2009)

The procedure for applying the half-cell method is presented in detail in ASTM C876 (2009) and is

briefly summarized here. In order to take voltage measurements a direct connection to the reinforcing

steel must be made. An electrical connection is made to connect the steel to the positive terminal on the

voltmeter and the reference electrode to the negative terminal (Figure 3.14). To effectively use this

method the concrete surface must be sufficiently moist to decrease the electrical resistance of the circuit.

The procedure to determine if the concrete is sufficiently moist involves attaching the multi-meter to a

strand and placing the half-cell electrode on the concrete surface somewhere along the strand. If the

reading is a near constant value then the concrete can be deemed sufficiently moist for testing. In the

event that the concrete does not meet these requirements pre-wetting of the concrete is necessary before

corrosion potentials measurements can be taken. The test measurements taken can be presented using an

equipotential contour map which provides graphical delineation of areas where corrosion activity may be

occurring or cumulative frequency distribution which provides an indication of the magnitude of affect

area of the concrete member.


Figure 3.15: Half-cell electrode and multimeter (Naito, Jones and Hodgson, 2010)

The potential difference measurements obtained during the testing depend on the type of reference

electrode used, the corrosion condition of the steel with in the concrete, cover depth, concrete resistivity

and oxygen availability (Elsener et al., 2003). Figure 3.15 shows an example of a reference electrode and

multi-meter used during half-cell testing of prestressed beams. In traditional reinforced concrete systems

the readings obtained from the potential difference measurements are compared to known threshold

values to determine the probability of corrosion of the steel. The following excerpt from ACI Committee

Report 222.2R-01, “Corrosion of Prestressing Steels,” explains how the measurements obtained using the

half-cell potential method are used identify corrosion of the reinforcement. According to ASTM C 876, a

measured half-cell potential more negative than -350 mV indicates a probability greater than 90% of

active corrosion at the test location. A potential less negative than -200 mV indicated a probability of

active corrosion less than 10%; while potentials between -200 mV and -350 mV indicate that corrosion

activity is uncertain at the test location (ACI Committee 222). It is important to note that these half-cell

potential values were established for normal reinforcement and are not definitive for prestressing steels,

for large concrete covers, or for concretes with certain constituents. It is recommended for prestressing

steels that a map of the potential of the beam be developed and that corrosion activity be identified by

looking at large relative changes in potential over the surface (Naito, Jones and Hodgson, 2010). A

sample half-cell potential map is presented in Figure 3.16. The different colors used in the plot represent

various levels of probable corrosion, with red shading indicative of high probability of corrosion.


Figure 3.16: Half-cell potential map (Naito, Jones and Hodgson, 2010)

3.4.3 Limitations

There are many issues that arise when applying the half-cell potential method externally to post-tensioned

systems. Half-cell potential is a powerful tool for the application of detecting corrosion in normal

reinforcement, but in the case of PT strand is only successful under very favorable conditions (Iyer,

Schokker and Sinha, 2002). In a summary of available NDE methods for PT systems by Azizinamini and

Gull (2012a) , it is stated that this method is probably not applicable to internal or external PT ducts at

this time and it is recommended that robust sensors and sensing systems that can be applied within the

duct be developed.

The major limitation that affects the ability of this method to produce accurate results when the electrode

is applied externally on the concrete surface is the presence of the duct systems required in grouted post-

tensioned systems. Traditional half-cell measurements are based on the electrical and electrolytic

continuity between rebar or PT strands in the concrete, reference electrode on the concrete surface and

voltmeter (Elsener et al., 2003). In the case of metal ducts it was found that the duct shields the strands

from the concrete surface and with plastic ducts this method cannot be used due to the fact that the plastic

duct creates an electrical barrier (Iyer, Sinha and Schokker, 2005; Elsnder et al., 2003). Additionally,

when attempting to take measurements in specimens with metallic ducts these readings can be confused

between corrosion of the strand, mild reinforcement, or duct depending on the electrical connectivity

between these components (Iyer, Sinha and Schokker, 2005). On the other hand in a report by DMJM

Harris (2003), in which half-cell testing was used to measure corrosion activity at the surface of PT

tendons embedded in grout inside metallic ducts, concluded that this method can be used to measure

corrosion activity. The application of this method to PT tendons with metallic ducts still needs to be

further explored and verified, but it appears that there are many potential negative effects caused by the

duct system.

Another limitation of this method for PT systems is that an electrical connection to the rebar or

prestressing strand is required. This requires access to the prestressing strand or reinforcement which may

not always be easily accessible in existing structures and may require invasive drilling. In addition care

must be taken when making the electrical connection during inspection. A bad electrical contact to the

reinforcement from the voltmeter and between the individual bars/strand of reinforcement can cause

errors in the measurements (Iyer, Sinha and Schokker, 2005). Also when applying this method the effect

of outside factors such as moisture content of the specimen, which can have a significant effect on the

voltage potential measurements and can vary by location along the same specimen and from inspection

dates, must be understood.


Based on the current technology it is not recommended to use traditional externally applied half-cell

potential methods for inspection of post-tensioned systems due to the masking effect created by the

presence of the ducts. One way to avoid this issue may be to embed internal half-cell probes into the duct


which would allow half-cell potential measurement to be taken without the masking effecting seen during

external applications. Commercially available half-cell probes are available and can be utilized for this

purpose. Electrical continuity of the strands within each duct would require one connection to each end

anchorage. This could be integrated into both metallic duct and fully isolated plastic duct systems.

Additional benefits include the ability to localize damage detection. Embedded half-cell probes can be

installed during construction at key locations allowing for long term monitoring and the ability to locate

damage along the length of a component. The application of commercially available internally embedded

half-cell sensors should be further explored through laboratory testing to see if this method can be

successfully applied to fully-grouted post-tensioned systems using both metallic and plastic duct systems.

3.4.5 References

Further information on half-cell potential methods can be found in the following references included in

this report




Strands in Concrete Bridge Components Task 3 - Forensic Evaluation and Rating Methodology

(Naito, Jones and Hodgson, 2010)


Balanced Cantilever Concrete Bridges (DMJM Harris, 2003)

5.1.24 Corrosion Evaluation of Post-Tensioned Tendons in Florida Bridges (Powers, Sagues and

Virmani, 1999)

5.2.1 Corrosion of Prestressing Steels (ACI 222.2R-01) (ACI Committee 222, 2001)


Tensioned Bridges (Salas et al., 2004)

5.5.7 ASTM C876 – 09: Standard Test Method for Corrosion Potentials of Uncoated Reinforcing

Steel in Concrete (ASTM C 876, 2009)

7.2 Corrosion Damage Quantification of Prestressing Strands using Acoustic Emission (Mangual

et al., 2012)

7.15 Half-Cell Potential Measurements – Potential Mapping on Reinforced Concrete Structures

(Elsener et al., 2003)





3.5 Impact Echo

3.5.1 Applications

The impact echo (IE) method has proven to be an effective tool for the detection and quantification of

subsurface defects in post-tensioned concrete structures, including cracking (Sansalone and Streett, 1997;

Tokai, Ohkuno and Ohtsu, 2009; Matsuyama, Yamada and Ohtsu, 2010), delamination and voids

(Sansalone and Streett, 1997; Ohtsu and Watanabe, 2002; Yeh and Liu, 2009), and partially grouted post-

tensioning steel ducts (Ata, Mihara and Ohtsu, 2007; Alver and Wiggenhauser, 2010). In addition, the

method can be used to locate embedded features, such as steel reinforcing and utilities, and to measure the

thickness of structural elements (Sansalone and Streett, 1997; Carino, 2001). If the geometry of the survey

region is well defined, the IE method can also be used to estimate mechanical properties for the

component materials, e.g. density and elastic modulus (Sansalone and Streett; Carino, 2001). The IE

method can accommodate a wide range of construction materials and only requires access to a single


surface of the structure. It is noted that data processing and interpretation for the IE method can be

difficult for embedded features/defects with complex geometries (e.g. post-tensioning steel anchorage

regions) due to complex wave interactions from multiple reflective surfaces.

3.5.2 Methodology

The IE method utilizes mechanical stress waves to identify changes in acoustic impedance within a solid

body. A schematic illustration of the procedure is presented Figure 3.17. The stress waves are generated

by mechanical impact of the structure (e.g hammer strike). These stress waves, which include longitudinal

P-waves, transverse S-waves, and guided surface waves (e.g. Rayleigh waves), propagate within the solid

body and are reflected or refracted from boundary surfaces and material interfaces. Interaction of the

reflected waves alters the wave energy content through mode conversion, e.g. p-wave energy converted to

s-wave energy. Figure 3.18 illustrates the wave energy content in a finite element impact echo simulation

(note: the deformed geometry has been scaled to visualize the wave fronts). The reflected waves

eventually set up resonant vibration modes that are measured at the surface using contact sensors (e.g.

displacement transducers or accelerometers). In order to identify dominant frequency content, a Fourier

transformation is performed to convert the time history surface response data into the frequency domain.

With knowledge of the wave propagation velocity for the component materials and the frequency content

for the surface recording, the depth to reflecting surfaces can be calculated from first principals, assuming

linear elastic response. In general, a subsurface feature/defect will act as an internal reflective boundary

that will reduce the travel distance for the stress waves, thereby increasing the characteristic frequency of

the surface recording. Figure 3.19 presents an illustration of this shift in the frequency spectrum that is

caused by a subsurface void.

Figure 3.17: Schematic illustration of the impact echo method (Carino, 2001)

Figure 3.18: Finite element simulation of stress wave propagation in a linear elastic

medium due to impact loading (Carino, 2001)


Figure 3.19: Frequency spectra from impact echo tests: (a) solid slab and (b) slab with a subsurface void

(Carino, 2001)

3.5.3 Limitations

Interpretation of IE data for embedded features/defects with complex geometries is complicated by the

presence of multiple reflective surfaces and wave interactions. As a result, the inspection of post-

tensioning steel anchorage regions may be problematic. In addition, steel reinforcing and other embedded

features in the vicinity of an inspected component (e.g. a post-tensioning steel duct) can generate

unwanted reflections that complicate signal processing and interpretation. Conventional procedures for

IE testing also require multiple test configurations in order to quantify planform geometry for the

subsurface feature/defect and are, therefore, not well suited for surveying large areas. It is noted,

however, that systems are currently being developed to improve test efficiency by employing a distributed

network of non-contact, air-coupled sensors (see, for example, Oh et al. 2012).


The IE method can be utilized for the detection and quantification of subsurface concrete defects (e.g.

cracking, delamination, and the presence of voids) and partially grouted regions of post-tensioning steel

ducts. In addition, the IE method can be used to estimate the depth of embedded features, such as

reinforcing steel and utilities, and to estimate material properties when specimen geometry is well

defined. The IE method is not recommended for subsurface defects and features with complex

geometries, as the number and orientation of reflective surfaces can significantly complicate data

interpretation. Current IE testing procedures are not readily amenable to surveying large areas, but can be

effective in the inspection of critical regions. It is noted that researchers are currently investigating

methods to improve the rapid assessment capabilities of IE testing.

3.5.5 References

Further information on the impact echo method can be found in the following references included in this

report:


Report –Volume 1(Azizinamini and Gull, 2012a)





et al., 2010)



Ducts – Final Report (Muszynski, Chini and Andary, 2003)





Bridge Post-Tensioning Ducts using Rolling Stress Waves for Continuous Scanning (Tinkey and

Olson, 2006)

6.2 Quantitative Evaluation of Contactless Impact Echo for Non-Destructive Assessment of Void

Detection within Tendon Ducts (Schoefs, Abraham and Popovics, 2012)

6.3 Non-Destructive Testing Methods to Identify Voids in External Post-Tensioned Tendons (Im

et al., 2012)



6.5 Concrete Bridge Condition Assessment with Impact Echo Scanning (Olson, Tinkey and

Miller, 2011)

6.6 Inspection of Voids in External Tendons of Posttensioned Bridges (Im, Hurlebaus and Trejo,

2010)

6.7 Modified SIBIE Procedure for Ungrouted Tendon Ducts Applied to Scanning Impact-Echo

(Alver and Wiggenhauser, 2010)

6.8 On-Site Measurement of Delamination and Surface Crack in Concrete Structure by

Visualized NDT (Matsuyama, Yamada and Ohtsu, 2010)

6.9 Identification of Ungrouted Tendon Duct in Prestressed Concrete by SIBIE (Ohtsu, Yamada

and Nakai, 2009)

6.10 Estimation of Surface-Crack Depth in Concrete by Scanning SIBIE Procedure (Tokai,

Ohkuno and Ohtsu, 2009)

6.11 Imaging of Internal Cracks in Concrete Structures Using the Surface Rendering Technique

(Yeh and Liu, 2009)

6.13 Imaging Concrete Structures Using Air-Coupled Impact-Echo (Zhu and Popovics, 2007)

6.14 Impact-Echo Scanning Evaluation of Grout/Void Conditions Inside Bridge Post-Tensioning

Ducts for Tendon Corrosion Mitigation (Tinkey and Olson, 2007a)

6.15 Impact-Echo Scanning for Grout Void Detection in Post-tensioned Bridge Ducts - Findings

from a Research Project and a Case History (Tinkey and Olson, 2007b)

6.16 Sensitivity Studies of Grout Defects in Posttensioned Bridge Ducts Using Impact Echo

Scanning Method (Tinkey and Olson, 2007c)

6.17 Imaging of Ungrouted Tendon Ducts in Prestressed Concrete by Improved SIBIE (Ata,

Mihara and Ohtsu, 2007)

6.18 Automated NDE of Post-Tensioned Concrete Bridges Using Imaging Echo Methods

(Streicher et al., 2006)

6.19 Impact Echo Scanning for Discontinuity Detection and Imaging in Posttensioned Concrete

Bridges and Other Structures (Tinkey, Olson and Wiggenhauser, 2005)



6.24 Guidance on the use of NDE on Voided Post-Tensioned Concrete Bridge Beams using

Impact Echo (Clark et al., 2003)


Aubagnac, Abraham, 2002)

6.27 Stack Imaging of Spectral Amplitudes Based on Impact-Echo for Flaw Detection (Ohtsu and

Watanabe, 2002)


6.28 Applications of Impact-Echo for Flaw Detection (Wouters and Poston, 2001)

Error! Reference source not found. Error! Reference source not found. (Sansalone and

reett, 1997)

6.31 Detecting Voids in Grouted Tendon Ducts of Post-Tensioned Concrete Structures using the

Impact Echo Method (Jaeger, Sansalone, Poston, 1996)





9.2 Estimation of Existing Prestress Level on Bonded Strand Using Impact-Echo Test (Kim, Lee

and Cho, 2012)

10.2 Comparison of NDT Methods for Assessment of a Concrete Bridge Deck (Oh et al., 2013)


2003)

10.24 The Impact-Echo Method: An Overview (Carino, 2001)


10.31 Automated NDE of PT Concrete Structures (Wiggenhauser, Streicher and Friese, 2011)

3.6 Infrared Thermography

3.6.1 Applications

Infrared thermography (IT) has been successfully used for the detection and measurement of surface and

subsurface defects in reinforced concrete structures, including cracking (Zenzinger, 2007) and

delamination (Maser and Roddis, 1990; Del Grande and Durbin, 1995; Durbin, Del Grande and Schaich,

1996, Bolleni, 2009; Vaghefi et al. 2011). IT accommodates a wide range of materials and only requires

access to a single surface. It is noted, however, that IT requires experimental or numerical simulation

data to calibrate depth measurements for a particular test configuration.

3.6.2 Methodology

IT produces a 2-D thermal image of the surveyed surface that can identify surface and subsurface defects,

as well as embedded features, through variations in emitted infrared radiation. The procedure can utilize

active and/or passive heat sources (e.g. electromagnetic or solar radiation) to generate thermal flows in

the structure. A heat sensor (e.g. infrared thermographic radiometer) is used to detect variations in surface

temperature within the surveyed region that are caused by spatial variations in material heat transfer

properties and heat transfer boundary conditions. For example, heat flow through a solid concrete

structure is controlled by thermal conduction, while heat flow through a subsurface void is controlled by

cavity radiation and convection. The rate of heat transport for these three mechanisms can vary

significantly depending on material properties, structure geometry, and heat transfer boundary conditions.

As a result, the presence of a subsurface void alters the temperature profile in the concrete directly above

the void, including the surface temperature. It is this contrast in surface temperature between voided and

solid regions that facilitates visualization of subsurface defects. Since variations in material properties

between concrete and embedded elements (e.g. reinforcing steel) generate similar contrasts in surface

temperature, a similar approach can be used for the identification and measurement of embedded features.

An illustration of this approach is presented in Figure 3.20, which shows a thermal image of a concrete

block with four subsurface voids at known depths. The variation in surface temperature provides

quantitative data regarding the planform area of the voids. It is noted that IT utilizing active heating

provides enhanced control over heating and cooling rates, and therefore temperature profiles in the

structure, compared to passive heating approaches. The active heating approach for IT can be calibrated

to target defects and/or features at specific depths.


Figure 3.20: Thermal image of a concrete block with subsurface voids (void depth shown)

(Washer, Fenwick and Nelson, 2013)

Physical parameters that influence thermal imaging of reinforced concrete structures include incident heat

flux and absorptivity, material thermal conductivity and volumetric heat capacity, structure geometry, and

ambient conditions (temperature, incident solar radiation, and air flow).

3.6.3 Limitations

IT provides a 2-D planform visualization of subsurface features and defects. However, the depth and

thickness of the subsurface object is not easily determined without supporting experimental or numerical

calibration data. In addition, due to the presence of the duct and other reinforcement in the section,

existing IT technologies are generally ineffective in identifying or quantifying grout and strand conditions

for embedded post-tensioning steel ducts.


IT is well suited for scanning large regions of a structure for delaminations and voids and can also be used

to locate embedded features (e.g. reinforcing steel and utilities). It is noted that supporting experimental

or numerical simulation data is needed to calibrate depth measurements. IT is not recommended for the

inspection of grouted post-tensioning steel ducts or embedded features with complex geometries (e.g.

post-tensioning steel anchorage regions).

3.6.5 References

Further information on IT can be found in the following references included in this report:







et al., 2010)


Penetrating Radar (Pollock et al., 2008)


Ducts – Final Report (Muszynski, Chini and Andary, 2003)




5.4.4 Demonstration of Dual-Band Infrared Thermal Imaging for Bridge Inspection (Durbin, Del

Grande and Schaich, 1996)



10.1 Guidelines for the Thermographic Inspection of Concrete Bridge Components in Shaded

Conditions (Washer, Fenwick and Nelson, 2013)

10.2 Comparison of NDT Methods for Assessment of a Concrete Bridge Deck (Oh et al., 2013)

10.6 Application of Thermal IR Imagery for Concrete Bridge Inspection (Vaghefi et al., 2011)

10.8 Environmental Effects on Subsurface Defect Detection in Concrete Structures Using

Infrared Thermography (Bolleni, 2009)

10.11 Thermographic Crack Detection by Eddy Current Excitation (Zenzinger et al., 2007)

10.28 Using Emissivity-Corrected Thermal Maps to Locate Deep Structural Defects in Concrete

Bridge Decks (Del Grande and Durbin, 1995)

10.29 Imaging of Reinforced Concrete: State-of-the-Art Review (Pla Rucki et al., 1995)

10.30 Principles of Thermography and Radar for Bridge Deck Assessment (Maser and Roddis,

1990)

3.7 Magnetic Flux Leakage

3.7.1 Applications

Magnetic flux leakage (MFL) is an established magnetic based method that is used to assess the condition

of reinforcement in concrete structures. This method can be used to detect location of reinforcing bars

and for the detection of corrosion and loss of steel cross section in cables (Ciolko and Tabatabai, 1999).

In post-tensioned systems this method can be applied to detect wire strand fracture and thinning in

internal ducts and external ducts, as well as stays and ropes (Azizinamini and Gull, 2012a). This method

has also been investigated for use with EIT systems to determine the location of a defect along the strand

(Elsener and Buchler, 2011).

3.7.2 Methodology

The magnetic flux leakage method is based on the principle that steel is a ferromagnetic material through

which magnetic flux lines can develop. Defects in the steel can be measured as variations in an induced

magnetic field (Ghorbanpoor et al., 2000). When a magnetic field comes near a steel material in concrete,

the magnetic flux lines pass through the steel bar or strand due to the fact that the steel offers a path of

least resistance as a result of its high magnetic permeability compared to the surrounding concrete and air.

When discontinuities or defects, typically caused by corrosion or fracture of the strand or rebar, are

present the low resistance path becomes blocked and the remaining steel may become saturated, forcing

some flux to flow through the air (Figure 3.21). Changes in the components of the flux can be detected

by sensors and can then be analyzed to determine the severity of the flaw (DaSilva et al., 2009). In the

application of MFL systems Hall-effect sensors are typically used to detect flux leakage.


Figure 3.21: Magnetic flux leakage due to defect (DaSilva et al., 2009)

Magnetic flux leakage can be implemented using two different methods; the active and the residual

method. In the active method the sensors are placed between the poles of the magnet. The MFL device

containing the magnet and sensors is passed over the specimen under inspection in order to collect

magnetic flux data. The active method is best suited when there are large areas of corrosion present. The

measured magnetic flux can be affected by a number of different variables including; the location of the

magnet and the magnetic field, concrete cover, the type of duct used (steel versus plastic), distance

between the ducts, number of strands in the ducts, level of corrosion, transverse reinforcement in the

vicinity of the corroded area, level of tension force in the steel strands and the distance between the sensor

and the corrosion point (DaSilva et al., 2009). The second way MFL can be applied is the residual

magnetic flux leakage method. When applying the residual method the specimen under investigation is

first magnetized until the steel becomes magnetically saturated. Once saturated a MFL device is passed

along the specimen to read the magnetic field. The residual method is primarily used to detect small

areas of corrosion where the active method is no longer applicable (DaSilva et al., 2009). Both the active

and residual magnetic flux method are established methods that can indicate local wire breaks (or member

thinning) in near surface tendons (Azizinamini and Gull, 2012a)

A typical MFL system used in prestressed and post-tensioned applications is comprised of a magnetic

field source, magnetic field detection sensors, structural supporting frame, mechanical control device,

electrical control device and circuits, wireless communication devices, software and notebook computer

(Ghorbanpoor et al., 2000). MFL devices can be constructed as hand held devices or beam-rider devices

depending on the inspection needs of the system under investigation. An example of a device constructed

to test post-tensioned bridge systems as presented in Azizinamini and Gull (2012a), can be seen in Figure

3.22, where the following parts are marked before assembly. (1) Aluminum housing, (2) Position

indicator (Proximity sensor to identify spatial location), (3) Magnetic yoke consisting of rectangular

blocks of neodymium magnets altering with wedges of highly permeable, high saturation, magnet iron,

(4) Array of Hall-effect sensors to measure the magnitude of the vertical component of magnetic flux

(placed upside to show sensor) and (5) sensor electronic and signal conditioning board.


Figure 3.22: MFL system for post-tensioned bridge inspection (Azizinamini and Gull, 2012a)

The data collected as the MFL device passes over a concrete specimen is used to produce graphs that

display the relative magnetic field amplitude versus the traveled time. Corrosion of the strand results in

erratic readings in the measured flux which can be detected from the graphs. Figure 3.23 and Figure 3.24

show an example of a MFL graph produced from a tendon with no corrosion compared to a MFL graph

produced by a corroded tendon. When analyzing the graphs smooth lines between successive peaks

indicate areas where no corrosion is present, while the presence of corrosion changes the magnetic field

resulting in lines between peaks that are no longer smooth (Jones et al., 2010). In addition to visual

techniques of interpreting the measurements, data analysis software can also be used to implement the

following techniques for the corrosion identification; the differencing technique, the correlation

technique, the two-dimensional profile technique, and the three-dimensional magnetic field disturbance

technique (Ghorbanpoor et. al., 2000).

Figure 3.23: MFL graph indicating no corrosion (Jones et al., 2010)

Figure 3.24: MFL graph indicating corrosion (Jones et al., 2010)

Magnetic flux leakage systems have been developed specifically for use in prestressed structures. The

MFL method has been tested for its ability to detect corrosion and wire fracture of prestressing strands in

both prestressed (Jones et al., 2010; DaSilva et al., 2009; Ghorbanpoor et al., 2000); and post-tensioned

systems (Azizinamini and Gull, 2012a; Mietz and Fisher, 2007; DMJM Harris, 2003). In general, the

conclusions of these studies found MFL to be a promising method for corrosion detection of prestressing

tendons. Testing on girders from a demolished PT bridge found that using MFL measurement techniques,


areas with several wire rupture could clearly be detected (Mietz and Fisher, 2007). Laboratory

experiments where 19 strands were broken at the middle and placed inside corrugated and galvanized

pipe found that under ideal conditions MFL can locate the damaged areas even in the presence of other

reinforcement (Azizinamini and Gull, 2012a). Although many of the tests of the MFL system have

shown positive results testing done in a report by DMJM Harris (2003) to assess the conditions of top

slab PT tendons found that the method was unable to identify losses of tendon area under truly blind

conditions. In addition, it was found that MFL was unable to locate tendons with induced flaws in the

trumpets. The failure of the method to identify these defects was attributed to the fact that the equipment

being used did not have a strong enough magnetic to saturate the tendons and consequently produce the

flux leakage (DMJM Harris, 2003).

3.7.3 Limitations

One of the major limitations of the magnetic flux leakage method when applied to PT systems is that it is

less effective for inspection of internal PT tendons due to the presence of other reinforcement and

increased embedment depth. Currently it is only typically used in the field for inspection of external PT

tendons. In field evaluations the following challenges are encountered: the masking effect of the duct,

disruption of the MFL signal due to the presence of additional layers of reinforcement, and limited access

to areas such as the anchorage zone (Azizinamini and Gull, 2012a). Another limitation of this method is

that the anchorage region is difficult to inspect. In many cases the tendons at these locations can be

embedded in thick layers of concrete making it difficult to achieve complete magnetic flux saturations

without increasing the strength of the magnetics used. Additionally the trumpet regions are difficult to

inspect due to high congestion of reinforcement steel (spiral and stirrups) and the end anchor plates which

make the signal difficult to interpret (DMJM Harris, 2003). Finally, the results produced by this method

can be hard to interpret and may require some expertise in order to get accurate results (Azizinamini and

Gull, 2012a).


Magnetic flux leakage can be used to determine strand corrosion and wire breaks in post-tensioned

tendons in some applications. This method can be potentially used with both steel and plastic ducts

systems. Although claims have been made that MFL can be used on both internal and external tendon

ducts at this time it has only been demonstrated to work well for applications of external ducts. The ideal

application of this method given the current technology is for identifying significant corrosion or wire

breaks in external plastic ducts. This method currently has limited application in the field for internal

post-tensioned tendons when applied using traditional methods. The potential application of MFL to

determine the location of defects in combination with internal EIT tendons is presented in Elsener and

Buchler (2011). The use of a SQUID array technology for magnetic inspection of prestressed concrete

girders is also being developed (Krause and Kreutzbruck, 2002; Krause et al., 2002). This type of system

would be more suited for periodic observation of tendons in order to record small changes, due to the

sensitivity to the SQUID sensors (Krause and Kreutzbruck, 2002). The development of the MFL method

for detecting strand corrosion may have success in future applications as the technology improves.

3.7.5 References

Further information on the magnetic flux leakage method can be found in the following references

included in this report:








et al., 2010)

5.1.5 Nondestructive Method to Detect Corrosion of Steel Elements in Concrete (DaSilva et al.,

2009)






System (Ghorbanpoor et al., 2000)


Bridges (Elsener and Buchler, 2011)

7.5 Evaluation of NDT Methods for Detection of Prestressing Steel Damage at Post-Tensioned

Concrete Structures (Mietz and Fischer, 2007)

7.18 Recent Developments in SQUID NDE (Krause and Kreutzbruck, 2002)

7.19 SQUID Array for Magnetic Inspection of Prestressed Concrete Bridges (Krause et al., 2002)

3.8 Radiography

3.8.1 Applications

Radiography has been used to detect grout voids, strand corrosion, and strand fracture in the tendons of

PT concrete bridges (Saravanan et al. 1996; Mariscotti et al. 2008 Pimentel and Mariscotti, 2010). In

addition, this method has been utilized for the verification of re-grouting operations, the location and

sizing of steel reinforcing bars and embedded utilities, and the visualization of unknown construction

details (Brown and St. Leger, 2003). Radiography accommodates a wide range of construction materials

(including plastic and metal ducts), embedded features with complex geometries, and both internal and

external post-tensioning configurations (Saravanan et al. 1996; Pimentel and Mariscotti, 2010). Access to

the front and back surfaces of the scanned region is required.

The recent development of portable high intensity MeV X-ray generators (Figure 3.25) now enable the

inspection of concrete sections up to 150 cm (5 ft) thick (Ueaska et al. 2013; Sentinel, 2014). Unlike

gamma ray-producing isotope sources, these portable X-ray machines only emit radiation during testing,

thereby providing better control over work site safety. The use of high intensity X-rays has also reduced

the required transmission time so that radiographic images with sufficient detail for defect/damage

detection can be generated in a matter of seconds. For example, Ueaska et al. (2013) were able to

generate radiographic images of a 400mm thick PT bridge section with transmission times of one second

using a newly developed 3.95MeV portable X-ray generator.

3.8.2 Methodology

Radiography utilizes the electromagnetic waves emitted from a radiation source (either an X-ray

generator or a radio isotope gamma-ray source) to penetrate the test object, exposing a photostimulable

detector on the opposing surface. Since the atomic structure of the surveyed material influences photon

attenuation and scattering phenomena, spatial variation in material composition leads to spatial variation

in radiation intensity reaching the detector (as illustrated in Figure 3.26). In modern digital radiographic

testing, these detector readings are digitized and converted to pixel intensity values, through which spatial

variations can be visualized on a computer monitor as color contrast. In NDE applications, this spatial

variation in pixel intensity is used to identify and measure defects or structural damage, and to visualize

embedded features for repair/retrofit operations. As a practical illustration, Figure 3.27 presents


radiographs of fully grouted and voided post-tensioning tendons, where the voided duct region is

discernible in the image (shown as a darker region due to higher incident radiation).

The ability of radiographic imaging to accommodate complex geometries and multi-layer material

interfaces (which present problems for other nondestructive test methods), and to provide full-field

subsurface visualization make it a powerful tool for structural condition assessment. However, its use in

concrete bridge inspection has historically been limited by the penetrating power of field deployable

radiation sources, as well as safety and logistical concerns associated with the use and transport of

radioactive materials. Recent advances in radiographic inspection equipment, such as the development of

portable high intensity MeV X-ray generators (discussed in the previous section); now enable the

inspection of concrete sections up to 150 cm (5 ft) thick with transmission times on the order of seconds

(Ueaska et al. 2013; Sentinel, 2014). Unlike gamma ray-producing isotope sources, these portable X-ray

machines only emit radiation during testing, thereby providing better control over work site safety. In

addition, the development of digital detectors and advanced image reconstruction algorithms for concrete

materials that reduce scatter-induced blurring (e.g. (Priyada, Ramar and Shivaramu, 2013)) have

improved imaging capabilities, and have enhanced data preservation and manipulation. As an illustration,

Figure 3.28 presents a radiograph of a post-tensioning anchorage region showing individual steel strands,

the reinforcing steel spiral (encircling the duct), and the tendon duct.

Figure 3.25: Portable 7.5MeV X-ray Betatron (www.Sentinelndt.com)

Figure 3.26: Schematic illustration of radiographic testing (adapted from Rao, 2007)

Radiation

Source

Detector

Subsurface Void

Variation

in Thickness

Specimen

Regions Exposed to Higher Incident Radiation


Figure 3.27: Radiographic images of post-tensioned concrete: (a) fully grouted post-tensioning steel duct,

(b) voided post-tensioning steel duct (Brown and St. Leger, 2003)

Figure 3.28: Radiograph of the anchorage region in a post-tensioned concrete bridge showing individual

strands, the reinforcing steel spiral (encircling the duct), and the tendon duct (Washer, 2003)

3.8.3 Limitations

Due to the procedure’s use of radioactive materials in certain applications, radiography may require

special preparations and planning to ensure public safety. It is noted that certain site conditions may

prohibit this type of testing, but instrumentation such as X-Ray generators exist where radioactive

material is not used. In addition, conventional methods for radiographic imaging do not provide depth of

field information for the surveyed structure, although the development of portable X-ray generators has

initiated research interest in adapting advanced 3-D imaging techniques, e.g. computed tomography (CT),

to structural inspection. The use of radiography also requires access to opposing sides of the surveyed

object.


Radiography is an imaging tool that can be used to detect and quantify subsurface features/defects in PT

concrete bridges. The testing procedure is able to handle complex geometries and multi-layer material

interfaces, which can present problems for other NDE methods and therefore can be a viable tool when a

more in-depth inspection of the strand anchorage and coupler regions is required. With further

development and verification, radiography could potentially identify corrosion on strands and grout voids.

Newly developed portable high intensity MeV X-ray machines, which can survey concrete bridge

sections up to 5ft thick with transmission times on the order of seconds, have improved the efficiency,

practicality, and imaging capabilities of radiographic testing (Ueaska et al. 2013). In addition, advanced

(a) (b)


digital image analysis methods (e.g. Priyada, Ramar and Shivaramu, 2013) have improved defect/damage

detection capabilities.

3.8.5 References

Further information on the radiography can be found in the following references included in this report:








5.4.2 Improving Bridge Inspections (Washer, 2003)

6.1 Application of Gamma Ray Scattering Technique for Non-Destructive Evaluation of Voids in

Concrete (Priyada, Ramar and Shivaramu, 2013)


and Hurlebaus, 2012)


2010)

6.12 Ultrasonic Imaging Methods for Investigation of Post-tensioned Concrete Structures: A

Study of Interfaces at Artificial Grouting and Its Verification (Krause et al., 2008)

6.25 Use of the MegascanTM

Imaging Process in Inspection Systems for Post-Tensioned Bridges

and Other Major Structures (Brown and St Leger, 2003)


Aubagnac and Abraham, 2002)

7.14 Location of Prestressing Steel Fractures in Concrete (Scheel and Hillemeir, 2003)

10.3 Use of Neutron Radiography and Tomography to Visualize the Autonomous Crack Sealing

Efficiency in Cementitious Materials (Van Tittelboom et al., 2013)

10.4 Commissioning of Portable 950 keV/3.95 MeV X-band Linac X-Ray Sources for On-Site

Transmission Testing (Ueaska et al., 2013)

10.5 Non-Destructive Radiographic Evaluation and Repairs to Pre-Stressed Structure Following

Partial Collapse (Reis and Dilek, 2012)

10.7 Gamma-Ray Inspection of Post Tensioning Cables in a Concrete Bridge (Pimentel and

Mariscotti, 2010)

10.9 Gamma-Ray Imaging for Void and Corrosion Assessment in PT Girders (Mariscotti et al.,

2008)


2003)

10.27 Non-Destructive Examination of Corroded Concrete Structures using Radiography

(Saravanan et al., 1996)

10.29 Imaging of Reinforced Concrete: State-of-the-Art Review (Pla Rucki et al., 1995)

3.9 Signal Processing for Defect Detection

3.9.1 Applications

The focus of this work is on data driven defect detection methods. In this context, NDE technologies that

are considered produce streams of data from an “unknown” state of the structure, and the objective is to

identify the existence, location and severity of defect by comparing this data to that of a healthy baseline.


The challenges are twofold. The first challenge is producing “features” from the collected data that are

sensitive to the defect but are not as sensitive or sensitive at all to other inevitable variations (changes in

environmental conditions, other benign changes, etc.) or measurement noise. The second challenge is in

monitoring this feature, detecting its change in a statistically meaningful way, and associating a particular

change to a particular defect.

A class of signal processing methods that are used to achieve this objective are statistical pattern

recognition (SPD) techniques. These methods have recently emerged as a promising alternative to system

identification for structural damage assessment. Their essence is to use well-known concepts in statistics

for boundary definition of different pattern classes, such as those for damaged and undamaged structures

(Jain et al., 2000).

3.9.2 Methodology and Applications

Statistical pattern recognition techniques have long been applied in many domains in science and

engineering. Examples of these applications include speech recognition (Ho and Baird, 1997), identifying

logical information from image documents (Jelinek, 1976; Schurmann et al., 1992), and reading DNA

sequences in bioinformatics (Liew, Yan and Yang, 2005). Their ability to process large volumes of

information produced by continuous and/or multichannel sensing is very beneficial, and in addition, these

techniques are adaptable to most fields of applied science as efficient mathematical tools. The range of

application of these techniques continues to expand to many new areas of natural and social sciences and

engineering (Yao and Pakzad, 2012).

The SPR paradigm for defect detection, proposed in Farrar et al. 1999, consists of four tasks: (1)

operational evaluation; (2) data acquisition; (3) feature selection and data compression; and (4) statistical

model development. This paradigm essentially focuses on the interdependence between structural data

collection and statistical data analysis procedures and their application. The feature extraction methods for

many of SPR applications in structural defect detection are based on AR/ARX/ARMA

(Autoregressive/Autoregressive with exogenous input/Autoregressive with moving average Brockwell

and Davis, 2009; Brockwell and Davis, 2002) modeling. This provides a powerful tool to process the data

collected by any NDE method, and produce decisions as to whether the data points out to a defect or

confirms that the test structure is healthy.

3.9.2.1 Estimation of Damage Feature

The objective of this step is to estimates parameters from the input/output data that are sensitive to

structural defect. Autoregressive time series with exogenous terms (ARX) can be used to detect these

defects. The general form of this model is y(𝑛) = ∑ 𝛼𝑖𝑦(𝑛 − 𝑖)𝑝𝑖=1 + ∑ 𝛽𝑗𝑢(𝑛 − 𝑗)𝑞

𝑗=0 + 𝜖(𝑛),

where 𝑦(𝑛) and 𝑢(𝑛) are the measured response and excitation at time index 𝑛, respectively, and

𝜖(𝑛) is the model residual. In this equation 𝑝 and 𝑞 are the orders of autoregressive and exogenous

terms, respectively. The parameters vector 𝑥 = [𝛼1×𝑝 𝛽1×(𝑞+1)] includes 𝑝 + 𝑞 + 1 terms, which are

essentially estimated parameters for the system transfer function: a structural defect will cause a change in

estimated autoregressive parameters.

3.9.2.2 Measure of Distance:

Once a set of feature parameters are estimated, they should be compared with their baseline values from

the healthy/undamaged state. The simplest way of comparing these values is by studying their difference

(𝑥𝑢𝑛𝑘𝑛𝑜𝑤𝑛 𝑠𝑡𝑎𝑡𝑒 − 𝑥𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒), which proves to be an inefficient measure of distance, unstable, and in

cases, not sensitive enough to structural defect. Two distant measures that can be used are: the

Mahalanobis distance of the AR coefficients and the Cosh distance of AR model spectra, between the


ARX models of the unknown state and the baseline/healthy state (Yao and Pakzad, 2012). Simulation

results show that a change in structural properties (change in stiffness and geometry) leads to changes in

these distance feature values.

3.9.2.3 Mahalanobis distance of AR coefficients for the ARX models

It has been proved that AR coefficient estimates from data are asymptotically normally distributed. From

the deviation statistics, a normal Gaussian statistical population in p-variants is usually described by a p-

dimensional frequency distribution:

𝑓(𝑥1, 𝑥2, … , 𝑥𝑝) =1

(2𝜋)𝑝 |𝛴12|

exp (1

2(𝑥 − 𝜇)𝑇𝛴−1(𝑥 − 𝜇)). (1)

where 𝜇 is the mean, and 𝛴 is the sample covariance matrix.

Mahalanobis distance is defined as twice the term inside the exponential brackets. The estimator of the

Mahalanobis distance between a potential outlier vector 𝑥𝜉 and the baseline sample set can be obtained as

𝐷𝜁 = (𝑥𝜉 − �̅�)Σ̂−1

(𝑥𝜉 − �̅�). (2)

�̂� is the estimated sample covariance matrix from baseline. When the incoming coefficient vector

deviates from the original distribution, the distance value will increase.

3.9.2.4 Cosh spectral distance of AR model spectra

Corresponding spectra plots can be constructed given an AR model:

𝑆𝐴𝑅(𝑝)

(𝜔) =1

|𝜙(𝑒𝑗𝜔)|2=

1

|∑ 𝜙𝑘𝑒−𝑗𝜔𝑘𝑝𝑘=0 |

2 (3)

Cosh spectral distance estimates can be used as a frequency domain alternative to Mahalanobis distance

of AR coefficients:

𝐶(𝑆, 𝑆̅) =1

2𝑁∑ [

𝑆(𝜔𝑗)

𝑆̅(𝜔𝑗)− log

𝑆(𝜔𝑗)

𝑆̅(𝜔𝑗)+

𝑆̅(𝜔𝑗)

𝑆(𝜔𝑗)−

𝑁

𝑗=1

log 𝑆̅(𝜔𝑗)

𝑆(𝜔𝑗)− 2]. (4)

𝑆(𝜔𝑗) is the power spectral density to be examined, and 𝑆 ̅(𝜔𝑗) is the average of the spectra estimates

from baseline signal collection. When the AR coefficients of the ARX model change as a result of a

defect, the Cosh distance value will increase.

3.9.2.5 Threshold Construction, Classification

Once an appropriate and damage-sensitive distance of the features from the unknown-state structure

compared with the baseline structure is calculated, it should be compared with a threshold to determine

whether damage has occurred or not. Statistical hypothesis testing (Koch, 1999) is the recognized

standard approach for threshold determination. It assumes that the features follow a certain probability

distribution, and the threshold is set at a point beyond which there is a very small chance for an event to

occur. This approach is theoretically optimal as long as the assumed feature distribution is valid.

Hypothesis testing does well for fault identification in machinery, as the excitation force is well known

and the damage types are well-defined. Although this approach is valid and should always be utilized

first, for civil engineering applications, however, there are more uncertainties. When the probability


distribution of damage features are too complex to be accurately represented by analytical distribution

functions, threshold constructed using hypothesis testing will yield poor results in damage identification

and an alternative approach is needed.

A data driven threshold determination scheme can be used, utilizing cross-validation and resampling

techniques (Good, 1999), and applied to the Mahalanobis distance and Cosh spectral distance features

introduced earlier. The data-driven method in combination with these distance features yields an effective

threshold for damage classification. The measure of distance features can be paired with the

autocorrelation function to improve the damage detection performance of time-series based methods. This

approach is described here for the Mahalanobis distance.

3.9.2.6 Threshold calculated from resampling: the ‘cross-one-out’ method

The ‘cross-one-out’ resampling technique can be adopted for threshold construction. First a segment is

cut from the baseline signal at a random time point and reserved for testing, and sample segments of the

same size are cut with a preset overlap from the remaining signal. The Mahalanobis distance between the

AR portion of model coefficients of the left-out segment and those of the sample set is computed and

stored. This process is repeated for a large number of times and the value beyond which 5% of the tests

occur is used as threshold in subsequent analysis. This approach is essentially an estimation of the feature

distribution by recomputing the statistic many times by leaving out a certain portion of observation, and

can be viewed as a combination of jackknife and cross-validation techniques (Shao and Tu, 1995).

3.9.3 References

Further information on signal processing for damage detection and sensor networks can be found in the

following references included in this report:

5.1.5 Nondestructive Method to Detect Corrosion of Steel Elements in Concrete (DaSilva et al.,

2009)



Olson, 2006)


System (Ghorbanpoor et al., 2000)

11.1 Automatic Delamination Detection of Concrete Bridge Decks Using Impact Signals (Zhang,

Harichandran and Ramuhalli, 2012))

11.2 Autoregressive Statistical Pattern Recognition Algorithms for Damage Detection in Civil

Structures (Yao and Pakzad, 2012)

11.3 Procedures for Fatigue Crack Growth Monitoring and Fatigue Life Prediction Using

Acoustic Emission Data and Neural Networks (Barsoum et al., 2011)

11.4 Time Series: Theory and Methods (2nd Edition) (Brockwell and Davis, 2009)

11.5 Discrete Wavelet Transform to Improve Guided-Wave-Based Health Monitoring of

Tendons and Cables (Rizzo and di Scalea, 2005)

11.6 Pattern Recognition Techniques for the Emerging Field of Bioinformatics: A Review (Liew,

Yan and Yang, 2005)

11.7 Introduction to Time Series and Forecasting (2nd Edition) (Brockwell and Davis, 2002)

11.8 Resampling Methods: A Practical Guide to Data Analysis (Good, 1999)

11.9 Parameter Estimation and Hypothesis Testing in Linear Models (Koch, 1999)

11.10 Large-Scale Simulation Studies in Image Pattern Recognition (Ho and Baird, 1997)

11.11 The Jackknife and Bootstrap (Shao and Tu, 1995)

11.12 Document Analysis- From Pixels to Contents (Schurmann et al., 1992)


11.13 Continuous Speech Recognition by Statistical Methods (Jelinek, 1976)

3.10 Time Domain Reflectometry

3.10.1 Applications

Time domain reflectometry (TDR) is an electrical measurement technique that was originally developed

as a method to locate and quantify defects in transmission lines. Recently TDR has been used in a

number of applications outside of investigating the condition of electrical cables including; monitoring of

water level in a dam, early detection of rock movement, soil moisture content and identifying defects

along post-tension ducts (Okanla et al., 1997). With regard to post-tensioned concrete structures time-

domain reflectometry methods have been used in laboratory experiments to identify grout voids within

the duct (Li et al., 2005; Chajes et al., 2003; Okanla et al.,1997) and to identify strand corrosion along

grouted tendons (Hunsperger et al., 2003; Liu et al., 2002). This method has the potential to determine

the location and relative size of a defect in the strand or grout (Liu et al., 2002). Time domain

reflectometry has been investigated for application in post-tensioned systems using internal or external

sensors, therefore it could be used on existing systems through the use of external sensors or internally

integrated into new construction using internal sensors (Li et al., 2005).

3.10.2 Methodology

The basic principle of the time domain reflectometry method involves sending high frequency electrical

pulses through a sensing cable. When an impedance discontinuity is encountered along the length of the

cable a partial reflection of the pulse is generated. These partial pulse reflections can then observed using

TDR cable test equipment. A basic functional block diagram for a typical time domain reflectometry

system is presented in Figure 3.28. In the case of post-tensioning systems the presence of physical

defects in the steel tendon, or the grout around the tendon, will cause a change in the electromagnetic

properties of the line (Chajes et. at, 2003). These changes in electromagnetic properties can be detected

by TDR identifying locations of corrosion and grout voids along the ducts. The application of the TDR

method for void detection and corrosion identification require a slightly different methodology regarding

the application of the transmission line. In general when using TDR for corrosion identification the steel

strand is used to establish a two-conductor transmission line and when identifying grout voids a separate

two conductor chord is used. This concept will be explained more thoroughly throughout this section.

Figure 3.28: Functional block diagram for typical time domain reflectometer (Liu et al., 2002)

When applying time domain reflectometry as a NDE method for corrosion detection, a sensor wire

(typically a coaxial cable) is run parallel with the steel strand to create an asymmetric two-conductor

transmission line (Figure 3.29) (Liu et al., 2002). A detailed explanation of the two-wire transmission

line model can be found in Chajes et al. (2003), but will be briefly summarized. In order to analyze the

wave propagation in the transmission line Maxwell’s equation with the proper imposed boundary

conditions needs to be solved. The analysis can be simplified by using a distributed parameter model to

study the wave propagation in the transmission line. Since the steel strand is used as the transmission line

the distributed properties in this case are calculated from the geometric and material properties of the

tendon. For this method the four distributed properties of interest are the capacitance, inductance,


resistance and conductance. From these parameters the impedance of strand can be calculated. It is

important to note that the impedance measurements depend on the radius of the strand and the dielectric

constant of the surrounding material. Any physical defects along the steel strand will therefore cause a

change in the impedance readings signifying tendon corrosion. In structures where there are multiple

strands present at the monitoring location this model needs to be altered in order to be applied. In

situations where multiple strands are closely spaced and electrically connected then they can be treated as

a single strand with a larger effective radius. (Chajes et al., 2003)

Figure 3.29: Twin-conductor transmission line (Liu et al., 2002)

For the application of time domain reflectometry to strand corrosion detection the damage sites within the

tendon need to be modeled as electrical discontinuities in the transmission line (Liu et al., 2002).

Laboratory experiments were conducted by Liu et al. (2002), to study the ability of TDR to detect the

presence and severity of pitting corrosion. Pitting corrosion can cause a large amount of localized

damage resulting in a significant loss to the cross-sectional area of the tendon. Theoretically, the

significant loss of cross sectional area along the tendon should cause a sudden increase in the localized

impedance signifying the presence of pitting corrosion. Additionally, the reflection amplitude created

when the wave encounters the site of pitting corrosion can provide information on the severity of the

damage. The location of the damage site along the tendon can also be obtained from the transit time of

the reflection wave. Figure 3.30 shows an example of a time domain reflectometry return where 50% of

the rebar was removed to simulate pitting corrosion at the center of bare rebar (not grouted). The pitting

corrosion is identified by the positive reflection indicated by point “C”. Based on this research the study

concluded that the steel/rebar and sensor wire can be modeled and evaluated analytically as a

transmission line and TDR can detect the location of damage site on a steel strand and reinforcing bar and

provide indications to the severity of the damaged region (Liu et al., 2002)

Figure 3.30: Time domain reflectometry return of 3m rebar sample with 50% pitting corrosion in middle

(Liu et al., 2002)


When applying the time domain reflectometry method for grout void detection the strand is typically not

used as part of the transmission line. In applications where only void detection monitoring is required

experiments have shown that standard transmission lines such as a 300 Ω twin-lead television cable and

two-wire lamp zip chord can be used for internal void detection (Li et al., 2005). The twin conductor cord

is run inside the duct, but does not need to be run parallel to the strand as was the case with the addition

sensor wire used for corrosion detection. This is due to the fact that the cord itself contains two parallel

sensor wires. The detection of grout voids using time domain reflectometry is based on the theory that

the presence of a grout void will dramatically change the dielectric constant of the medium through which

the transmission line passes (in this application the grout), therefore affecting the characteristic impedance

of the line (Chajes et al., 2003). The cause the change in the dielectric constant is due to the presence of

air or water in the voids which have different electrical properties then the grout. Typically, the

characteristic impedance at locations of voids will increase because of the decrease in the dielectric

properties caused by the void (Liu et al., 2002).

Laboratory experiments were conducted by Chajes et al. (2003) and Li et al. (2005) to investigate the

ability of the TDR method to detect voids in post-tensioning ducts. In these experiments a positive step

pulse is generated and sent down the transmission line. When a void is located near the transmission line

a reflection is generated by the void and recorded as a signal bump by the oscilloscope (Li et al., 2005).

An example of a reflected voltage wave signal can be seen in Figure 3.31. In this figure the horizontal

axis represents the time at which the voltage change is recorded and the vertical axis represents the

voltage. In this particular experiment a 200 mV pulse was generated and sent down the transmission line

(marked in the figure as “step pulse”). The first pulse reflection is caused by the impedance mismatch

between the transmission line and the 50 Ω coaxial cable used and indicated as the bump labeled

“oscilloscope-transmission line connection”. The second pulse reflection is caused by a 5.6 cm void in the

grout indicated in the figure as “Void (diameter = 5.6 cm).” The unreflected portion of the pulse continues

along the transmission until it reaches the end of the specimen indicated by “the end.” The known signal

wave velocity on the transmission line can also be used to determine the location of the void along the

length of the tendon. The time scale, shown on the horizontal axis, can be converted to a distance so that

the location of the defect (in this case a void) can be determined (Li et al., 2005).

Figure 3.31: Reflected TDR voltage wave signal (Li et al., 2005)

Based on the experimental work performed on grout void detection with TDR the following conclusions

can be made. Voids can be detected using a single wire in conjunction with an existing tendon or through

the use of commercially available transmission line (TV cable or lamp cord). Voids can be detected using

transmission lines internal and external to the tendon, however, the location of the lines need to be

optimized to provide the best resolution. If appropriate internal senor wire geometry is selected for


corrosion detection it can also be used to detect grout voids in the vicinity of the cable. The application of

external sensors is more difficult due to reduced reflected signal strength and the creation of noise

reflections from other structural components. It is also recommended for external sensors that different

transmission line geometries and larger pulse voltage be used (Li et al., 2005). The effectiveness of TDR

for void detection was demonstrated through test specimens with built-in voids. Factors affecting void

detection such as void size, void content and corrosion were identified (Chajes et al., 2003).

It should be noted that there are other forms of time domain reflectometry outside of electrical time

domain reflectometry described in this section that are being explored for the use of NDE monitoring in

civil structures. Optical time domain reflectometry (OTDR) is also being explored as a possible

technique. The basic principles of OTDR are described by Chang and Liu (2003). The OTDR technique

is based on intensity measurements of distributed sensors. Measurements are taken based on the time of

flight of light signals, if strain is induced at any point along the length of the fiber a change of intensity is

reflected back. The OTDR records the time it take for the light signal to travel back from the location

where the strain was encountered and then converts this time to a distance. Similar to TDR these

measurements are used to identify the location at which the anomaly occurred (Chang and Liu, 2003).

This technology is under development.

3.10.3 Limitations

When utilizing time domain reflectometry one limitation that arises is the sensitivity to the proximity of

the defect to the sensor wire. In the case of detecting strand corrosion the measurement sensitivity is

related to the distance between the cable and sensor wire. The characteristic impedance depends on this

distance, therefore the closer the two conductors the more sensitive the measurement will be (Liu et al.,

2002). For the application of detecting grout voids the sensitivity is dependent on the location of the

defect with respect to the transmission line. Voids far away from the strand/wire transmission line might

be more difficult to detect because of their negligible effect on the line impedance (Chajes et al., 2003).

The sensitivity of the method to defect size is also a concern when using TDR. In the case of grout voids

the reflection magnitude is related to void size, the larger the grout void the larger the reflection

magnitude (Chajes et al., 2003). Experimental results for strand corrosion identification have indicated

the same relationship between defect size and reflection magnitude; the magnitude of the reflection

depends on the severity of the damage (Liu et al., 2002). This indicates that there is a limit to the size of

defect that can be identified using this method, which means small grout voids or surface corrosion may

be difficult or impossible to detect.

Another limitation of this method is that there is a lack of testing and verification of this method in field

applications. The lack of field application may be due to the following issues presented in Hunsperger et

al. (2003). Existing structures do not embody features designed for up to date TDR implementation.

External detection cannot avoid the presence of concrete layers which cause strong attenuation to the

signals, obstructing TDR measurements. Large pulse generators must be used in order to enhance the

visibility of voids in corrosion sites in the field. Additional further research is needed to find more

practical sensor geometries of transmission lines that can meet the requirements of field practice

(Hunsperger et al., 2003). Another concern with regards to field application is the implementation of the

sensor wire for corrosion detection in the field. The wire should remain parallel to the strand for

monitoring purposes but in many field applications the post-tensioning wire is draped at various locations

along its length. This may make it difficult to receive consistent measurements along a strand.

Weather, temperature and moisture content also effects the readings obtained using this method, so these

factors should be noted and taken into consideration during the inspection (Li et al., 2005). In addition

the sensitivity of the measurements tends to decrease when external sensors are used (Liu et al., 2002).

The accuracy and applicability of this method using internal or external sensors should be explored


further and verified in the field or in testing mock up that more similarly mimic field conditions, such as

grouted tendons embedded in concrete with in the presence of a rebar network.


Time domain reflectometry can potentially be used to identify grout voids and strand corrosion in post-

tensioned concrete systems. When integrated into new construction this method can be applied to both

internal and external ducts. TDR can be used for both quality control to help eliminate the presence of

grout voids and long term monitoring of grout and strand condition within the tendon ducts allowing for

early detection of defects. Being able to detect and correct defects at an early stage can help increase the

life span of post-tensioned bridge systems. Based on the reviewed literature this method shows promise

for the application of grout void detection when using sensors embedded into the duct system. Currently

there is no commercially available TDR system for use in post-tensioned inspection. The development of

such a system should be explored and developed in order to make this a viable inspection method for use

with PT systems.

3.10.5 References

Further information on the time domain reflectometry method can be found in the following references

included in this report:

5.1.1 Improved Inspection Techniques for Steel Prestressing/Post-tensioning Strand – Final Report –

Volume 1(Azizinamini and Gull, 2012a)



10.13. Time-Domain Reflectometry to Detect Voids in Posttensioning Ducts (Li et al., 2005)

10.16. Recent Research in Nondestructive Evaluation of Civil Infrastructures (Chang and Liu, 2003)

10.17 Detecting Corrosion in Existing Structures Using Time Domain Reflectometry (Hunsperger et

al., 2003)

10.19. Time Domain Reflectometry for Void Detection in Grouted Posttensioned Bridges (Chajes et

al., 2003)

10.23. Corrosion Detection of Steel Cables Using Time Domain Reflectometry (Liu et al., 2002)

10.26. Detecting Faults in Posttensioning Ducts by Electrical Time-Domain Reflectometry (Okanla et

al., 1997)

3.11 Ultrasonic Testing

3.11.1 Applications

Ultrasonic testing (UT) of post-tensioned concrete structures can be subdivided into two approaches: (1)

ultrasonic guided wave testing (GWT) and (2) ultrasonic imaging. Ultrasonic GWT has been investigated

in a laboratory scale to measure prestress force (Rizzo, 2006; Chaki and Bourse, 2009), and to detect wire

breaks and cross-sectional loss in anchorage zones due to corrosion (Beard, Lowe and Cawley, 2003;

Bartoli et al. 2009). The GWT procedure not only identifies the presence of defects in post-tensioning

steel, but can also be used to locate defects (Bartoli et al., 2009). It is noted that the GWT procedure

requires access to ends of post-tensioning steel tendons and therefore is viable only for new construction.

The procedure can accommodate both steel and plastic duct material.

Ultrasonic imaging utilizes an array of surface-mounted transducers to visualize subsurface conditions.

The procedure requires access to a single surface, and has been used to locate and quantify subsurface

defects and features, such as voids, delaminations, and partially grouted post-tensioning steel ducts

(Schickert, Krause and Muller, 2003; Krause et al., 2008). Similar to GWT, the procedure can

accommodate both steel and plastic duct material.


3.11.2 Methodology

Ultrasonic GWT of post-tensioned concrete structures involves the generation of ultrasonic sound waves

in the post-tensioning steel. As illustrated in Figure 3.32, these ultrasonic waves are generated at the

exposed ends of the steel tendons using an electronic device that can produce high voltage electrical

pulses. The propagating waves, which are confined and guided by the geometric boundaries of the

tendon, are reflected and refracted by local cross-sectional changes (e.g. broken wires or mass loss due to

corrosion). As a result, measurement of the frequency content and amplitude of the reflected waves

(using a transducer mounted to the tendon end) provides information regarding defect size, orientation,

and location. GWT utilizes relatively low-frequency waves (5-250 kHz) that have a long wavelength

with respect to the tendon diameter in order to reduce attenuation for long-range inspection.

Ultrasonic imaging uses an array of surface-mounted transducers to generate and receive ultrasonic shear

waves in the surveyed region of the structure. As illustrated in Figure 3.33, multiple intersecting shear

wave paths can be generated across the transducer array. The reflected wave content is processed using

the synthetic aperture focusing technique (SAFT), which shifts the individual transducer signals in time to

superpose on a coherent wave reflector, i.e. an embedded feature or defect. SAFT is able to locate wave

reflecting targets but is unable to distinguish further characteristics (e.g. interface type). This deficiency

has led to the development of phase modified SAFT. By using the phase information of the received

wave pulses, phase modified SAFT can distinguish between material interfaces, e.g. concrete-steel or

concrete-air. Figure 3.34 presents conventional and phase modified SAFT images for a post-tensioned

concrete beam using a shear wave transducer array.

Figure 3.32: Pulse-echo test configuration for the inspection of post-tensioning steel using guided

ultrasonic waves (Beard, Lowe and Cawley, 2003)


Figure 3.33: Illustration of ultrasonic imaging: equipment (top) and test configuration (bottom) for a shear

wave array (Azizinamini and Gull, 2012a)

Figure 3.34: Illustration of a conventional SAFT image (top) and a phase modified SAFT image (bottom)

collected from a post-tensioned concrete beam. The color in the phase modified image indicates the

phase of the reflected wave (Azizinamini and Gull, 2012a)

3.11.3 Limitations

Ultrasonic GWT requires that the ends of the post-tensioning steel tendons are exposed and accessible.

Testing, analysis, and data interpretation requires experienced personnel. Conventional ultrasonic imaging

procedures using SAFT are well suited for the location of embedded features and defects, but do not

provide information regarding the type of material interface. As a result, they are unable to characterize

grout conditions in post-tensioning steel ducts. Procedures that utilize phase modified SAFT, however,

are able to distinguish between material interfaces and can be used to locate and quantify grouting defects

in post-tensioning steel ducts. Ultrasonic methods are not readily adaptable to the inspection of

geometrically complex details such as post-tensioning anchorage and strand coupler regions, where signal

interpretation is complicated by wave interactions.



Ultrasonic GWT is well suited for monitoring the condition of post-tensioning steel strands in concrete

bridges. In order to accommodate GWT, post-tensioned bridge designs should provide anchorage details

that leave the ends of the tendons exposed and accessible. Ultrasonic imaging utilizing phase modified

SAFT provides a robust visualization tool that can be used to locate and measure subsurface features and

defects, including grout conditions in post-tensioning steel ducts.

3.11.5 References

Further information on ultrasonic testing can be found in the following references included in this report:







et al., 2010)


Ducts – Final Report (Muszynski, Chini amd Andary, 2003)



Olson, 2006)




and Hurlebaus, 2012)




2010)

6.12 Ultrasonic Imaging Methods for Investigation of Post-tensioned Concrete Structures: A

Study of Interfaces at Artificial Grouting and Its Verification (Krause et al., 2008)


(Streicher et al., 2006)



6.22 Ultrasonic Imaging of Concrete Elements Using Reconstruction by Synthetic Aperture

Focusing Technique (Schickert, Krause and Muller, 2003)

6.23 Ultrasonic Guided Waves for Inspection of Grouted Tendons and Bolts (Beard, Lowe and

Cawley, 2003)

6.29 Ultrasonic Tomography of Grouted Duct Post-Tensioned Reinforced Concrete Bridge

Beams (Martin et al., 2001)



9.4 Non-Destructive Evaluation of the Stress Levels in Prestressed Steel Strands using

Acoustoelastic Effect (Chaki and Bourse, 2009)


Bridges (Bartoli et al., 2009)

9.8 Ultrasonic Wave Propagation in Progressively Loaded Multi-Wire Strands (Rizzo, 2006)



(Maierhofer et al., 2008)


Bridges (Kohl and Streicher, 2006)



10.15 Progress in Ultrasonic Imaging of Concrete (Schickert, 2005)


2003)

10.18 Ultrasonic C-scan Imaging: Preliminary Evaluation for Corrosion and Void Detection in

Posttensioned Tendons (Iyer, Schokker and Sinha, 2003)

10.21 Non-Contact Ultrasonic Imagining for Post-Tensioned Bridges to Investigate Corrosion

and Void Status


3.12 Visual Inspection

3.12.1 Applications

Visual inspection techniques are among the oldest and most commonly used forms of NDE in bridge

inspection today. Visual inspection is used in some capacity in almost all types of bridge inspections and

can be used to identify cracking, spalling, fretting, surface corrosion, exfoliation, pitting and inter-

granular corrosion (Azizinamini and Gull, 2012a). In post-tensioned systems, due to the encapsulated

construction details used, visual inspection with the naked eye is limited to cases where significant

damage is present. This damage can include but is not limited to splitting of external PT ducts and

external corrosion of anchorage blocks. Typically PT details require invasive drilling and the use of

borescopes to provide localized information on tendon corrosion and grout condition.

3.12.2 Methodology

Visual inspection typically involves accessing a structural component based on the exterior appearance.

Post-tensioned systems can be more difficult to inspect visually because often corrosion of the strands

does not result in noticeable external changes until the structure is severely damaged. In grouted post-

tensioned systems the tendon and grout condition cannot be examined visually without performing

invasive drilling since they are encapsulated by the duct. Typically, a hole is drilled at various locations

along the tendon duct and a borescope is inserted in order to obtain images of the internal condition of the

tendon. Based on these images grout voids and strand corrosion can be identified. Figure 3.35 shows

images or a partially grouted duct obtained using a borescope. As illustrated the quality of the images

allow for identification of strand condition when voids are present. This method of visual inspection for

grouted post-tensioned ducts tends to be easier to perform on external ducts since they duct itself is more

easily accessible. In the case of internal PT ducts the location of the duct and potential voids must be

known to ensure that the drilling process does not cause damage to the tendon.


Figure 3.35: Photos a and b show duct partially grouted with strand exposure. Photo c

shows voided duct with strands fully exposed. Photo d shows partially grouted duct, no

strands exposed. (DMJM Harris, 2003)

3.12.3 Limitations

Visual inspection of post-tensioned systems can be difficult because there is limited access to the tendon.

Visual inspection of external components is often not possible because typically signs of distress are not

externally visible until the strands are severely damaged. Inspection involving borescope requires

invasive drilling which can cause damage to the tendon if care is not taken during this process.

Additionally, only localized information on the grout condition and tendon corrosion can be obtained at

each drill site. This inspection process tends to be time consuming and requires that the location of

tendons be known. It is not practical to inspect the full length of a tendon with just the use of visual

inspection.


Visual inspection is a relatively inexpensive method that is able to provide accurate localized inspection

information. For these reasons it is recommended that borescope inspections be used in conjunction with

other NDE methods. Once defective areas along a tendon have been identified by other NDE methods

that can more rapidly inspect the entire tendon length, borescope inspection can be used at the identified

locations to verify and determine the severity of the damage.

3.12.5 References

For further information on visual inspection the following references identified below can be examined.

Details on each reference can be found in the identified section of this report.





Concrete Bridges- Final Report – Volume II (Azizinamini and Gull, 2012a)



et al., 2010)



(Naito, Jones and Hodgson, 2010)




Maintenance of Florida Post-Tensioned Bridges (Volume 9) (Corven Engineering, 2002)

5.1.21 Mid-Bay Bridge Post-Tensioning Evaluation (Corven Engineering, 2001)

5.2.1 Corrosion of Prestressing Steels (ACI 222.2R-01) (ACI Committee, 2001)


2010)

7.7 Corrosion of the Strand-Anchorage System in Post-Tensioned Grouted Assemblies (Wang,

Sagues and Powers, 2005)

7.14 Location of Prestressing Steel Fractures in Concrete (Scheel and Hillemeir, 2003)




4 Summary of NDE methods and tools

Readily available nondestructive evaluation methods applied to post-tensioned systems were investigated

based on the most current published literature. The current applications, methodology, limitations and

potential applications for the following methods were explored in chapter 3 of this report; Visual

Inspection, Ground Penetrating Radar, Half-Cell Potential, Electrically Isolated Tendons, Time Domain

Reflectometry, Magnetic Flux Leakage, Impact Echo, Acoustic Emission, Radiography, Infrared

Thermography and Ultrasonic Testing. Based on the information gathered from the literature review it

was identified that the accuracy and applicability of many of the techniques are affected by the duct type

(metal or plastic), duct location (internal or external), and the surrounding components (i.e. reinforcing

bars) with in the post-tensioned system. A summary of the NDE methods is presented in Table 4.1.

Table 4.1: Summary of NDE Methods

Met

ho

d Application in PT

Systems

Limitations Current Uses in PT

Bridge Inspections

Recommendations

Aco

ust

ic E

mis

sio

n

Mainly used for the

detection of wire

fracture occurrence

through continuous

monitoring

Cannot detect

existing damage

Produces large

amounts of data that can

be difficult to interpret

Fractures in

grouted tendons are

more difficult to detect

then in ungrouted or

partially grouted

specimens

Not used for

monitoring of grouted

tendons

The ability to accurately

detect a break in a fully grouted

PT tendon has not been

adequately demonstrated and

therefore is not currently

recommended for long term

monitoring

Ele

ctri

call

y I

sola

ted

Ten

do

ns Detection of

breaches in the

corrosion protection

system

Provides enhanced

levels of corrosion

protection of the

tendon

Allows for quality

control and long

term monitoring

Requires plastic

duct and special

isolation hardware for

anchorages.

If electrical

isolation is not achieved

during construction

future NDE monitoring

cannot be performed

Not used in the US

PT systems

Applications exist in

Switzerland (required

for most PT systems)

and Italy for quality

control and long term

monitoring of the PT

corrosion protection

systems

The method appears to be

viable for detection of a breach

in the corrosion protection

system of the duct during

service life.

This approach appears to be

viable and should be

investigated further for the US

market

Gro

un

d P

enet

rati

ng

Rad

ar

External detection of

metallic duct

location

Potential use in

detecting grout voids

in plastic ducts

Potential use in

external detection of

plastic ducts

Difficult to identify

duct/tendon in areas

with high reinforcement

congestion

Cannot inspect

conditions within metal

ducts

Accuracy is

reduced with increase in

embedment depth.

Widely used to

locate metal ducts

during inspections

This method provides a

well-established tool for

location of metallic ducts and

reinforcement

GPR has also shown

promise for locating plastic

ducts in laboratory testing.

This method provides the

possibly for identification of

voids in plastic ducts. This

approach should be investigated

further.


Table 4.1: Summary of NDE Methods M

eth

od Application in PT

Systems


Bridge Inspections

Recommendations H

alf-

Cel

l P

ote

nti

al

Detection of regions

of high potential,

indicative of active

corrosion of

tendons.

Ineffective for

plastic or steel duct

tendon systems when

applied externally due

to the masking effect of

the ducts

External applications

not typically used for

the inspection of PT

tendons

Due to the shielding

provided by the duct this

method is not viable for external

assessment of corrosion

Half-cell probes internally

embedded into the tendons are

commercially available and

should be investigated as a

viable method

Imp

act

Ech

o

Detection of Grout

Voids

Strand Location

Difficult to use in

areas with congested

rebar

Not well suited for

inspecting large areas

unless automated

systems are used.

Not typically used

for tendon inspection This method has the

potential to be used as a quality

control tool to ensure proper

grouting of the tendon in known

problem areas and is currently

best suited for metal ducts.

It is not currently

recommended as a tool for

strand location.

Infr

ared

Th

erm

og

rap

hy

Grout Void

Detection

Strand Location

Ineffective for steel

duct systems

Depth and

thickness measurements

require experimental or

numerical calibration

data.

Not typically used

for tendon inspection Based on the current

technology and lack of

successful applications in PT

systems this method is not

currently viable for grout void

or strand location assessment.

Mag

net

ic F

lux

Lea

kag

e

Strand Corrosion

Wire Fracture

More difficult to

use when the duct is

embedded in concrete

Presence of rebar

can affect the accuracy

Ducts can create a

masking effect

Primarily used for

external ducts.

Field readings for

internal ducts have been

found to be less

accurate.

This method is viable for

the assessment of external

tendons.

Due to the ability for this

method to detect corrosion of

the strand it should be further

investigated for applicability to

internal tendons.

Rad

iog

rap

hy

Strand Corrosion

Grout Voids

Strand Location

Requires access to

both sides of the

specimen

Gamma ray

devices use radioactive

materials

X-ray and Gamma

ray require safety

precautions during

transmission.

Not typically used

for tendon inspection May be suited for detection

strand corrosion and grout

voids. If verified, it would be

suited for the inspection of

tendon anchorage regions and

strand couplers where complex

geometries and multi-layer

material interfaces present

problems for other NDE

methods.

Portable high intensity X-

ray machines have improved the

efficiency and imaging

capabilities of radiography


Table 4.1: Summary of NDE Methods M

eth

od Application in PT

Systems


Bridge Inspections

Recommendations T

ime

Do

mai

n R

efle

cto

met

ry

Grout Voids

Strand Corrosion

Method is sensitive

to the size of the defect

Best results are

obtained with internal

sensors which need to

be integrated into the

duct

Content in voids

can affect the accuracy

Pulse generator

required can be

expensive.

No commercially

available system for PT

inspection

Not typically used or

currently installed in PT

bridge systems

This method appears

promising and should be

investigated further for internal

integration in PT systems.

The ability to detect both

corrosion and grout voids while

providing information on the

location of the defect should be

verified

Ability to detect moisture

levels in grout should be

explored further

Ult

raso

nic

Tes

tin

g

Strand Corrosion

Grout Voids

PT Tendon Location

Ends of PT tendons

need to be accessible for

GWT

Signal

interpretation difficult

for complex geometries

Not commonly

used for tendon

inspection.

GWT has been

used for condition

assessment of steel

strand and prestress

monitoring

Ultrasonic imaging

has been used for

grout void

detection in PT

tendons.

GWT is a viable method for

identifying strand breaks.

Phase modified SAFT is a

reliable method for detecting

grout voids in PT tendons, but

methods cannot handle complex

geometries such as the tendon

anchorage regions.

This approach appears to be

viable and should be

investigated further for the US

market

Vis

ual

In

spec

tio

n

Strand Corrosion

Grout Voids

Access to the

interior of the duct

requires invasive

drilling

Each access point

to the tendon only

allows for a small

amount of the tendon to

be inspected

Location of the

tendon must be known.

Currently one of

the most typically

used methods

Borescopes are

widely used to

inspect internal

conditions of

tendons

This method provides

accurate information on the

condition of the tendon. Due to

the fact that this method

requires invasive drilling and

allows only a small portion of

the tendon to be inspected it is

best suited as a tool to verify

localized damage identified by

other methods.

4.1 Future Work

As part of the next task of this project a comprehensive study of PT systems, components and details

(subtask 11.2) and bridge superstructure, components and details (subtask 11.3) will be conducted. The

purpose of this study is to determine if any of these details can be altered to better accommodate NDE

testing. This will include but is not limited to modification of congested reinforcement areas, location of

diaphragms and other detailing concepts that will improve the access and inspection of the bridge regions

sensitive to damage. Based on the investigation of the techniques that are currently available for field and


the academic applications of these techniques in research applications NDE methods that can be

integrated into PT systems appear to show the most promise for both quality control and long term

monitoring. The following NDE methods which can be internally integrated into PT systems will be

explored in the subsequent tasks. This includes electrically isolated tendons, time domain reflectometry,

ultrasonic, and internal half-cell measurements.

The primary goal of the future subtasks is to identify PT system component details and bridge

superstructure details that restrict the ability to effectively evaluate the post-tensioned tendons. Practical

solutions on how to alter the PT system components and bridge superstructure for each of the methods

selected will be developed. The ultimate goal is to find a combination of NDE, PT system details and

bridge superstructure details that can be implemented into future systems to increase the accuracy and

ease of inspection for post-tensioned systems.


5 Agency Sponsored Studies

5.1 Department of Transportation Reports


Report –Volume 1

Author(s): Atorod Azizinamini and Jawad Gull

Publication: FDOT Contract No. BDK80 977-13

Publication Date: June 2012

Abstract/Summary: Post-tensioned bridges require a detailed inspection of their post-tensioning systems

since damage in these systems in not evident and can result in costly repairs/replacements, loss of

integrity and reduction in safety of the bridge. Different nondestructive evaluation (NDE) techniques can

be used for the inspection of post-tensioning systems however; there is no systematic way by which a

particular NDE technology may be selected for a particular job.

This project presents elements and foundation for development of a systematic, job specific approach for

the selection of NDE technology for inspection of the post-tensioning systems.

In order to achieve this goal, factors affecting the performance of NDE techniques used for the condition

assessment of the post-tensioning systems are identified. NDE techniques are then grouped according to

their underlying phenomenological basis to put forth the working principles, and pros and cons that can

be used for the development of ranking system.

It has been found that major difference in grouting practice before and after 2000 affects the selection of

NDE technique for the inspection of post-tensioning systems. Other factors affecting the performance of

the NDE techniques are the duct system, geometrically difficult zones, and the types of defects. Separate

sets of NDE technique are recommended for the internal/external ducts positioned along the traffic and

geometrically difficult zones. A job specific “Ranking Index” is proposed for the selection of NDE that

takes into account the not only above mentioned factors but also the cost of conducting NDE. Based on

the project findings a roadmap to develop comprehensive methodology to effectively asses the condition

of post-tensioned bridge systems is presented.


Concrete Bridges- Final Report – Volume II

Author(s): Atorod Azizinamini and Jawad Gull

Publication: FDOT Contract No. BDK80 977-13


Abstract/Summary: Post-tensioned bridges require a detailed inspection of their post-tensioning systems

since damage in these systems in not evident and can result in costly repairs/replacements, loss of

integrity and reduction in safety of the bridge. Different nondestructive evaluation (NDE) techniques can

be used for the inspection of post-tensioning systems however; there is no systematic way by which a

particular NDE technology may be selected for a particular job.

This project presents elements and foundation for development of a systematic, job specific approach for

the selection of NDE technology for inspection of the post-tensioning systems.

In order to achieve this goal, factors affecting the performance of NDE techniques used for the condition

assessment of the post-tensioning systems are identified. NDE techniques are then grouped according to


their underlying phenomenological basis to put forth the working principles, and pros and cons that can

be used for the development of ranking system.\

It has been found that major difference in grouting practice before and after 2000 affects the selection of

NDE technique for the inspection of post-tensioning systems. Other factors affecting the performance of

the NDE techniques are the duct system, geometrically difficult zones, and the types of defects. Separate

sets of NDE technique are recommended for the internal/external ducts positioned along the traffic and

geometrically difficult zones. A job specific “Ranking Index” is proposed for the selection of NDE that

takes into account the not only above mentioned factors but also the cost of conducting NDE. Based on

the project findings a roadmap to develop comprehensive methodology to effectively asses the condition

of post-tensioned bridge systems is presented.


Strands in Concrete Bridge Components Task 2 – Assessment of Candidate NDT Methods

Author(s): Lawrence Jones, Stephen Pessiki, Clay Naito, and Ian Hodgson

Publication: Pennsylvania Department of Transportation

Publication Date: 2010

Abstract/Summary: Catastrophic failures of non-composite prestressed precast concrete adjacent-box

beam bridges have occurred in several states due to corrosion of the prestressing steel. These failures

have highlighted the need to improve methods used to detect corrosion damage and subsequently load

rate the damaged members. In light of this, PennDot initiated a research program aimed at improving

inspection techniques through evaluation of the off-the-shelf non-destructive testing (NDT) technologies

and correlation of surface conditions with non-visible strand corrosion. Funding for the project was

provided by the departments of transportation of Pennsylvania (the lead agency), New York, and Illinois.

Currently, inspection of concrete box girder sections relies on visual methods which correlate longitudinal

and transverse cracking, spalling and exposed strands with the rated level of performance of the member.

While the visual method provides a qualitative estimate of the amount of damage, the specific location

along a strand and the amount of damage to the strands is not clearly defined. As a result, the assessment

of the condition of the bridge could in some cases results in an un-conservative or overly-conservative

estimate of remaining strength. Furthermore, without a high level of accuracy in locating damage to the

strands, remediation and rehabilitation is difficult to accomplish. To improve on the current inspection

techniques the visual inspection requirements are revisited through an extensive destructive evaluation

study. In addition, NDT methods are evaluated and compared with actual damage present in a group of

40-50 year old box beams removed from service. The goal of this project is to determine if visual

inspection techniques or currently available NDT technologies will allow for accurate identification of

non-visible corrosion of prestressing strands.

To perform this evaluation, seven non-composite adjacent prestressed box beam segments were procured.

The beams were recovered from three decommissioned bridges in the state of Pennsylvania. The beams

selected were chosen to have variety of different construction details and levels of damage to provide a

spectrum of corrosion conditions.

Six NDT experts evaluated the beams prior to destructive evaluation. Each team compiled their data and

reported their findings for comparison with the in-house destructive evaluation. This report presents the

background of each NDT method used and discusses the accuracy and feasibility of using the method for

field inspection of prestressed concrete bridge components. Based on the results of the study it was found

that Magnetic Flux Leakage and Remnant Magnetism evaluation methods are potentially viable for the

detection of non-visible corrosion of prestressing strands. Ground penetrating radar techniques may also

be viable with improvements in the resolution of the hardware. The results produced by Line Scanning


Thermography, Electromagnetic Corrosion Detection, Galvanostatic Pulse Corrosion and Ultrasonic

Shear Wave Testing do not provide better accuracy than visual observation techniques.

The accuracy and reliability of two methods were examined more thoroughly. Magnetic Flux Leakage

and Remnant Magnetism were re-evaluated using a series of manufactured test slabs. The results of this

study indicated that both methods are capable of detecting section loss in strands with good accuracy.



Author(s): Clay Naito, Larry Jones and Ian Hodgson

Publication: Pennsylvania Department of Transportation


Abstract/Summary: Catastrophic failures of non-composite prestressed precast concrete adjacent-box

beam bridges have occurred in several states due to corrosion of the prestressing steel. These failures

have highlighted the need to improve methods used to detect corrosion damage and subsequently load

rate the damaged members. In light of this, PennDot initiated a research program aimed at improving

inspection techniques through evaluation of the off-the-shelf non-destructive testing (NDT) technologies

and correlation of surface conditions with non-visible strand corrosion. Funding for the project was

provided by the departments of transportation of Pennsylvania (the lead agency), New York, and Illinois.

Currently, inspection of concrete box girder sections relies on visual methods which correlate longitudinal

and transverse cracking, spalling and exposed strands with the rated level of performance of the member.

While the visual method provides a qualitative estimate of the amount of damage, the specific location

along a strand and the amount of damage to the strands is not clearly defined. As a result, the assessment

of the condition of the bridge could in some cases results in an un-conservative or overly-conservative

estimate of remaining strength. Furthermore, without a high level of accuracy in locating damage to the

strands, remediation and rehabilitation is difficult to accomplish. To improve on the current inspection

techniques the visual inspection requirements are revisited through an extensive destructive evaluation

study. In addition, NDT methods are evaluated and compared with actual damage present in a group of

40-50 year old box beams removed from service. The goal of this project is to determine if visual

inspection techniques or currently available NDT technologies will allow for accurate identification of

non-visible corrosion of prestressing strands.

This report presents the results of the visual inspection, material testing, half-cell potential mapping, and

the destructive evaluation of the beams. The research results indicate that fabrication techniques used for

box beam construction in the 1950-1960 time period allowed for large variations in construction

tolerance. Half cell methods were shown to not provide an accurate or reliable method of identifying

corrosion of prestressing strands. Longitudinal cracking was shown to provide an accurate and reliable

means of identifying corrosion of prestressing strands. Probabilities of corrosion on strands adjacent to

longitudinal cracks are determined and discussed. Additionally, a new recommendation for inspecting

beams and its impact on operating and inventory rating is provided.

5.1.5 Nondestructive Method to Detect Corrosion of Steel Elements in Concrete

Author(s): Marcelo DaSilva, Saeed Javidi, AaronYakel, and Atorod Azizinamini

Publication: National Bridge Research Organization & Nebraska Department of Roads

Publication Date: April 2009

Abstract: This report provides an outline of a novel approach, methodology, and equipment needed to

detect corrosion of any steel element that is embedded in the concrete bridges. Further, within this project,


a complete non-destructive system is developed that can detect the presence and extent of corrosion in

any steel elements in concrete bridges.

Possible applications include:

a) Detecting corrosion of reinforcing bars in concrete deck

b) Detecting corrosion of prestressing strands in prestressed girders

c) Detecting corrosion of prestressing strands in post tensioned concrete bridges and placed in steel or

plastic duct

d) Evaluating the condition of bridge deck with respect to corrosion

The method is specifically developed for post tension concrete bridges, which is the most challenging

condition. Application to other conditions will be an easy transition. The method is referred to as non-

destructive since it can detect the presence of corrosion without drilling holes or similar approaches for

visual inspection. It is in that sense a “blind” technique, where condition of the embedded steel is assessed

similar to taking x-ray or having an MRI on human body. The foundation of the method is based on a

simple concept in physics called The Hall effect. The principle is based on creating a magnetic field

around the embedded steel element and studying the resulting magnetic field changes using Hall-effect

sensors. The idea is that the magnetic field will change in the presence of corrosion. The development of

the concept and non-destructive equipment started with a very primitive device and evolved into a system

that can be applied to field conditions. A series of laboratory tests were carried out to develop the

methodology and the non-destructive device itself. An extensive amount of numerical analyses were

carried out to comprehend the meaning of different signal types that were being obtained from the non-

destructive device. Finally, limited field studies were carried out to develop a comprehensive

methodology adoptable to field conditions. Corrosion of steel reinforcing bars or strands in concrete

bridges present a great challenge for inspection and safety evaluation of these structures. The non-

destructive equipment and associated methodology developed in this project allows effective and rapid

assessment of bridges for the presence of corrosion.

5.1.6 Effect of Voids in Grouted, Post-Tensioned Concrete Bridge Construction: Volume 1 –

Electrochemical Testing and Reliability Assessment

Author(s): David Trejo, Mary Beth D. Hueste, Paolo Gardoni, Radhakrishna G. Pillai, Kenneth

Reinschmidt, Seok Been Im, Suresh Kataria, Stefan Hurlebaus, Michael Gamble and Thanh Tat Ngo

Publication: Texas Department of Transportation (Report No. FHWA/TX-09/0-4588-1 Vol.1)

Publication Date: September 2009

Abstract/Summary: Post-tensioned (PT) bridges are major structures that carry significant

traffic. PT bridges are economical for spanning long distances. In Texas, there are several

signature PT bridges. In the late 1990s and early 2000s, several state highway agencies

identified challenges with the PT structures, mainly corrosion of the PT strands. The Texas

Department of Transportation (TxDOT) performed some comprehensive inspections of its

PT bridges. A consultant’s report recommended that all ducts be re-grouted. However, the

environment in Texas is very different than the environments in which the corrosion of the

PT strands were observed. The objective of this research was to evaluate the corrosion

activity of strands for PT structures and to correlate this corrosion activity with general

environmental and void conditions. To achieve this objective, time-variant probabilistic

models were developed to predict the tension capacity of PT strands subjected to different

environmental and void conditions. Using these probabilistic models, time-variant structural

reliability models were developed. The probability of failure of a simplified PT structure

subjected to HS20 and HL93 loading conditions was assessed. Both flexural failure and

serviceability were assessed. Results indicate that the presence of water and chlorides can


lead to significant corrosion rates and failure is dependent on this corrosion activity and the

number of strands exposed to these conditions. Volume 1 of this report presents these results.

To assist TxDOT with developing a plan to mitigate this corrosion, studies were performed

to assess repair grout materials, inspection methods, and repair methods. In addition, a

general methodology is presented on optimizing repairs. These topics are presented in

Volume 2 of this report. An Inspection and Repair Manual was also developed from this

research and is presented in a separate report. Results indicate that TxDOT should prevent

water and chlorides from infiltrating the tendons; this can be achieved in part by repairing

drain lines and ducts and protecting anchor heads, as these conditions can lead to early

failure of PT bridges. Recommendations on inspections, repairs, and materials are provided;

however, further research on the potential formation of galvanic coupling of strands

embedded in both existing and new repair grouts needs to be assessed.


Penetrating Radar

Author(s): David G. Pollock, Kenneth J. Dupuis, Benjamin Lacour, and Karl R. Olsen

Publication: Washington State Department of Transportation (Report No. WA-RD 717.1)

Publication Date: December 2008

Abstract/Summary: Thermal imaging and ground-penetrating radar was conducted on concrete specimens

with simulated air voids. For the thermal imaging inspections, six concrete specimens were constructed

during the month of June 2007 to simulate the walls of post-tensioned box girder bridges. The objective

was to detect simulated air voids within grouted post-tensioning ducts, thus locating areas where the post-

tensioning steel strands are vulnerable to corrosion. The most important deduction taken from these

inspections was that PT-ducts and simulated voids were more detectable in the 20 cm (8 in.) thick

specimens than in the 30 cm (12 in.) thick specimens. While inspections of the 20 cm (8 in.) thick

specimens revealed the majority of their simulated voids, only one thicker specimen inspection (12c)

indicated the presence of simulated voids (four voids in two ducts). Also, PT-ducts were much clearer and

visible in the thermal images of the thinner specimens.

Ground-penetrating radar (GPR) inspection was conducted on fourteen concrete specimens between

August and October 2007. Based on the GPR surveys conducted in this study, it is apparent that the

detection of post-tensioning strands and simulated voids within grouted ducts embedded in concrete is

possible with a 1.5 GHz GPR system. The layout of the top layer of steel reinforcement in each concrete

specimen was evident in the GPR images, but the bottom layer of reinforcement was not clearly detected

since it was effectively “hidden” beneath the top layer of rebar. Although none of the post-tensioning

strands and simulated air voids within the grouted steel ducts was detectable, simulated voids within

plastic ducts were generally detectable in GPR images. The high dielectric constant of the steel ducts did

not allow the microwaves to transmit through the surface of the duct and reach the simulated voids.

However, the general location of the duct, its orientation and its depth in the concrete were accurately

determined using GPR. Thus it can be inferred that the void orientation is critical for detection in GPR

images.

5.1.8 New Directions for Florida Post-Tensioned Bridges – Load Rating Post-Tensioned Concrete

Segmental Bridges (Volume 10A)

Author(s): Corven Engineering

Publication: Florida Department of Transportation


Publication Date: October 8, 2004

Abstract/Summary: Condition Inspection and Maintenance of Florida Post-Tensioned Bridges addresses

the specifics of ensuring the long-term durability of tendons in existing and newly constructed bridges.

The types of inspections and testing procedures available for condition assessments are reviewed, and a

protocol of remedies are presented for various symptoms found.


Concrete Beam Bridges (Volume 10B)









Ducts – Final Report

Author(s): Larry C. Muszynski, Abdol R. Chini, and Elie G. Andary

Publication: Florida DOT (Report No. DOT HS 809 412)

Publication Date: May 30, 2003

Abstract/Summary: The use of post-tensioning in bridges provides durability and structural benefits to the

system while expediting the construction process. However, there is considerable interest to determine

whether a tendon duct is properly filled with grout or not. Implementing non-destructive testing can be

vital to the integrity of the structure because loss of post-tensioning can result in catastrophic failure.

Objectives: The purpose of this work was to develop and validate a non-destructive testing and evaluation

(NDT&E) method that can be used in the field to detect internal concrete conditions such as voids and

cracks in grouted tendon ducts during bonded post-tensioned applications.

Methods: These are three techniques that were evaluated in this research effort: Impact Echo (IE),

Spectral Analysis of Surface Waves (SASW), and Ultrasonic Tomography Imaging (UTI).

Results: Based on the results for all three tests, we suggest that the IE scanning should be used to evaluate

the internal condition of the grouted duct. Impactechograms may also be used as a way to present the

results since the plot provides more information in IE frequency data. The image results from UTI

showed locations of both ducts clearly in all three cases. Unfortunately, the images did not show details

inside the ducts.

Conclusion: Scanning Impact Eacho, IE, tests showed the most promise for assessing internal grout

conditions of the steel duct. For a plastic duct, it was more difficult to identify grout conditions due to

partial debonding conditions between the plastic duct and concrete wall.

5.1.11 Test and Assessment of NDT Methods for Post-Tensioning Systems in Segmental Balanced

Cantilever Concrete Bridges

Author(s): DMJM Harris

Publication: Florida Department of Transportation Central Structures Office


Publication Date: February 2003

Abstract/Summary: The failure of several post-tensioning tendons in the Niles Channel and Mid-Bay

bridges in Florida due to poor workmanship and inadequate grouting has raised questions about the

integrity of post-tensioning tendons in existing concrete segmental bridges. The need to assess the

condition of post-tensioning tendons in existing Florida bridges has prompted the Florida Department of

Transportation to fund a study, with collaboration from the FHWA, on the accuracy of several Non-

Destructive Testing (NDT) methods in a real case scenario. The program involves the use of selected

NDT methods to assess the status of the top slab post-tensioning tendons of Ramp D located in the

interchange at the Fort Lauderdale-Hollywood International Airport. This precast balanced cantilever

concrete box girder bridge is being demolished as part of the airport expansion thus permitting the

verification of the NDT findings via dissection of the concrete segments. The NDT methods to be

examined are: Impulse Radar, Impact-Echo, Magnetic Flux Leakage and High-Powered X-Ray

Imagining. The tests were performed in late March 2002 by three independent sub-consultants with

overall project management provided by DMJM+HARRIS. This report will provide a description of the

procedures used by the sub-consultants to utilize these NDT methods to evaluate an existing concrete

bridge, and will present conclusions on the accuracy of the NDT findings. The accuracy of thee NDT

findings have been evaluated by core drilling in the deck and visually inspecting the

tendons.

The assessment of the NDT methods provided the following conclusions:

Endoscope Inspection

The use of the endoscope to evaluate the condition of top slab tendons was found, in this testing program,

to be a reliable testing method. Testing, at a given point in the deck, was done in an average of 10 minutes

and required a four-person crew. The endoscope inspection should be preceded by more economical NDT

testing methods that locate areas where tendon flaws (void, corrosion, loss of section, etc) are most likely

to exist. Also, it is critical for drilling to be done with much care in order to avoid damaging the tendons

at the time of inspection. The use of special concrete drills capable to detect the steel duct and stop before

damaging it is recommended. And finally, after inspection, drilled holes should be appropriately patched

to avoid any future maintenance and durability problems.

Impulse Radar Testing Method

The impulse radar testing method provided quick and accurate location of the tendons. The method

requires small size equipment that can be operated by a two-person crew. A test at a given point can be

done in less than one minute. Although the location of the tendons at the segment joints was performed

accurately based on the contract drawings information, the location of these tendons between segment

joints could not be ascertained based on this information only. At these locations Impulse Radar was 80%

reliable in locating the tendons. The method can provide not only the horizontal location of the tendon but

also the depth into the concrete, which can be of tremendous value in the interpretation of the Impact

Echo results.

Impact-Echo Testing Method

The Impact-Echo testing method was found to be a reliable method to identify grout voids in tendon ducts

provided that a combination of techniques including impulse radar and rebar locators are used. In

addition, invasive endoscopy tests are required to correlate the interpretation of the signals with the

existing conditions (deck 3-D geometry, nearby tendons and mild steel, etc.). The reliability of the

method (defined as detecting large voids) was found to be higher than 60% in this testing program.

Locating the testing point and performing the test can be done in less than 3 minutes with very small

equipment operated by a two-person crew. The method is effective in providing a clear indication of a

sudden discontinuity in material properties and distinguishing whether this discontinuity represents a void


or a stiffer material (the tendon). However, the size of the void, (an essential factor in assessing its

importance and possible consequences), is difficult to ascertain.

Magnetic Flux Leakage Method

The testing performed using the MFL method was, for practical purposes, found inadequate to identify

losses of tendon area. The method failed to locate the tendons with the induced flaws in anchor trumpets.

The reason being that the equipment used, did not have magnets strong enough to magnetically saturate

the tendons and consequently, produce the flux to leak. The method, as this stage, does not provide the

necessary confidence in the method (in its current condition) for practical applications. The MFL method

is fast in terms of data acquisition. However, it requires careful and expert interpretation of the test record.

A major drawback of this method is that it requires a very accurate depiction of the tendon path at the

roadway surface, which, in turn, requires the extensive use of another testing methods such as Impulse

Radar.

High Energy Linear Accelerator

This procedure was found to have the potential to be a very effective method for locating flaws in tendons

deeply embedded in the concrete. It provided a relatively clear view of the elements inside the concrete.

To be most effective, the interpretation of the film should be performed by an expert in both concrete

bridges and x-rays. At this moment, the method is very expensive, very cumbersome to use, and requires

a large amount of heavy equipment and a large crew size. In addition, the scatter of the x-ray beam

requires that a large radius around the testing area to be evacuated to avoid health issues. In the future, if

more compact equipment is developed for use in bridges, this method could be a valuable tool for the

inspection of post-tensioned bridges.

Based on the results of the study, the authors recommend the following steps for the inspection of tendon

in existing balanced cantilever concrete box girder bridges:

Step 1 – Examination of existing records and information, such as Contract Plans, Shop Drawings, As-

built Plans and previous inspection reports.

Step 2 – Perform a detailed visual inspection of the bridge. The recommendations stated in the Florida

Department of Transportation document titled “Post Tensioned Bridges Walk Through Inspections”, can

be used for this purpose.

Step 3 – Depending on the results of the visual inspection the following scenarios are possible: a) If the

visual inspection does not reveal deficiencies that may affect the integrity of the post-tensioning system,

no further action is needed. On the other hand, if the bridge has been in service for a number of years (say

10) and an in-depth inspection is warranted, then prepare a plan for inspecting the bridge using a

combination of NDT testing (Impulse Radar and Impact-Echo) and invasive techniques (Endoscopy

Inspection). The testing should be done on a representative sample of the tendons, at most 10%, 2002.

The tendons to be tested and the test location on the tendons should be based on their structural

importance. b) If the visual inspection reveals significant deficiencies such as water leakage at segment

joints, efflorescence, concrete cracking or spalling; prepare an inspection plan combining impact echo an

endoscopy inspection. In this case, however, the areas with significant deficiencies should be inspected in

detail and, if deemed necessary, all tendons should be inspected. Other areas should be inspected

following the 5% rule stated above.

Step 4 - If an inspection combining NDT testing techniques and invasive techniques is deemed necessary,

then proceed as follows: a) Use a combination of as-built plans, impulse radar and rebar locators to locate

the embedded steel components including both reinforcing steel and post-tensioning tendons. Mark the

location of the embedded steel on the concrete surface. b) Artificially divide the tendons in sections


(approximately five feet long each) and select a sample based on an statistically-based method like those

employed in quality control programs. c) Investigate the selected sample for tendon voids using the

Impact- Echo method. Calibrate the signal interpretation using the knowledge of embedded steel

components and deck 3-D geometry with drilling and endoscopy. Using the calibrated signal

interpretation complete the inspection of the selected samples. If the inspection does not reveal significant

deficiencies and a high percentage of the test locations (say 95%) indicate no relevant voids, take no

further actions. If other conditions exist, verify void relevance and strand integrity by drilling and

inspecting with a flexible shaft endoscope. d) If the flexible shaft endoscope inspection find significant

voids and strand corrosion, then expand the sample size. e) At each drilled hole determine the volume of

the void by using a vacuum or a pressure device. If this volume is large then repair the void using vacuum

grouting. f) Upon completion of the inspection clean the hole and repair the drilled hole with a fluid

epoxy for the repair of old structures (like FDOT Type E).

5.1.12 New Directions for Florida Post-Tensioned Bridges – Post-Tensioning in Florida Bridges

(Volume 1)



Publication Date: February 15, 2002

Abstract/Summary: Post-Tensioning in Florida Bridges presents a history of post-tensioning in Florida

along with the different types of post-tensioned bridges typically built in Florida. This volume also

reviews the critical nature of different types of post-tensioning tendons and details a new five-part

strategy for improving the durability of post-tensioned bridges.

5.1.13 New Directions for Florida Post-Tensioned Bridges – Design and Construction Inspection of

Precast Segmental Balanced Cantilever Bridges (Volume 2)



Publication Date: September 1, 2002

Abstract/Summary: Design and Construction Inspection of various types of post-tensioned bridges applies

the five-part strategy of Volume 1 to bridges in Florida. Items such as materials for enhanced post-

tensioning systems, plan sheet requirements, grouting, and detailing practices for watertight bridges and

multi-layered anchor protection are presented in detail. The various types of inspection necessary to

accomplish the purposes of the five-part strategy are presented from the perspective of CEI along with

detailed checklists of critical items or activities.

5.1.14 New Directions for Florida Post-Tensioned Bridges –Design and Construction Inspection of

Precast Segmental Span-By-Span Bridges (Volume 3)











5.1.15 New Directions for Florida Post-Tensioned Bridges- Design and Construction Inspection of

Precast Concrete Spliced I-Grider Bridges (Volume 4)










5.1.16 New Directions for Florida Post-Tensioned Bridges - Design and Construction Inspection of

Cast-In-Place Segmental Balanced Cantilever Bridges (Volume 5)










5.1.17 New Directions for Florida Post-Tensioned Bridges - Design and Construction Inspection of

Bridges Cast-In-Place on Falsework (Volume 6)










5.1.18 New Directions for Florida Post-Tensioned Bridges – Design and Construction of Post-

Tensioned Substructures











5.1.19 New Directions for Florida Post-Tensioned Bridges – Design and Construction of

Transverse Post-Tensioning of Superstructures (Volume 8)











Maintenance of Florida Post-Tensioned Bridges (Volume 9)



Publication Date: March 22, 2002





5.1.21 Mid-Bay Bridge Post-Tensioning Evaluation


Publication: Florida Department of Transportation District 3


Abstract/Summary: The Mid-Bay Bridge, Florida Bridge No. 570091, is a precast segmental bridge

crossing Choctawhatchee Bay in Okaloosa County, Florida. The bridge carries FL 293 between US 98

near Sandestin and SR 20 east of Niceville. A location map of the bridge is given in Figure 1.1.

On August 28, 2000, during a routine inspection of the Mid-Bay Bridge, a post-tensioning tendon in Span

28 was observed to be significantly distressed. The polyethylene sheathing surrounding the tendon was

cracked, exposing the tendon’s high strength prestressing strands and surrounding cementitious grout.

Several of the strands of the post-tensioning tendon were fractured.

Concern raised from this observation led to an immediate “walk-through” inspection to verify if


other post-tensioning tendons were exhibiting similar signs of distress. A post-tensioning tendon in Span

57 was found to have failed completely at the north end of the tendon, as evidenced by pull out from the

expansion joint diaphragm.

As a result of these preliminary findings, a more complete inspection, testing and analysis program was

developed to identify the source and extent of corrosion in the post-tensioning

tendons and to develop necessary remedial action. This report presents the findings of these

inspections, tests and analyses, as well as the repairs performed.

5.1.22 Initial Development of Methods for Assessing Condition of Post-Tensioned Tendons of

Segmental Bridges

Author(s): A.A Sagues, S.C Kranc and R.H. Hoehne

Publication: Florida Department of Transportation (Contract No. BC374)

Publication Date: May 17, 2000

Abstract/Summary: Examination of post tensioned tendons of the Niles Channel Bridge during Spring,

1999 indicated severe corrosion damage and strand separation near the anchorage points on two tendons.

This investigation was conducted to determine the suitability of non destructive mechanical and electrical

testing of tendons to detect strand failure.

Mechanical testing consisted of measuring the vibrational response of tendons to mechanical excitation,

and using the results to estimate the tendon tension and stiffness. Comparisons among these values were

used to identify indicators of possible distress. One of the indicators (relative difference in tension at

opposite ends of the tendon) was strongest for a tendon previously known to have broken strands. Other

indicators (low tension, low stiffness) may serve as additional or alternative indicators of distress.

Accordingly, a list of tendons exhibiting exceptional tension conditions has been formulated.

Specialized procedures for vibration data acquisition were developed to permit characterization of an

entire large bridge in a short time (days). Data processing and equation solution procedures tailored to this

analysis were developed and implemented. A baseline of vibrational behavior for all tendons in the Niles

Channel bridge was developed so that comparative measurements may be conducted over the remaining

service life of the bridge. The results also revealed global trends of tension as a function of position in the

bridge. For example, the tendons in the Atlantic side of the bridge were found to have typically lower

tension (by about 5%) than those in the Gulf side.

Electrical testing consisted of measuring the electrical resistance of the tendon as a function of distance

from the anchoring plate and determining whether the initial extrapolated value at zero distance and the

slope conformed to those expected for an ideally sound tendon. This procedure is slower than the

vibrational tests and has been conducted on a limited number of control tendons and others identified as

suspects by the vibrational test. The preliminary tests showed a strong indication of distress by this

method only in the one tendon known to have failed strands.

The procedures evaluated to date appear to be suitable for quick screening of the structure (vibrational)

followed by more detailed analysis of suspects (electrical). Examination of the results available to date

yielded consistent indications of distress only for the one tendon known to have failed strands. Electrical

testing of the remaining suspects and periodic vibrational testing of the entire bridge is recommended.

Due to substantial evidence and previous history, it is recommended that all tendon segments adjacent to

open expansion joints be subject to more intense scrutiny, including direct examination of the tendons.


Conditions leading to strand corrosion in the Niles Channel bridge need to be assessed, with special

attention to the source and accumulation mechanisms of chloride ions, and to the factors that may affect

corrosion performance after any remedial action is taken.

5.1.23 Tensile Test Results of Post Tensioning Cables from the Midbay Bridge

Author(s): Thomas E. Beitelman

Publication: Florida Department of Transportation Structures Research Center


Abstract/Summary: The Midbay bridge, located on State Road 293 between State Road 20 and U.S. 98 in

Niceville, Florida is a concrete segmental box bridge with approximately 140 spans. The bridge uses a

total of six external post-tensioning cables as the primary live load resistance system, with three of the six

cables on either side of the boxes. Each cable is composed of 19 – 0.6” diameter steel cables that are

stressed to 31,000 pounds each at the time of construction. The cables are held in place between steel

anchors and surrounded by a protective sleeve. The primary anchorage system consists of end steel blocks

with post-tensioning wedges, while the secondary anchorage system relies on a grout that is pumped into

the casing surrounding the strands.

An inspection of the post-tensioning system during the year 2000 revealed that a significant number of

these cables were exhibiting signs of corrosion and possibly, improper grout. The corrosion has been

speculated to have been caused by trapped moisture in voids where grout should have existed. Inspection

using a borescope inserted into a hole drilled through the anchorage assembly revealed that several wires

in various individual strands have ruptured and others have had some level of corrosion.

5.1.24 Corrosion Evaluation of Post-Tensioned Tendons in Florida Bridges

Author(s): Rodney G. Powers, Alberto A. Sagues and Yash Paul Virmani

Publication: Florida Department of Transportation (Report No. FL/DOT/SMO/04-475)


Abstract/Summary: Severe corrosion distress and failures in post-tensioned tendons has been found in two

major bridges in the State of Florida. Corrosion distress and complete tendon failure has been identified in

horizontally oriented tendons that support pre-cast bridge superstructure box segments. In virtually all

instances, the observed corrosion has been associated with the presence of grout voids and visual

evidence of grout bleed water having been present for indeterminate periods of time. With few

exceptions, corrosion induced strand failures have occurred in the immediate vicinity of the anchorage.

The anchorage systems in both bridges utilize a proprietary multi-plane anchorage (housing) comprised of

ductile cast iron and forged steel wedge plate. The presence of these metals coupled with high strength

strands gives rise to concerns relative to galvanic corrosion thus prompting this preliminary investigation

focusing on dissimilar metals corrosion. The investigation involves laboratory tests that examine the

corrosion aspects of grout bleed water and re-charge water in contact with dissimilar metals comprising

the tendon and anchorage system. The preliminary results indicate that the high strength post-tensioned

strands are mostly anodic to the anchorage system when exposed to either grout bleed water or recharge

water such as that which may be experienced through leakage. The preliminary findings of the

investigation are presented along with the implications on existing structures and on future design and

materials selection for post-tensioning systems.


5.2 ACI Reports

5.2.1 Corrosion of Prestressing Steels (ACI 222.2R-01)

Author(s): ACI Committee 222

Publication: ACI 222.2R-01


Abstract/Summary: This report reflects the current understanding of corrosion of prestressing steels in

concrete. The report includes chapters that cover the various types of prestressing steel, including some

discussion on metallurgical differences. Deterioration mechanisms are discussed, including hydrogen

embrittlement and stress-corrosion cracking. Methods to protect prestressing steel against corrosion in

new construction are presented, along with a discussion of field performance of prestressed concrete

structures. Finally, field evaluation and remediation techniques are presented.

5.3 NCHRP Reports


Bridge Post-Tensioning Ducts using Rolling Stress Waves for Continuous Scanning

Author(s): Yajai Tinkey and Larry D. Olson

Publication: Final Report for Highway IDEA Project 102


Abstract/Summary: The objective of the research project is to develop reliable nondestructive near-

continuous scanning methods for condition assessment of the internal grout conditions inside bridge

ducts. Different sizes of ducts were included in this study as well as varying sizes of void defects. In

addition, detailed sensitivity studies of nondestructive grout defect detection with Impact Echo Scanning

of 8-four inch diameter ducts with constructed defects were the main research focus. Two specimens were

used in this research project. The first specimen used for this study was a large mock-up slab located at

the BAM facility in Berlin, Germany. The size of the slab is 32.8 x 13.1 ft(10 x 4 m) with a nominal

thickness of 11.8 inches (30 cm). The mock-up slab was constructed in2002 for the purpose of blind

studies of grout defect detection with different non-intrusive methods. Half of the mock-up slab includes

ducts with the diameters ranging from l.57 to 4.72inches (40to120mm). Concrete cover depths above the

ducts varied from 2.75 to 7.5 inches (70 to 190 mm). The other half of the slab includes different types of

internal voids and other simulated defects.

The research also included the first attempt to develop a complete stress wave scanner by adding another

rolling displacement transducer 8 inches (20 cm) in a line from the first rolling transducer. This additional

rolling transducer allows Spectral Analysis of Surface Wave tests for concrete quality/condition/velocity

to be performed at the same time as thickness/flaw detection tests are conducted with the Impact Echo

Scanning test. Improvements in software were implemented to support simultaneous analysis of data from

both tests.

A compete stress wave scanner was used to perform SASW and IE tests on the BAM mock-up slab. The

tests were performed in a line fashion parallel to the direction of the ducts every 5 cm. A total of 200 test

lines were performed to cover the whole slab area. Table I summarizes the grout defect size that can be

detected in ducts of different diameters and concrete covers. Reviews of Table I show that half size and

full size voids can be detected with the IE tests in 4.72 and 3.94 inches in diameter. Only full size voids

can be detected inside ducts with a diameter of 3.15 inches. However, once the concrete cover is 5.5


inches and higher, the IE results become intermittent and unreliable. In summary, it is easier to detect

grout defect in ducts with bigger diameters. In addition, the deeper the duct is inside the concrete, the

harder it is to detect grout defects with the IE tests.

Technical problems occurred during the use of the stress wave scanner with variability in contact

conditions between the second rolling transducer and the test surface in SASV/ tests. Consequently, data

from the second rolling transducer were intermittent. However, good data from the second rolling

transducer were still generally obtained. These data showed an approximate 11% reduction in surface

wave velocity at locations associating with grout void.


Bridges, Final Report Phase I: Technology Review

Author(s): Adrian T. Ciolko and Habib Tabatabai

Publication: NCHRP Web Document 23 (Project 10-53)

Publication Date: March 1999

Abstract/Summary: This report contains the findings of a study performed to determine whether a

practical and economical method for quantitative nondestructive condition evaluation of bonded

prestressing systems in highway bridges exists. The report provides a comprehensive summary of a global

technology review made to identify NDT methods developed in the time period commencing in 1990.

The noted NDT advances of the decade, which possessed some potential for assessing strand condition,

were characterized and evaluated based on technical, accuracy, operational, logistical, safety, and other

factors. The contents of this report will be of interest to bridge maintenance engineers, researchers, and

others concerned with assessing the condition of concrete bridges and the degree of strength and

serviceability impairment created by deteriorating prestressing systems.

The second specimen used in this study is a full scale U-Shaped bridge girder. The length of the girder is

100 ft. However, only the first 20 ft was included in this study. There were four empty steel ducts inside

each wall of the girder (a total of 8 ducts). The diameter of each duct is 4 inches. Several pieces of

Styrofoam were inserted inside the duct. The foam was positioned on the roof of the duct to simulate real

world grout defect. The size of the foam used ranged from as small as 16% duct perimeter lost or 60lo

depth lost to 84% perimeter lost or 94% depth lost (void). The use of 3D surface plotting of the IE

thickness results was helpful with interpretation and visualization of grout defects. A grout defect as small

as 20% perimeter lost or 11% depth lost in 4" duct was detected by the IE tests with the interpretation

using 3D surface plotting. The 3-D visualization with a color scale of the thickness change from normal

(fully grouted duct) to thicker (partial to full void) proved to be an important tool for imaging sound grout

versus partial to full void conditions for both the BAM and U-Shaped girder test specimens. The 3-D

color scales proved to indicate very good precision at indicating the size of the internal voids as reflected

by increasing thickness echo depths with increasing void size as reported herein. Such visualization of

Impact Echo Scanning results allows for much greater sensitivity and economical, near-continuous testing

of real-world bridge ducts.

The last part of the research project focused on the use of the Ultrasonic Pulse Echo test. A commercial

unit (Low Frequency Flaw Detector - A1220) was used to perform the UPE test. The UPE test was able to

detect the thickness of the wall where no ducts exist inside correctly. However, the UPE test was unable

to detect beyond the duct once the ducts are present. This is potentially because of debonding problem

between grout and the metal ducts which had occurred by the time of the UPE testing of the

comparatively old ducts. Thus, no information was gained from UPE tests on the internal grout conditions

in terms of the degree of the voiding in the duct.


5.4 Federal Highway Administration (FHWA)


Tensioned Bridges

Author(s): R.M. Salas, A.J. Schokker, J.S. West, J.E. Breen, and M.E. Kreger

Publication: Report No. FHWA/TX-04/0-1405-9

Publication Date: February 2004

Abstract/Summary: The effectiveness of cement grout in galvanized or polyethylene ducts, the most

widely used corrosion protection system for multistrand bonded post-tensioned concrete tendons, has

been under debate due to several reported examples of significant tendon corrosion damage. While

experience in the USA has been generally good, some foreign experience has been less than satisfactory.

This report is the last technical report from a comprehensive research program started in 1993 under

TxDOT Project 0-1405. The objectives were to examine the use of post-tensioning in bridge

substructures, identify durability concerns and existing technology, develop and carry out an experimental

testing program, and conclude with durability design guidelines.

Four experimental programs were developed: improved and high-performance grout studies, to develop

grout with desirable fresh properties to provide good corrosion protection to the prestressing strands; a

long-term macrocell corrosion test series, to investigate corrosion protection for internal tendons in

precast segmental construction; a long-term beam corrosion test series, to examine the effects of post-

tensioning on corrosion protection as affected by crack width; and, a long-term column corrosion test

series, to examine corrosion protection in vertical elements.

This report includes the final results after completion of exposure testing, performing comprehensive

autopsies and updating the durability design guidelines to reduce the corrosion risk of the post-tensioning

system.

After autopsies were performed, overall findings indicate negative durability effects due to the use of

mixed reinforcement, small concrete covers, galvanized steel ducts, and industry standard or heat-shrink

galvanized duct splices. The width of cracks was shown to have a direct negative effect on specimen

performance. Grout voids were found to be detrimental to the durability of both galvanized ducts and

strand. Relying on epoxy and galvanized bar coatings was also found inappropriate because of local

attack. On the other hand, very positive effects were found with the use of high performance concrete,

high-performance grouts, high post-tensioning levels, plastic ducts, and sound epoxy filling at the joints.

5.4.2 Improving Bridge Inspections

Author(s): Glenn A. Washer

Publication: FHWA Public Roads – Vol. 67 No. 3

Publication Date: Nov/Dec 2003

Abstract/Summary: N/A


System

Author(s): A. Ghorbanpoor, R. Borchelt, M. Edwards, and E. Abdel Salam

Publication: Report No. FHWA-RD-00-026


Publication Date: May 2000

Abstract/Summary: This report describes all aspects of a study to develop a nondestructive evaluation

(NDE) system based on the concept of magnetic flux leakage (MFL) to detect corrosion and fracture of

prestressing steel in pretensioned and post-tensioned

concrete bridge members. The basic methodology is based on introducing a direct-current magnetic field

in close proximity of the prestressing or post-tensioning steel and monitoring the variations of the field

due to loss of cross-sectional area of steel from corrosion or fracture.

The mechanical and electrical components of the MFL system include a lightweight structural frame that

supports the source of the required magnetic field and an array of sensors for measuring the magnetic

field variation. The frame also supports a series of mechanical and electrical components that facilitate the

operation of the system. Two strong permanent magnets provide the required magnetic field. A set of 10

Hall-effect sensors in the system measure the variations in the magnetic field due to the presence of flaws

in prestressing or post-tensioning steel. Software is developed to acquire and analyze the MFL data as

well as to control all hardware, including the mechanical and electrical components of the system. The

system is designed and fabricated to offer ease of use during the field operation. The operation of the

system includes attaching the structural frame of the system to a test beam and conducting the test by

controlling the frame and its components by a notebook computer from a remote site via wireless

communication.

Although most of the effort made during this study was associated with the development of the MFL

system, limited laboratory and field investigations were conducted to assess the capabilities and

limitations of the system. During both the laboratory and field investigations, it was demonstrated that the

installation and operation of the MFL system were successful. System installation on a test beam may be

accomplished easily and in a time period not longer than a few minutes. During the laboratory study, steel

prestressing strands with partial localized cross-sectional area losses from 7% to 71% were used as test

specimens. Also, prestressing strands with real corrosion were used for the same purpose. It was found

that the smallest flaw in a strand that could be detected had a 7% reduction in the cross-sectional area.

This capability was demonstrated for the strands that were placed at a distance of up to 128 nun (5 in)

from the magnet and sensor assembly of the system. A field demonstration was conducted that showed

that the installation and operation of the MFL system were successful.

It is recommended that additional laboratory and field investigations beyond this study be conducted with

the use of the new MFL system in order to fully evaluate its capabilities and limitations. This would also

facilitate the establishment of a more comprehensive database that can enhance the data interpretation

capability and the overall reliability of the system.

5.4.4 Demonstration of Dual-Band Infrared Thermal Imaging for Bridge Inspection

Author(s): Philip F. Durbin, Nancy K. Del Grande and Paul C. Schaich

Publication: FHWA Nondestructive Evaluation Research and Development Program


Abstract/Summary: Developing and implementing methods of effective bridge rehabilitation is a major

issue for the Federal Highway Administration (FHWA). The nation spends $5 billion annually to replace,

rehabilitate or construct new bridges. According to the National Bridge Inventory, over 100,OOO U.S.

bridges are structurally deficient. About 40,000 of these bridges have advanced deck deterioration. The

most common cause of serious deck deterioration is delamination Delaminations result when steel

reinforcements within the bridge deck corrode, creating gaps that separate the concrete into layers. A


reliable inspection technology, capable of identifying delaminations, would represent a powerful new tool

in bridge maintenance.

To date, most bridge inspections rely on human interpretation of surface visual features or chain dragging.

These methods are slow, disruptive, unreliable and raise serious safety concern. Infixed thermal imaging

detects subsurface delaminations and surface clutter, which is introduced by foreign material on the

roadway. Typically, foreign material which is not always evident on a video tape image, produces a

unique IR reflectance background unlike the thermal response of a subsurface delamination.

Lawrence Livermore National Laboratory (LLNL) uses dual-band infrared (DBIR) thermal imaging to

identify and remove nonthermal IR reflectance backgrounds from foreign material on the roadway.

DBXR methods improve the performance of IR thermal imaging by a factor of ten, compared to single-

band infrared (SBXR) methods. DBIR thermal imaging allows precise temperature measurement to

reliably locate bridge deck delaminations and remove wavelength-dependent emissivity variations due to

foreign material on the roadway.

We conducted a two-phase study to develop and demonstrate DBIR imaging for bridge deck inspection.

The first phase demonstrated the DBIR method on a surrogate bridge deck containing synthetic

delaminations. The second phase demonstrated the DBIR method at the Grass Valley Creek Bridges near

Redding CA. We designed and fielded a mobile DBIR bridge inspection laboratory and drove this self-

contained unit at limited highway speeds over 0.4 lane miles of bridge deck We demonstrated the power

of DBIR thermal image by removing the bridge deck clutter, which had unique spatial, spectral, thermal,

thermal inertia, emissivity and temporal responses, unlike the IR responses which characterize bridge

deck delaminations.

The LLNL precise thermal imaging method provides an enabling technology for rapid, reliable, bridge

deck inspections while minimizing fane closures. The LLNL method can indicate the fractional area of

the bridge that is delaminated as well as locate and characterize the damaged regions. This technique is

expected to help prioritize bridges for repair and then to direct the repairs to specific locations.

5.5 ASTM Standards

5.5.1 ASTM A 36 -12: Standard Specification for Carbon Structural Steel

Scope: This specification covers carbon steel shapes, plates, and bars of structural quality for use in

riveted, bolted, or welded construction of bridges and buildings, and for general structural purposes

5.5.2 ASTM A 53 -12: Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-

Coated, Welded and Seamless

Scope: This specification covers seamless and welded black and hot-dipped galvanized steel pipe in NPS to NPS 26

[DN 6 to DN 650] (Note 1), inclusive, with nominal wall thickness (Note 2) as given in Table X2.2 and Table X2.3.

It shall be permissible to furnish pipe having other dimensions provided that such pipe complies with all other

requirements of this specification. Supplementary requirements of an optional nature are provided and shall apply

only when specified by the purchaser.

5.5.3 ASTM A 240 -13: Standard Specification for Chromium and Chromium-Nickel Stainless

Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications

Scope: This specification covers chromium, chromium-nickel, and chromium-manganese-nickel stainless

steel plate, sheet, and strip for pressure vessels and for general applications.


5.5.4 ASTM A 416 -12: Standard Specification for Steel Strand, Uncoated Seven-Wire for

Prestressed Concrete

Scope: This specification covers two types and two grades of seven-wire, uncoated steel strand for use

prestressed concrete construction. The two types of strand are low-relaxation and stress-relieved (normal-

relaxation). Low-relaxation strand is to be regarded as the standard type. Stress-relieved (normal-

relaxation) strand will not be furnished unless specifically ordered. Grade 250 [1725] and Grade 270

[1860] have minimum tensile strengths of 250 ksi [1725 MPa] and 270 ksi [1860MPa], respectively,

based on the nominal area of the strand.

5.5.5 ASTM A 653 -13: Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-

Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process

Scope: This specification covers steel sheet, zinc-coated (galvanized) or zinc-iron alloy-coated

(galvannealed) by the hotdip process in coils and cut lengths.

5.5.6 ASTM A 722 -12: Standard Specification for Uncoated High-Strength Steel Bars for

Prestressing Concrete

Scope: This specification covers uncoated high-strength steel bars intended for use in pretensioned and

post-tensioned prestressed concrete construction or in prestressed ground anchors. Bars are of a minimum

ultimate tensile strength level of 1035 MPa (150 000 psi).

5.5.7 ASTM C876 – 09: Standard Test Method for Corrosion Potentials of Uncoated Reinforcing

Steel in Concrete

Scope: This test method covers the estimation of the electrical corrosion potential of uncoated reinforcing

steel in field and laboratory concrete, for the purpose of determining the corrosion activity of the

reinforcing steel.

5.5.8 ASTM D 1693 -13: Standard Test Method for Environmental Stress-Cracking of Ethylene

Plastics

Scope: This test method covers the determination of the susceptibility of ethylene plastics, as defined in

TerminologyD883, to environmental stress-cracking when subjected to the conditions herein specified.

Under certain conditions of stress and in the presence of environments such as soaps, wetting agents, oils,

or detergents, ethylene plastics may exhibit mechanical failure by cracking.

5.5.9 ASTM D4101-14: Standard Specification for Polypropylene Injection and Extrusion

Materials

Scope: This specification covers polypropylene materials suitable for injection molding and extrusion.

Polymers consist of homopolymer, copolymers, and elastomer compounded with or without the addition

of impact modifiers (ethylene-propylene rubber, polyisobutylene rubber, and butyl rubber), colorants,

stabilizers, lubricants, or reinforcements.

5.5.10 ASTM F 405 – 13: Standard Specification for Corrugated Polyethylene (PE) Pipe and

Fittings

Scope: This specification covers requirements and test methods for materials, marking dimensions,

workmanship, elongation, brittleness, pipe stiffness, and perforations for corrugated polyethylene (PE)

pipe and fittings in nominal sizes of 3 to 6 in., inclusive.


5.6 Other

5.6.1 Post-tensioned Multistrand Anchorage Capacity Deterioration Due to Corrosion: John Day

Lock Project

Author(s): Robert Ebeling, Richard Haskins, David Scofield, John Hite and Ralph Strom

Publication: U.S. Army Engineer Research and Development


Abstract/Summary: A Research Work Unit (WU) has been initiated in the Navigation Systems Research

Program to investigate post-tensioned multistrand anchors. A significant number of COE projects have

installed multistrand high-capacity post-tensioned foundation anchors over the last three decades. These

anchors are embedded and access is limited to the top anchor head for inspection purposes. Due to the

evolution of corrosion protection criteria for ground anchors, the early installed anchors may have

inadequate corrosion protection that does not meet current corrosion protection standards. The older

anchors are approaching the end of their design life and are showing various degrees of deterioration,

corrosion, and broken strands. Current load capacity and remaining life of the anchors are unknown. One

procedure used to test post-tensioned tendons involves lift-off tests, which are both dangerous and

expensive. The applicability of lift-off testing to most existing ground anchorage is severely restricted to

the very few existing ground anchors that were not grouted for corrosion protection along the free length

of the anchor and which also have special provisions for the connection of the jacking equipment to the

anchor head. This severely restricts the practical use of lift-off testing of existing ground anchorage as a

viable testing procedure. Additionally, testing deteriorated anchors has been avoided in the past because

of greater danger of breaking anchors. To meet reliability analysis required for major rehabilitation

studies, estimates of load capacity, rate of decrease, and remaining life are required.

5.6.2 Guide Specification for Grouted Post-Tensioning

Author(s): PTI/ASBI

Publication: PTI/ASBI M50.3-12


Abstract/Summary: The purpose of this document is to establish a unified, nationally recognized

specification for grouted post-tensioning. It is meant to apply to buildings, bridges, storage structures, and

other structures using grouted post-tensioning tendons except as follows: Stay cables and rock anchors

which are already covered by other PTI documents.


Bridges

Author(s): Bernhard Elsener and Markus Buchler

Publication: Swiss Association of Road and Transportation Experts (VSS)

Publication Date: July 2011

Abstract/Summary: Tendons contribute decisively to the serviceability, safety and durability of pre

stressed concrete structures. Despite a generally good long-term behaviour, some corrosion problems and

rare collapses are documented; thus industry, designers and owners were looking for a more durable

solution. Electrically isolated tendons with plastic ducts for internal grouted post-tensioning were

developed about 20 years ago. The progress achieved in the meantime and the implementation of this new

approach into the framework of fib recommendation is also based on previous research reports and the

first Swiss guideline “Measures to ensure the durability of post-tensioning tendons in bridges”.


Since 1995 an increasing number of bridges, flyovers and viaducts have been constructed with electrically

isolated tendons (EIT) according to the fib protection level 3 (PL3). Quality control and long-term

monitoring is performed with electrical resistance measurements between tendon and reinforcement.

Experience with practical application showed some difficulties in reaching the acceptance value specified

in the guideline, further the time of measurement of 28 days was found not to be adequate for practice.

Together with the revision of the Swiss guideline the present research project was undertaken.

The research report establishes the scientific and technical background for the revision of the guideline,

especially with regard to the limit of the electrical resistance specified and its evolution with time after

grouting. The project shows that the anchor head might not be “a priori” a systematic defect in the

electrical isolation, problems can arise in application on site. Larger defects or short-circuits in the

electrical isolation can be located with magnetic flux measurements from the concrete surface.

Finally the research report documents some typical application of electrically isolated tendons on bridges

and viaducts in Switzerland. Thanks to the international collaboration in the framework of COST 534 a

large number of results from prefabricated segments of viaducts of the new high-speed lines of the Italian

railways are available. Both type of structures show the importance of a careful quality control system,

including the design process, material specification and training of the workers on site. The most

important conclusion for practice is that also tendons that did not reach the 28-day acceptance criteria can

be considered as better protected against corrosion and can be included in the long-term monitoring

strategy.


Author(s): Swiss Federal Roads Authority ASTRA

Publication: ASTRA 12 0010


Abstract/Summary:

Objective

The Codes SIA 262:2003 and SIA 262/1:2003 contain the fundamental information for prestressing

systems. The present Guideline shall serve as additional reference for the Code SIA 262/1.

Since the introduction of prestressing systems, much data has been accumulated, especially in areas

pertaining to construction, testing and monitoring. This data is presented in this present Guideline as

specifications and supplements to the above mentioned codes.

Code Basis

The Code SIA 262:2003 forms the basis. The most significant details on durability issue can be found in

the following clauses of the code:

Basic Principles – Durability

Construction Material – Prestressing systems – Durability

Structural Analysis and Dimensioning – Verification of Structural Safety – Fatigue

Construction – Processing of prestressing steel and prestressing tendons

Construction – Prestressing

Scope


The present Guideline is applicable for bonded post-tensioning tendons and is applicable to all

Government financed (full or in part) road construction projects, as well as in other construction projects

within the responsibility of SBB Ltd.

Application and Revision of Guideline

The present Guideline “Measures to ensure durability of post-tensioning tendons in structures” is

applicable from 1st September 2007. The list of revisions can be found in page 49.


6 Grout Condition

6.1 Application of Gamma Ray Scattering Technique for Non-Destructive Evaluation of Voids in Concrete

Author(s): P. Priyada, R. Ramar and Shivaramu

Publication: Applied Radiation and Isotopes


Abstract/Summary: This paper describes application of the gamma ray scattering technique for NDE of

concrete voids. A novel nonlinear extrapolation method is employed to correct for self-absorption and

multiple scattered intensities. The attenuation data obtained from transmission method is employed for

reconstruction of scattered images and the results show a good agreement in size and position of the voids

with good spatial resolution. Intercomparison of the results of scattering and transmission techniques

shows a good agreement in the position of the voids.

6.2 Quantitative Evaluation of Contactless Impact Echo for Non-Destructive Assessment of Void Detection within Tendon Ducts

Author(s): Franck Schoefs, Odile Abraham and John S. Popovics

Publication: Construction and Building Materials


Abstract/Summary: Owners of pre-stressed concrete structures must realize preventive maintenance in

order to maintain structural safety and limit economic losses. Detection voids in tendon ducts, where

corrosion could occur, is key in this effort. This paper focuses on the quantification of the performance of

the impact echo method (IEM), applied using a new laser interferometer contactless robot, for duct void

detection in a reinforced concrete wall. We show first the influence of the wall stiffness on IEM

(resonance) frequency. We use a probabilistic modeling to evaluate the IEM. We illustrate a way for

accounting on-site uncertainties of NDT measurements.

6.3 Non-Destructive Testing Methods to Identify Voids in External Post-Tensioned Tendons

Author(s): Seok Been Im and Stefan Hurlebaus

Publication: KSCE Journal of Civil Engineering


Abstract/Summary: A considerable number of Post-Tensioned (PT) bridges have been constructed

because PT systems enable them to carry significant traffic loads and have an aesthetical structure.

However, strand corrosion has been a long-standing issue because it may lead to the failure of tendons

and the deterioration of structural performance. The corrosion typically occurs in voided locations with

exposed strands; thus, the inspection of voids in external PT tendons is important and necessary in order

to protect strands before corrosion occurs. Based on literature review, several Non-Destructive Testing

(NDT) methods are compared for effectiveness of identifying voids in external PT tendons, and the

Impact Echo (IE), ultrasonic, and sounding inspection methods are then selected and assessed using

small-scale and mock-up specimens. From the experimental results, the wave-based inspection methods,

IE and ultrasonic methods, are difficult to apply in the field because the imperfect bonds between ducts


and grouts obstruct the transmission of waves. However, the sounding inspection method is not affected

by the discontinuities and successfully identifies voids in test specimens. Thus, the sounding inspection

can be an effective tool for identifying voids because of its easy application in the field.

6.4 Detecting Voids in Grouted Tendon Ducts of Post-Tensioned Concrete Structures Using Three Different Methods

Author(s): Xianyan Zhou, Zhifeng Wang and Dahai Zhang

Publication: CICE Conference Proceedings


Abstract/Summary: An increasing amount of post-tensioned concrete structures are used widely in China,

and the grout condition inside tendon ducts attracts people’s attentions. In order to assess internal grout

quality of grouted tendon ducts in post-tensioned concrete structures accurately, and guarantee the

lifetime of prestressed concrete structures, three different nondestructive testing (NDT) methods, Impact-

Echo Scanning (IES), Ultrasonic Transmission Method (UTM), and Ground Penetrating Radar (GPR),

have been employed to investigate the grouting defects qualitatively and quantitatively. A series of indoor

model testing, in which different types of soft foams were placed in tendon ducts to simulate the flaws

during in-situ construction, were carried out. Results show that the IES method can do a better job in

situation where the walls of the ducts are metal rather than plastic. In contrast, and GPR technology may

achieve a better performance in detecting voids within plastic ducts.

6.5 Concrete Bridge Condition Assessment with Impact Echo Scanning

Author(s): Larry D. Olson, Yajai Tinkey, and Patrick Miller

Publication: ASCE Geotechnical Special Publication No. 218


Abstract/Summary: The paper discusses the use of the Impact Echo Scanning nondestructive evaluation

(NDE) technique for quality assurance and condition assessment of the grouting of posttensioned bridge

ducts and concrete bridge decks. Both research and case history results are presented. Impact Echo

Scanning was first done for detecting void/grout conditions in post-tensioned ducts. Recent research and

development of a Bridge Deck Scanner using the Impact Echo Scanning technique is compared with

Ground Penetrating Radar and Acoustic Chain Drag Sounding to identify concrete delaminations due to

rebar corrosion. The research on both Impact Echo Scanning applications was sponsored by the US

National Cooperative Highway Research Innovations Deserving Exploratory Analysis program. The use

of the Bridge Deck Scanner to evaluate concrete deck conditions in terms of void/honeycomb and

thickness is also discussed.

6.6 Inspection of Voids in External Tendons of Posttensioned Bridges

Author(s): Seok Been Im, Stefan Hurlebaus, and David Trejo

Publication: Journal of the Transportation Research Board


Abstract/Summary: Segmental posttensioned bridges are major structures that carry significant

traffic. Recent investigations of these bridges identified voids in their ducts. The exposed strands at these

void locations can undergo corrosion. The corrosion of strands may lead to the failure of tendons. As

such, an effective inspection process for identifying these voids is needed. From a literature review,

several nondestructive testing methods are compared for applicability to void inspection in external

tendons. The impact echo, ultrasonic pulse velocity, and sounding inspection methods are then selected


and assessed for further preliminary testing. The sounding inspection method is further assessed for its

effectiveness in identifying voids in a full-scale, external tendon system. The results indicate that

sounding inspection slightly underestimates the size of the voids. However, the inspected size and

locations of voids have a close correlation with actual voids in ducts. Thus, sounding inspection can be

an effective tool for identifying voids because of its easy application in the field.

6.7 Modified SIBIE Procedure for Ungrouted Tendon Ducts Applied to Scanning Impact-Echo

Author(s): Ninel Alver and Herbert Wiggenhauser



Abstract/Summary: The impact-echo method has been successfully applied to identify defects inside

concrete. In addition, to detect ungrouted tendon ducts in a large concrete slab, a scanning impact-echo

technique is developed. However, since resonant frequencies in the spectrum responsible for the travel

paths via defects are only taken into account, the method could lead to erroneous results due to

complicated spectra obtained in the tests. Consequently, Stack Imaging of spectral amplitudes Based on

Impact-Echo (SIBIE) procedure has been developed to improve the data interpretation. Conventionally,

SIBIE is applied to a single measurement data and a point information of defects is obtained at the area,

where the impact test is performed. In this study, SIBIE is applied to scanning impact-echo data.

Locations of ungrouted tendon ducts embedded in a large concrete specimen are investigated. In order to

visualize the whole cross-section tested, the SIBIE analysis is modified, introducing an elliptical

integration mode. It is demonstrated that ungrouted tendon ducts are successfully located by the modified

SIBIE analysis, whereas results of the conventional B-scan analysis are not so good as the modified

SIBIE analysis.

6.8 On-Site Measurement of Delamination and Surface Crack in Concrete Structure by Visualized NDT

Author(s): Kimitoshi Matsuyama, Masahiko Yamada and Masayasu Ohtsu



Abstract/Summary: On-site measurement in concrete piers of a highway structure was conducted by

visualized non-destructive testing (NDT). First, the radar technique was applied to identify delaminated

areas between prestressed concrete (PC) panels covered and original concrete piers. By applying the

impact-echo method, Stack Imaging of spectral amplitudes Based on Impact-Echo (SIBIE) procedure is

developed as an imaging technique, which can visually display a cross-sectional view based on the

impact-echo data. In order to identify voids in delaminated areas and depths of surface cracks, then the

impact-echo tests were conducted. From these results, a superior applicability of the SIBIE procedure to

on-site measurement is demonstrated in existing concrete structures.

6.9 Identification of Ungrouted Tendon Duct in Prestressed Concrete by SIBIE

Author(s): Masayasu Ohtsu, Masahiko Yamada, and Yoko Nakai

Publication: 2009 NDTCE Conference Proceedings

Publication Date: June-July 2009

Abstract/Summary: In the impact-echo method, the presence and the locations of defects in concrete are


estimated from identifying peak frequencies in the frequency spectra, which are responsible for the

resonance due to time-of-flight from the defects. In practical applications, however, spectra obtained

include so many peak frequencies that it is fairly difficult to identify the defects correctly. SIBIE (Stack

Imaging of spectral amplitudes Based on Impact Echo) procedure is developed as an imaging technique

applied to the impact-echo, where defects in concrete are identified visually at the cross-section. In this

study, the SIBIE procedure is applied to identify ungrouted post-tensioning ducts in prestressed concrete.

Concrete slabs containing an ungrouted duct, a partially-grouted duct, and a fully-grouted duct of metal

and polyethylene sheaths were tested. It is demonstrated that the defect can be identified with reasonable

accuracy by SIBIE in all the cases tested.

6.10 Estimation of Surface-Crack Depth in Concrete by Scanning SIBIE Procedure

Author(s): Masanobu Tokai, Taro Ohkubo and Masayasu Ohtsu

Publication: NDTCE’09 Non-Destructive Testing in Civil Engineering


Abstract/Summary: For maintenance of concrete structures, the depth of a surface crack is an important

factor to be estimated in inspection. As the nondestructive testing (NDT), the impact-echo method has

been extensively applied to the concrete structures. Wave motions resulting from a short duration

mechanical impact are recorded, and peak frequencies of their spectra are identified. However, frequency

spectra cannot always be interpreted successfully as many peaks are often observed. Thus, an imaging

procedure is developed, which is named as SIBIE (Stack Imaging of spectral amplitudes Based on Impact

Echo). The procedure provides two dimensional (2-D) image of a specimen cross-section.

In the present paper, SIBIE procedure is applied to estimating a surface-crack depth of actual cracks

generated by a bending test. An accelerometer was employed for the detection. Because the cracks were

created in a zigzag manner, a scanning procedure was applied in addition to the conventional one-point

detection. It is demonstrated that curved extension of the surface crack is reasonably estimated by a

scanning procedure. Thus, it is confirmed that the SIBIE analysis is a promising technique for the crack-

depth evaluation in concrete.

6.11 Imaging of Internal Cracks in Concrete Structures Using the Surface Rendering Technique

Author(s): Po-Liang Yeh and Pei-Ling Liu

Publication: NDT&E International


Abstract/Summary: The impact echo method is effective in the inspection of concrete defects. If the test

area is large and many tests are performed, it is difficult to get a picture of the concrete interior by

examining a series of test spectra. In order to provide the engineers with a more direct way of detecting

the defects in the structure, this study proposes a three-dimensional (3D) imaging method to depict the

internal cracks in concrete structures. To acquire the test data, a mesh is drawn on the surface of the

concrete. Then, impact echo tests are performed at the grids. The recorded signals are processed to obtain

the depth spectra of the concrete. Finally, the surface rendering technique is adopted to construct the 3D

image of the concrete interior. Both numerical simulations and model tests are used to verify the proposed

imaging method. It is seen that surface rendering technique can be used to show the internal cracks in the

concrete specimens successfully.


6.12 Ultrasonic Imaging Methods for Investigation of Post-tensioned Concrete Structures: A Study of Interfaces at Artificial Grouting and Its Verification

Author(s): Martin Krause, Boris Milmann, Frank Mielentz, Doreen Streicher, Bernhard Redmer, Klaus

Mayer, K.J. Langenberg and Martin Schickert

Publication: Journal of Nondestructive Evaluation


Abstract/Summary: This paper presents the progress of successful location of grouting faults in tendon

ducts with ultrasonic imaging. The examples were obtained in the research group FOR 384 funded by

DFG (German Research Foundation). The co-operation of experimental research and modeling allowed

imaging and identification of grouted and ungrouted areas of tendon ducts (including strands) in a large

test specimen (40 m2). In addition to the criteria for indicating grouting faults in post-tensioned ducts

known until now the phase evaluation of reflected ultrasonic pulses is described. Experiments and

modeling of wave propagation are presented for reflections at metal plates in concrete (thickness range

0.5 mm to 40 mm) and for tendon ducts including strands. The main part of the progress was achieved by

automated measurements using dry contact transducers, 3D-SAFT reconstruction including phase

evaluation and modeling considering wave propagation for typical elastic parameters and exact

experimental site conditions. The results for shear waves as well as for pressure waves are compared in

the frequency range from 50 kHz to 120 kHz.

6.13 Imaging Concrete Structures Using Air-Coupled Impact-Echo

Author(s): Jinying Zhu and John S. Popovics

Publication: ASCE Journal of Engineering Mechanics


Abstract/Summary: In this paper, air-coupled impact-echo is successfully applied for nondestructive

evaluation of concrete. The air-coupled sensor is a small _6.3 mm diameter_ measurement microphone

located several centimeters above the top surface of the concrete being evaluated. Unwanted ambient

acoustic noise is attenuated by a specially designed sound insulation enclosure. Test results show that air-

coupled sensors are effective for impact-echo when appropriate impactors are used. Impact-echo data

obtained by air-coupled sensors are equivalent to those obtained by conventional contact sensors. Test

results from concrete slabs containing artificial delaminations and voids are reported, where an air-

coupled impact-echo scan is conducted over the entire slab area. Defects are located in the generated

twodimensional contour image. The areal size of defects are accurately determined when the

measurement point spacing in the scan is smaller than half of the expected defect size. Test results from

air-coupled impact-echo scans carried out over internal metal and plastic ducts within another concrete

slab are also reported. The goal of the experiment is to investigate the grouting condition inside the ducts.

Impact-echo line scan images differentiate poorly grouted sections from the well-grouted sections within

the metal duct.

6.14 Impact-Echo Scanning Evaluation of Grout/Void Conditions Inside Bridge Post-Tensioning Ducts for Tendon Corrosion Mitigation


Publication: Concrete Repair Bulletin

Publication Date: May/June 2007

Abstract/Summary: Post-tensioned (PT) systems have been widely used for bridge transportation systems

since the late 1950s. If a good quality control plan is not implemented, however, there is a strong


possibility that during construction the ducts may not be fully grouted. This results in voids in some areas

and the associated lack of cover for PT tendons that increase their risk of corrosion. Thus, over the long-

term, water can enter the tendon ducts in the void areas, resulting in corrosion of the tendon. The collapse

of the Brickton Meadows Footbridge in Hampshire, UK, in 1967 is the first serious case of corrosion of

tendons leading to a major catastrophe. 1 In 1985, the collapse of the Ynys-y-Gwas Bridge, a precast

segmental PT bridge in Wales, was attributed to the corrosion of the internal prestressing tendons at

mortar joints between segments.1,2 Corrosion-related failures of PT tendons have been found in several

major segmental bridges such as the Niles Channel Bridge near Key West, FL, in 1999 and Midway

Bridge near Destin, FL, in 2000.3 In addition to actual failures, corrosion damage was found in many PT

bridge ducts in bridges still in use in Florida and on the East Coast.4 Impact-Echo (IE) scanning research

and consulting results for grout/void in PT ducts are discussed in the following.

6.15 Impact-Echo Scanning for Grout Void Detection in Post-tensioned Bridge Ducts - Findings from a Research Project and a Case History


Publication: 2007 Structures Congress Proceedings


Abstract/Summary: This paper presents the findings from a research project funded by the NCHRP –

IDEA Program. This paper discusses the experimental results from the studies which involved a defect

sensitivity study of an Impact-Echo (IE) Scanner to detect and image discontinuities in post-tensioned

ducts of a mockup U-shaped bridge girder and a mockup slab. Different sizes of ducts were included in

this study as well as varying sizes of void defects. Detailed sensitivity study of nondestructive grout

defect detection with Impact-Echo Scanning of 8-four inch diameter ducts with constructed defects was

the main focus in this study. Comparisons of the IE defect interpretation and the actual design conditions

of the ducts inside the bridge girder/slab are presented. The IE results are presented in a three-dimensional

fashion using thickness surface plots to provide improved visualization and interpretation of the internal

grout to void defect conditions inside the ducts of the girder. The Impact-Echo tests were performed with

a Scanner which greatly facilitates the Impact-Echo test process by allowing for rapid, near continuous

testing and true “scanning” capabilities to test concrete structures. The paper summarizes the general

background of the Impact-Echo technique and the Impact-Echo Scanner. Descriptions of two mock-up

specimens used in the experiment and the discussion of the results from the Impact- Echo Scanner are

presented herein. Finally, a case study using an Impact Echo Scanner to locate grout voids inside the

Orwell Bridge in UK is included in this paper.

6.16 Sensitivity Studies of Grout Defects in Posttensioned Bridge Ducts Using Impact Echo Scanning Method




Abstract/Summary: Findings are presented from an NCHRP Innovations Deserving Exploratory Analysis

Program project, Nondestructive Evaluation Method for Determination of Internal Grout Conditions

Inside Bridge Posttensioning Ducts Using Rolling Stress Waves for Continuous Scanning. The study

involved a defect sensitivity study of an impact echo scanner to detect and image discontinuities in

posttensioned ducts of a mock-up U-shaped bridge girder and a mock-up slab. Various sizes of ducts were

included in this study as well as various sizes of void defects. A detailed sensitivity study of

nondestructive grout defect detection with impact echo scanning of eight ducts 4 in. in diameter with

constructed defects was the main focus in the study. Comparisons of the impact echo defect interpretation


and the actual design conditions of the ducts inside the bridge girder or slab are presented. The impact

echo results are presented threedimensionally by using thickness surface plots to provide improved

visualization and interpretation of the internal grout to void defect conditions inside the ducts of the

girder. The impact echo tests were performed with a scanner, which greatly facilitates the impact echo test

process by allowing for rapid, near continuous testing and true scanning capabilities to test concrete

structures. The general background of the impact echo technique and the impact echo scanner is

summarized. Descriptions of two mock-up specimens used in the experiment and the discussion of the

results from the impact echo scanner are presented.

6.17 Imaging of Ungrouted Tendon Ducts in Prestressed Concrete by Improved SIBIE

Author(s): Ninel Ata, Shinichi Mihara and Masayasy Ohtsu

Publication: NDT&E International


Abstract/Summary: The impact-echo method has been extensively applied to nondestructive evaluation of

defects in concrete structures. The presence and the location of defects in concrete are estimated by

identifying peak frequencies in the frequency spectra. To detect ungrouted tendon ducts, the method is

known to be available. However, because post-tensioning prestressed concrete members usually have thin

web portions, spectra obtained could include many peak frequencies. As a result, it is often problematic to

select appropriate peak frequencies associated with the presence of ungrouted ducts. Stack imaging of

spectral amplitudes based on impact-echo (SIBIE) is developed, in order to improve the impact-echo and

to visually identify the locations of such reflectors as voids and defects. In the present paper, SIBIE is

successfully applied to identify ungrouted metal and plastic sheaths at the hunch portion of a prestressed

concrete beam. Two dimensional dynamic BEM analysis is performed to investigate the relations between

peak frequencies and locations of reflectors. At the peak frequencies in the spectra, locations of stress

concentration are correlated with the response modes.


Author(s): Doreen Streicher, Daniel Algernon, Jens Wostmann, Matthias Behrens, and H. Wiggenhauser

Publication: Proceedings of the 9th European Conference on NDT


Abstract/Summary: Test problems for NDE of post-tensioned concrete bridges are to determine the

thickness of concrete, to locate metallic tendon ducts and reinforcement bars as well as to determine the

grouting condition in tendon ducts. Impulse-radar, ultrasonic echo and impact-echo are applied in

combination within a scanning system. For applying the imaging echo methods it is necessary to take data

in a two dimensional, rather dense grid (mesh width between 2 cm and 5 cm). Therefore time and

personnel requirements for manual measurements are extensive. In order to enhance the efficiency of the

non-destructive measurements several automated systems were developed at BAM. Applying those

systems, sensors for different methods can be automatically moved in a pre-selected grid width. Using

these systems large measurement areas up to 4 m x 10 m were investigated on several post-tensioned

concrete bridges. Automated measurements were carried out also in areas with limited accessibility (e.g.

box girders). Special data processing and data imaging is used for a more detailed interpretation of the

results. Constituents of the data processing are 3D-reconstruction and fusion of data sets, derived from

measurements with different methods or configurations. The detected reflectors and scatters of the

acoustic and electromagnetic measurements can be visualized in slices and movies. From radar data sets,

measured in perpendicular directions of antenna polarization, images of the perpendicularly arranged bars

of a reinforcement layer can be generated. The results of 3D-reconstructions of ultrasonic echo data allow


to recognize tendon ducts up to a measurement depth of approximately 55 cm. In summary the

combination of the electromagnetic and acoustic methods allows improving and simplifying the

interpretability of data.

6.19 Impact Echo Scanning for Discontinuity Detection and Imaging in Posttensioned Concrete Bridges and Other Structures

Author(s): Y. Tinkey, L.D. Olson and H. Wiggenhauser

Publication: Materials Evaluation

Publication Date: January 2005

Abstract/Summary: This paper focuses on experimental results from two scanning impact echo systems

on the internal condition of posttensioned ducts. The first system uses an impact echo head attached to an

X/Y scanner and the second system is a rolling impact echo scanning system. The experimental tests

were performed by two different research agencies and comparisons of the blind interpretation and the

actual design conditions of the posttensioned ducts and slab are included herein. Background of the

impact echo technique and its implementation with a rolling scanning transducer are discussed in the

paper. The impact echo technique is generally used to either determine the internal condition of concrete

structures or to measure the thickness of concrete members. The rolling transducer is the impact echo

scanner expedites the test process by allowing for rapid, near continuous testing. The results from the

rolling impact scanning system are presented in a three dimensional fashion to provide better

interpretation of the internal conditions of the ducts.

In these studies, the impact echo results from both research agencies show good agreement in correctly

identifying grouting discontinuities in tendon ducts. Discontinuities of grout in bridge ducts are located

based on an indirect indication of a void due to an apparent impact increase in bridge wall/slab thickness

that actually reflects the lower resonant echo frequency due to the decreased stiffness associated with the

duct void. No direct reflection from the duct with grouting discontinuities was observed in these

experiments.

6.20 Complementary Application of Radar, Impact-Echo and Ultrasonics for Testing Concrete Structures and Metallic Tendon Ducts

Author(s): Christiane Maierhofer, Martin Krause, Frank Mielentz, Doreen Streicher, Boris Milmann,

Andre Gardei, Christoph Kohl and Herbert Wiggenhauser



Abstract/Summary: Nondestructive testing of concrete structures plays an increasing role in civil

engineering, although until now the full potential of such techniques has not been tapped. For

posttensioned structures, the investigation of tendon ducts is one of the most essential testing problems.

The location of tendon ducts, the determination of concrete cover and, especially, the detection and

quantification of ungrouted areas inside the ducts are the relevant questions. Recent developments and

opportunities of radar, impact-echo, and ultrasonics for the investigation of tendon ducts are presented.

Although the obtained results on positioning and concrete cover determination are sufficient, the location

of ungrouted areas is still a matter of research. Thus, new approaches for this testing problem have to be

considered. Additionally, the combined use of complementary techniques offers a high potential to

increase the reliability of results. Data will be displayed on the combined application of acoustic and

electromagnetic impulse-echo methods and of data fusion related to the investigation of tendon ducts.


6.21 Contribution of Capacitance Probes for Nondestructive Inspection of External Post-Tensioned Ducts

Author(s): J. Iaquinta

Publication: 16th World Conference on NDT


Abstract/Summary: In bridges, external post-tension habitually comes as cables placed in ducts for which

the residual internal space is (imperfectly) filled with a fluid cement grout. Detecting the problems of

injection is not practicable visually from the outside, and no effective auscultation tools were found yet. A

recent laboratory experiment established that capacitance probes can be employed, but the main difficulty

is to provide a correct interpretation of the measurements in terms of deterioration of the coating, along

with the occurrence of water or grout voids. In order to understand if the presence of the cable itself can

disturb the diagnosis in such proportions that any inspection is destined to failure, the subject was tackled

here from a numerical point of view. It is shown that the capacitance probe is sensitive to the location of

the cable, but that it is still possible to distinguish typical defects present at low depth. This result is

confirmed, from a qualitative point of view, by tests performed with an actual probe.

6.22 Ultrasonic Imaging of Concrete Elements Using Reconstruction by Synthetic Aperture Focusing Technique

Author(s): Martin Schickert, Martin Krause, and Wolfgang Muller

Publication: ASCE Journal of Materials in Civil Engineering


Abstract/Summary: Ultrasonic reconstruction by the synthetic aperture focusing technique ~SAFT! has a

great potential to image concrete elements and detect embedded objects. Its algorithm focuses ultrasonic

signals received at many aperture points by coherent superposition, yielding a high-resolution image of

the region of interest. Using this approach, several problems caused by the strongly inhomogeneous

structure of concrete are diminished, where scattering of transmitted pulses leads to disturbing phenomena

such as attenuation and structural noise. This contribution is intended to review the work of the writers on

the application of SAFT reconstruction to concrete testing. First, consequences of scattering of ultrasonic

waves in concrete are qualitatively explained. Then the use of SAFT is discussed in comparison to

traditional A-scan and B-scan techniques. Different reconstruction algorithms and implementations are

presented for one-, two-, and three-dimensional SAFT. Pulse-echo measurement systems are described,

which are able to acquire large sets of data on linear and planar apertures employing single transducer,

transducer array, and scanning laser Doppler vibrometer arrangements. To illustrate the application of the

SAFT techniques, examples from laboratory and field experiments are described comprising imaging of

back walls, tendon ducts containing faults, layers, and reinforcement in concrete elements.

6.23 Ultrasonic Guided Waves for Inspection of Grouted Tendons and Bolts

Author(s): M. D. Beard, M. J. S. Lowe, and P. Cawley



Abstract/Summary: There is currently a need to improve the nondestructive testing techniques that are

used to inspect grouted steel reinforcing tendons, anchors, and rock bolts for corrosion and fracture. A

method of inspection using guided ultrasonic waves has been proposed, which uses a pulse-echo

technique carried out from the free end of the structure. The maximum inspection range is determined by

the amount of attenuation that the wave experiences as a result of leakage into the embedding material


and material losses. However, previous work has identified high frequency modes that have low

attenuation and so increase the inspection range. Research has been carried out with a focus on the

inspection of the posttensioning tendons used to reinforce concrete. The research presented in this paper

uses experimental techniques to measure the attenuation in short lengths of grouted tendons, to evaluate

the reflection coefficient of the modes from different geometry breaks, and to assess the impact of tendon

curvature. The outcome of this research shows that the inspection range for tendons is limited, but the

outlook for the inspection of the larger diameter grouted bolts and rebars that are used in the construction

industry is promising. Considerable success has already been achieved in the testing of epoxy bonded

rock bolts using this method.

6.24 Guidance on the use of NDE on Voided Post-Tensioned Concrete Bridge Beams using Impact Echo

Author(s): M. Clark, J. Halliday, J. Watson, and M.C. Forde

Publication: 82nd

TRB Meeting Proceedings


Abstract/Summary: Guidance is given on the use of Impact Echo (IE) testing, which can be useful in

detecting voids in grout in post-tensioned tendon ducts in prestressed concrete bridge elements. It is well

known that voids in grouted ducts can lead to ingress of water and to corrosion of the tendon wires. Voids

can also reduce the integrity of the post-tensioned member in that fractured wires are not encased in grout

and cannot therefore rebond themselves either side of the fracture. This paper should assist the bridge

maintenance engineer in preparing a survey of posttensioned members to establish the likelihood of there

being voids in the grouted ducts.

6.25 Use of the MegascanTM Imaging Process in Inspection Systems for Post-Tensioned Bridges and Other Major Structures

Author(s): Kevin Brown and John St Leger

Publication: International Symposium on Nondestructive Testing


Abstract/Summary: The safety of post-tensioned structures is reliant on the integrity of steel tendons

located in ducts within the main structural elements. This paper describes the key role MegaScanTM

Imaging currently plays in the inspection system.

6.26 Comparison of NDT Techniques on a Post-Tensioned Beam Before its Autopsy

Author(s): X. Derobert, C Aubagnac and O. Abraham

Publication: Journal of NDT & E International


Abstract/Summary: Following the complete demolition of a prestressed concrete bridge in southern

France, a suspected weak post-tensioned beam was retained for non-destructive testing (NDT). Ground

penetrating radar, Ferroscan (covermeter), gamma-ray radiography and impact-echo methods have all

been tested and their results then discussed after the autopsy of the beam by means of hydro-demolition.

This paper describes the extent to which NDT surveys are able to respond to the needs of structural

engineers, through the use of complementary NDT approaches. The introduction of a second technique

should provide a more pertinent response while eliminating certain ambiguities either by improving

measurement reliability or by focusing on questionable zones to obtain more precise local measurements.


Afterwards, the problems still not adequately resolved by these techniques are pinpointed. This step

concludes with the set of needs heretofore unmet by such techniques.

6.27 Stack Imaging of Spectral Amplitudes Based on Impact-Echo for Flaw Detection

Author(s): Masayasu Ohstu and Takeshi Watanabe

Publication: NDE&T International


Abstract/Summary: The impact-echo method recently has drawn a remarkable amount of attention for

nondestructive evaluation of defects in concrete structures. Extracting resonance frequencies responsible

for the locations of reflectors, the depth and the presence of defects are estimated. So far, however, the

technique has some limitations for practical applications. This is because there exist unresolved problems

in the application of the impact-echo to concrete structures in service. Consequently, the method is

studied theoretically on the basis of the elastodynamics and the signal analysis. In order to circumvent the

difficulty to identify peak frequencies in the conventional procedure, a new procedure to evaluate defects

in concrete is investigated, applying an imaging procedure. Thus, stack imaging of spectral amplitudes

based on the impact-echo is developed. This procedure is applied to a prestressed concrete beam to

classify a grouted duct and an ungrouted duct. The location and presence of the ungrouted duct can be

visually identified.

6.28 Applications of Impact-Echo for Flaw Detection

Author(s): Jeffrey Wouters and Randall W. Poston



Abstract/Summary: Impact-echo testing has been highly successful in locating and evaluating numerous

types of flaws in concrete and masonry structures. While the requisite knowledge and experience for

successful testing is not overly difficult to obtain, it is not uncommon to encounter improper application

of impact-echo for flaw detection. This paper presents both theory and application of impact-echo for

flaw detection, with an emphasis on actual field application of the method. Examples of application of the

method for flaw detection are also shown. It has been found that a thorough understanding of the actual

testing parameters and variables will result in more accurate and successful flaw detection.

6.29 Ultrasonic Tomography of Grouted Duct Post-Tensioned Reinforced Concrete Bridge Beams

Author(s): J. Martin, K.J. Broughton, A. Giannopolous, M.S.A Hardy and M.C. Forde

Publication: NDT & E International


Abstract/Summary: Some concern exists over the safety and durability of the 600 post-tensioned bridges

in the UK, and the much larger number worldwide. The objective of the work reported herein was to

identify voiding in the metallic tendon ducts in these bridges. Voiding can give rise to two sets of

problems: (a) possible ingress of chlorides, which would cause corrosion; and (b) a lack of redistribution

of stress within the beam. It was against this background that it was important to first of all identify the

extent of voiding in post-tensioned bridges.

The new technique of ultrasonic tomography was used for the trials reported in this paper. Two test

beams were examined: a 10 m long beam at the Transport Research Laboratory (TRL), Crowthorne, UK


and a short test beam constructed at Stanger Science and Environment, Elstree, UK. The ducts in the

TRL beam were 40 mm in diameter. This is smaller than would normally be encountered in a post-

tensioned bridge beam. A more usual duct diameter would be 100-110 mm with a cover of around

125mm. The second test beam at Stanger Science and Environment, Elstree contained 100-mm diameter

ducts.

The time-of-flight tomography data obtained demonstrated that it is a potentially highly successful

method of investigating post-tensioned concrete beams. The method is somewhat time consuming and so

should be used in conjunction with a simpler testing method, e.g. sonic impact-echo, which identifies

areas of interest. The smaller the ducts to be investigated, the smaller the required distances between

testing stations. this therefore significantly increases the testing time.

6.30 : Nondestructive Evaluation of Concrete and Masonry

Author(s): Mary Sansalone and William Streett

Publication:



6.31 Detecting Voids in Grouted Tendon Ducts of Post-Tensioned Concrete Structures using the Impact Echo Method

Author(s): Barbara J. Jaeger, Mary J. Sansalone, Randall W. Poston

Publication: ACI Structural Journal

Publication Date: July/August 1996

Abstract/Summary: An ongoing research program at Cornell University is aimed at developing the

theoretical basis and practical applications for impact-echo, a nondestructive testing technique based on

the use of transient stress waves. This paper discusses a recently completed phase of the program that

involved detecting voids in grouted tendon ducts in post-tensioned concrete structures. Results from

numerical finite element studies, controlled-flaw laboratory studies, and a field study are presented.

Three-dimensional dynamic finite element analyses were performed to examine the response of fully

grouted, partially grouted, and ungrouted tendon ducts to transient stress waves. Laboratory specimens

were built and tested to verify the numerical results. Numerical and laboratory studies demonstrated that

the impact-echo method could be used successfully to detect both complete and partial voids in grouted

tendon ducts. Afield study was then conducted on an existing post-tensioned bridge. Fully grouted,

partially grouted, and ungrouted tendon ducts located in actual bridge girders were examined. The results

of the impact-echo tests were confirmed by invasive testing; the ducts were opened up and visually

inspected. The impact-echo results correctly located fully grouted, partially grouted, and ungrouted

tendon ducts.


7 Strand Corrosion

7.1 Acoustic Emission Monitoring of Reinforced Concrete under Accelerated Corrosion

Author(s): M. Di Benedetti, G. Loretoa, F. Mattab, and A. Nannia


Publication Date: August 2012

Abstract/Summary: The development of techniques capable of evaluating the deterioration of reinforced

concrete (RC) structures is instrumental to the advancement of structural health monitoring (SHM)

techniques and service life estimate methodologies for constructed facilities. One of the main causes

leading to degradation is the corrosion of steel reinforcement. This process can be modeled

phenomenologically, while laboratory tests aimed at studying durability responses are typically

accelerated to provide usable results within a realistic period of time. A number of nondestructive

methods have been recently studied. Acoustic emission (AE) is emerging as a nondestructive tool to

detect the onset and progression of deterioration mechanisms associated with concrete cracking. In this

paper, an accelerated corrosion and continuous AE monitoring test setup is presented, providing relevant

information on the characteristics of the corrosion circuit, continuous measurement procedure, selection

of AE sensors and AE parameter setting for data acquisition. The effectiveness of AE in detecting and

characterizing the initiation of the corrosion process is discussed on the basis of results from small-scale,

pre-cracked RC

7.2 Corrosion Damage Quantification of Prestressing Strands using Acoustic Emission

Author(s): Jesé Mangual, Mohamed ElBatanouny, Paul Ziehl, and Fabio Matta


Publication Date: November 2012

Abstract/Summary: Steel degradation due to corrosion in prestressed concrete bridges has been of major

concern as it presents a threat to the integrity of structures adjacent to marine environments or where de-

icing salts are regularly used. To assess the potential for monitoring of the corrosion process an

accelerated corrosion testing program has been conducted. A series of specimens with dimensions 114

mm x 114 mm x 508 mm (4.5 in. x 4.5 in. x 20 in.) were subjected to constant potential application

through the embedded steel strand while continuously monitoring with acoustic emission (AE).

Depassivation of the strand was detected by monitoring the fluctuations in applied anodic current. Half-

cell potential measurements assessed the probability of corrosion and all results obtained were compared

to acoustic emission data. The mass loss of the corroded strands was correlated to acoustic emission

intensity analysis to quantify the degree of damage. Results show that acoustic emission is as sensitive as

half-cell potential for detecting the presence of corrosion and may be used to effectively locate corroded

areas. The intensity analysis proved useful for categorizing the level of damage; making it a strong

candidate for structural prognosis.

7.3 Detection of Corrosion of Post-Tensioned Strands in Grouted Assemblies

Author(s): Luciana V. Taveira, B.V. Joseph, A.A. Sagues and J Lopez-Sabando

Publication: NACE International Corrosion 2008


Publication Date: November 2008

Abstract/Summary: The feasibility and sensitivity of electrochemical noise (EN), electrochemical

impedance spectroscopy (EIS), and linear polarization resistance (LPR) for detection of corrosion in post-

tensioning (PT) components was investigated. The use of the electrical resistance (ER) technique, as well

EIS and LPR, to detect air space corrosion at the grout voids was also explored. The results showed that

high sensitivity noise measurements (in the μV range) are feasible for the strand-anchorage systems. The

potential and galvanic current trends for the assemblies suggest the presence of activation-passivation

cycles linked to each water ingress event. The EN method was adequate to identify only one modality

of corrosion, and failed to detect other potentially important forms of corrosion despite the presence of

significant macrocell current. In contrast, the EIS and LPR methods more reliably detected ongoing

corrosion. The ER method could sensitively detect the deterioration of grouted and bare steel strands

exposed to a high humidity environment as in the air space of a grout void. The air space corrosion

experiments showed that an aggressive environment may evolve in the grout void, resulting in

appreciable corrosion rates.

7.4 Monitoring of Electrically Isolated Post-Tensioning Tendons

Author(s): B. Elsener

Publication: Taylor & Francis Group, London


Abstract: Electrically Isolated Tendons (EIT) have been introduced as one possible solution to reach the

highest protection level (PL3) in the framework of fib recommendation for grouted post-tensioned

tendons. This approach allows to check the integrity of the plastic duct during and after construction and

to monitor the corrosion protection of the high-strength steel during the whole service life with electrical

impedance measurements. The paper presents results on PC structures with EIT regarding quality control,

long term monitoring and location of defects. Practical experience in Switzerland over the last six years

was included in the revision of the Swiss Guideline “Measures to ensure the durability of post-tensioning

tendons in bridges.

7.5 Evaluation of NDT Methods for Detection of Prestressing Steel Damage at Post-Tensioned Concrete Structures

Author(s): J. Mietz and J. Fischer

Publication: Materials and Corrosion


Abstract/Summary: For an assessment of the risk potential of existing structures, where in the case of

prestressing steel damage the load-bearing capacity could be significantly affected, non-destructive

testing techniques which enable reliable evaluation of the condition of the prestressing steels are of

utmost importance. During the demolition of a post-tensioned concrete bridge in Berlin where damage of

prestressing wires due to hydrogen-induced stress corrosion cracking were found in former investigations,

parts of the girders of the bridge superstructure could be taken out. After non-destructive investigations by

means of two testing techniques based on magnetic leakage flux measurement and one technique based

on electromagnetic resonance measurement, subsequent removal of the concrete cover up to the ducts,

opening the ducts and evaluation of the prestressing steels allowed a verification of the results obtained by

the NDT methods. From the results it can be concluded that areas with fractures of several wires can be

detected by the two techniques based on magnetic leakage flux measurement.


7.6 Enhanced Durability, Quality Control and Monitoring of Electrically Isolated Tendons

Author(s): M. Della Vedova and B. Elsener

Publication: Proceedings of the 2nd

International fib Congress


Abstract/Summary: Post-tensioning tendons contribute decisively to the serviceability, safety and

durability of prestressed concrete (PC) bridges. In order to reach the goals of durability (optimum

corrosion protection) and of monitoring requested by the Italian Railway [1] and the Swiss Federal Roads

and Railway Authorities, the new system of electrically isolated tendons according to the Swiss Guideline

[2] has been adopted. In Switzerland about 80 bridges of different length have been constructed since

1995 with thick-walled corrugated plastic ducts and electrically isolated anchorages [3, 4]. Similar

systems have been massively applied for the first time in Italy for the design and construction of several

bridges and viaducts of the new highspeed lines [5, 6]. In Italy, traditional choice for railway bridges is

the use of simply supported spans and about 90% of the viaducts of the new lines are realized with partial

or total precasting of PC decks; this allowed to carry out test programs on the construction site and

extensive quality control during construction – a point that has been recognized to be more difficult on

continuous span bridges as those usually constructed in Switzerland. The requirements, test methods and

acceptance criteria in agreement with international regulations [7], have been applied. Electrically isolated

tendons have been introduced as one possible solution to reach the highest protection level (PL3) in the

framework of fib recommendations [8, 9]. Using electrically isolated tendons allows to check the

electrical isolation of the tendons and the integrity of the plastic duct during and after construction [4, 10]

and to monitor the corrosion protection of the tendons during service life with impedance measurements

[10, 11].

7.7 Corrosion of the Strand-Anchorage System in Post-Tensioned Grouted Assemblies

Author(s): Hongbin Wang, A.A. Sagues, R.G. Powers



Abstract/Summary: Commercial ductile iron post-tension anchorage assemblies containing unstressed

high strength strand, two types of grout and simulated grout voids were subject to simulated water

intrusion events with fresh and salty (0.01N NaCl) water. Galvanic current, strand-anchor conductance

and potentials were monitored to identify corrosion location and magnitude. Potentials of the galvanic

systems at different locations along the assemblies were recorded. The results from showed that external

water intrusion can be a major source of corrosion tendon failure. Conditions for strand steel

depassivation can develop even if only modest carbonation of the grout occurs. Fresh water could initiate

corrosion if the native chloride content of the grout exceeded a relatively small amount (e.g. 500 ppm).

Currently allowable chloride limits may need revision. Galvanic coupling between strand steel and

anchorage iron could significantly aggravate corrosion of the strands. Significant corrosion of strands in

the void space was observed, especially in a grout that supported high internal relative humidity.

Projections of the combined effects of the deterioration mechanisms identified were consistent with the

observation of tendon failures in the field after as little as 7 years.

7.8 Long-Term Monitoring of Electrically Isolated Post-Tensioning Tendons

Author(s): Bernhard Elsener


Publication: Structural Concrete


Abstract/Summary: Electrically isolated tendons with plastic ducts for internal grouted post-tensioning

were developed about 15 years ago. This new generation of tendons offers enhanced corrosion protection

of the steel strands and the possibility to monitor the corrosion protection by simple non-destructive

measurements (electrical impedance). This paper reports practical experience on quality control and

long-term monitoring of two flyovers with electrically isolated tendons in Switzerland. The results of

impedance measurements are rationalized on the basis of a simple model of a capacitance C ( of the

polymer duct) in parallel to a resistance R (leak, defects) that both have a clear physical meaning and

depend on the length of the tendon. The penetration of (chloride-containing) water at defects of the duct

will lead to a decrease of the resistance R of that tendon. Thus for the first time the corrosion protection

of the structurally important post-tensioned tendons can be monitored during the whole service life of the

structure.

7.9 Electrical Isolation as Enhanced Protection for Posttensioning Tendons in Concrete Structures (PL3)


Publication: First Workshop of COST 534: NDT Assessment and New Systems in Prestressed Concrete

Structures

Publication Date: October 2004




measurements (electrical impedance). This is the reason that the fib draft "Durability specifics for

prestressed concrete structures" proposes this technology for the highest protection level (PL3) for post-

tensioning tendons. The measurement principle and the development of the monitoring technique are

discussed.

7.10 Experience with Electrically Isolated Tendons in Switzerland



Structures





measurements (electrical impedance). In this paper the Swiss experience will be presented and discussed

with respect to long term monitoring, the most frequent failures and the value of the

limiting values of the specific resistance given in the Guideline.

7.11 Protection Against Corrosion and Monitoring of Posttensioning Tendons in Prestressed Concrete Railway Bridges in Italy

Author(s): M. Della Vedova and L. Evangelista


Structures



Abstract/Summary: The paper describes the state of the art and future trends in Italy with regard to

durability and monitoring of post-tensioning tendons in prestressed concrete bridges and viaducts for

highspeed and ordinary railways. Two major tasks have been considered in the planning and design of

these structures: durability and possibility of monitoring. In order to reach these goals in the frame of

post-tensioning systems for prestressed concrete bridge decks, the thick-walled corrugated plastic ducts

for bonded internal tendons have been applied and electrically isolated anchorages have been adopted, as

first applications of these systems in Italy. The first data on quality control are presented.

7.12 Corrosion Protection and Monitoring of Electrically Isolated Post-Tensioning Tendons

Author(s): M. Della Vedova, B. Elsener and L. Evangelista

Publication: Proceedings Third European Conference on Structural Control


Abstract/Summary: The paper describes the experience and future trends in Switzerland and Italy with

regard to durability and monitoring of post-tensioning tendons in prestressed concrete bridges and

viaducts and fly-overs already built in Switzerland. Two major tasks have been considered in the planning

and design of these structures: durability and possibility of monitoring. In order to reach these goals in

the frame of post-tensioning systems for prestressed concrete bridge decks, the thick-walled corrugated

plastic ducts for bonded internal tendons have been applied and electrically isolated anchorages have

been adopted, as first applications of these systems in Swiss and Italy.

7.13 Mechanism of Corrosion of Steel Strands in Post Tensioned Grouted Assemblies

Author(s): A.A Sagues, Rodney G. Powers and Hongbin Wang



Abstract/Summary: In this preliminary study, the complexity of the corrosion phenomenon of post-

tensioning strands in grouted anchorage assemblies was examined and both physicochemical and

electrochemical key technical issues were identified. Measurements of oxygen reduction efficiency in

high pH electrolytes were conducted to obtain polarization parameters to be used in modeling. The time

evolution of electrical resistivity of 5 low-bleed commercial grouts was measured also for model input. A

mathematical model for a simple grout-strand system was proposed and dimensionless equations were

formulated, to solve the combined polarization and oxygen transport problem. Within the range of

validity of the model assumptions, initial computations indicated that oxygen availability was a key factor

in determining corrosion severity while grout resistivity was secondary. Predicted corrosion rates were in

general agreement with field and laboratory observations. Issues for subsequent model

development were identified.

7.14 Location of Prestressing Steel Fractures in Concrete

Author(s): H. Scheel and B. Hillemeir

Publication: Journal of Materials in Civil Engineering


Abstract/Summary: The remanent magnetism method allows the identification of potentially unsafe

conditions in pretensioned and posttensioned concrete structures by locating fractures in the prestressing


steel. This nondestructive method identifies fractures of single wires, even when they are bundled with

intact wires. The magnetic field of tendons is measured at the concrete surface, once they have been

premagnetized with an electromagnet. Fractures produce characteristic magnetic leakage fields, which

can be measured with appropriate sensors at the concrete surface. The parameters associated with

fractured wires have been quantitatively identified in the laboratory and have been confirmed in the field.

The knowledge of these parameters allows us to draw conclusions about the reduction of a crosssectional

area or the number of fractured wires in a tendon. The method has been successfully applied outside of

the laboratory on full size bonded and unbonded posttensioned structures.

7.15 Half-Cell Potential Measurements – Potential Mapping on Reinforced Concrete Structures

Author(s): B. Elsener, C.Andrade, J. Gulikers, R. Polder and M. Raupach

Publication: Materials and Structures


Abstract/Summary: This RILEM Technical Recommendation intends to provide the background, a

description of the application and guidelines for the interpretation of half-cell potential measurements on

reinforced concrete structures. It covers both: point measurements (mostly used during inspection, thus in

the project phase of a restoration) and potential mapping.

7.16 Ultrasonic Imaging – A Novel Way to Investigate Corrosion Status in Post-Tensioned Concrete Members

Author(s): Shivprakash Iyer, Andrea J. Schokker, Sunil K. Sinha

Publication: Journal of the Indian Institute of Science

Publication Date: September-December 2002

Abstract/Summary: The problem of durability in reinforced concrete structures is most prevalent in

chloride-induced corrosion of reinforcing steel. Traditional corrosion monitoring techniques such as half-

cell potential and corrosion rate measurements often fail when used in this type of structure and standard

nondestructive testing methods such as impact-echo have also encountered problems. This study

introduces a new method called C-scan imaging to evaluate grouted post-tensioned tendons. Preliminary

investigations on lab specimens show promise for the technique.
















7.18 Recent Developments in SQUID NDE

Author(s): H.J Krause and M.V. Kreutzbruck

Publication: Physica C


Abstract/Summary: By presenting brief summaries of recent application highlights, an overview of NDE

methods using SQUIDs is given. Bridge inspection with a SQUID array integrated with a yoke magnet

excitation was shown by scanning along the pre-stressed steel of bridges and verified by opening the

bridge deck. As the construction of the megaliner Airbus aircraft progresses, testing procedures for

extremely thick-walled structures are needed. Defects at a depth of up to 40 mm were measured in a

bolted three-layer aluminum sample with a total thickness of 62 mm. For the investigation of aircraft

wheels, a remote eddy current (EC) excitation scheme yields better depth selectivity. Defects with an

inside penetration of only 10% could be detected. SQUID magnetometers are well suited for pulsed EC

techniques which cover a broader depth range than standard single frequency EC. An inversion procedure

is presented providing a tomographic-like conductivity image of stacked aluminum samples. A recent

SQUID application is nondestructive testing of niobium sheets used for superconducting cavities of

particle accelerators. The detection of tantalum inclusions and other impurities which lower the cavity

performance is based on the measurement of local current inhomogeneities caused by EC excitation or

thermal gradients. Alternate techniques using SQUID sensors, such as modulated excitation arrays,

rotating field schemes, sensor multiplexing, magnetic moment detection, and microscopy setups, are

discussed.

7.19 SQUID Array for Magnetic Inspection of Prestressed Concrete Bridges

Author(s): H.J Krause, W.Wolf, W. Glaas, E. Zimmermann, M.I. Faley, G. Sawade, R. Mattheus, G.

Neudert, U. Gampe and J. Krieger

Publication: Physica C


Abstract/Summary: For detection of tendon ruptures in prestressed members of bridges, a four-channel

SQUID system was developed. The tendons are magnetized by scanning a yoke electromagnet over the

concrete surface along the hidden member. Four HTS dc-SQUID magnetometers with ramp-type

junctions, optimized for high-field performance, are mounted in an orientation-independent liquid

nitrogen cryostat. The SQUIDs are integrated as a linear array within the yoke and operated in magnetic

fields up to 15 mT, recording the stray field during magnetization as well as the remanent field after

switching off the excitation. Unwanted signals from stirrups of the mild steel reinforcement are

suppressed with two types of techniques: either the comparison of remanent field signals after changing

the magnetization direction of the stirrups, or a best fit of typical stirrup signals to the stray field signal

and their subtraction. Subsequent correlation analysis with the dipolar signal of a typical void yields

rupture signal amplitudes. A finite element program was written to simulate stray field and remanent field

traces of typical steel configurations. Excellent agreement with measured data was found. Results of

measurements on a prestressed highway bridge are presented. Signal amplitudes above the threshold

values were verified as originating from ruptures of the steel tendon by opening the bridge deck.

7.20 Continuous Acoustic Monitoring of Grouted Post-Tensioned Concrete Bridges

Author(s): D.W. Cullington, D. MacNeil, P. Paulson and J. Elliott

Publication: NDT & E International



Abstract/Summary: The paper describes the first UK site installation of a monitoring system that detects

the fracture of wires in post-tensioning tendons by listening with acoustic sensors attached to the surface

of the concrete. Trials have shown the system to work reliably for grouted and ungrouted tendons.

Acoustic events from other sources such as road traffic are discarded using software and hardware filters

at the unattended site. Data from possible wire-fracture events are sent off site for final identification and

positioning. The system is running continuously on site, on a viaduct, with close to 100% up-time. In

open and blind trials on the viaduct, 41 out of 44 wire break or facsimile events were correctly located

and identified and a further two were correctly located. The system can assist in the management of

bridges where the post-tensioning system is at risk from corrosion.


8 Strand Location

8.1 Application of Ground Penetrating Radar (GPR) as a Diagnostic Technique in Concrete Bridge

Author(s): Eleni Cheilakou, Panagriotis Theodorakeas, Maria Koui, Serafeim Moustakidis and Christos

Zeris

Publication: http://www.ndt.net/article/defektoskopie2012/papers/107_p.pdf


Abstract/Summary: The inspection of reinforced and pre-stressed concrete bridges is a critical task and

fundamental element continuing overall safety. Since the service life of those structures is mainly

dependent on the normal age-related degradation and integrity loss of the embedded metallic

reinforcement bars and tendon ducts, a detailed knowledge of the internal structural state of is essential

for the prevention of further damage and the improved planning of maintenance and rehabilitation. Smart

methods for assessing the structural integrity of such concrete bridges are therefore essential to ensure the

safety of the structure, as well as to reduce the huge manufacturing costs and out of service time of the

structure due to maintenance. Ground Penetrating Radar (GPR) is a well-established and among the

leading diagnostic technologies in the field of NDT&E especially prepared for these purposes. In the

last few decays, GPR has evolved as a powerful tool for the non destructive investigation of concrete

bridges, as it is one of the fastest and most cost-effective non invasive methods, available to provide

efficient information about the true position and condition of embedded reinforcement bars and tendons

ducts. The present research work evaluates the potential of GPR for the inspection of pre-stressed

concrete bridges and its usefulness to provide non visible information of the interior structural condition

required for strengthening and rehabilitation purposes. For that purpose, different concrete blocks with

embedded steel reinforcement bars and plastic ducts were investigated by means of GPR in order to locate

the internal structural elements and verify the original drawings. A 3D survey was also performed with

the aim to produce a 3D map of the interior concrete structure. The results obtained showed the

effectiveness and reliability of GPR technique for concrete bridge investigations.

8.2 Rebar Detection Using GPR: An Emerging Non Destructive QC Approach

Author(s): D.C. Bala, R.D. Garg, and S.S. Jain

Publication: International Journal of Engineering Research and Applications


Abstract/Summary: Civil infrastructure especially the rigid pavements have been increasingly covering a

huge part of the infrastructure around us with the demand of rapid urbanization. The quality and condition

assessment of it after its construction is an important issue with the engineers. The need for defect

diagnosis and verification of their various design parameters is vital and important for quality control and

decision making steps because of various economic, quality and safety reasons. In India, the pavement

evaluation is done solely by conventional destructive methods involving a lengthy procedure of core

drilling, sampling and laboratory testing which, due to the same reason, is time consuming, costly and

traffic disturbing in nature. An innovative non-destructive high resolution subsurface imaging technique

(ground penetrating radar ‘GPR’) has been used to monitor the RCC (reinforced cement concrete)

pavements parameters at IIT Roorkee, India, to overcome the existing difficulties. It has substantially

reduced manpower requirement, time, money and traffic disturbances. In this paper a successful attempt

has been made nondestructively using 1000 MHz antenna based GPR to verify the presence and array size


of rebars used in RCC roads along with its depth, concrete thickness and the masking effects of rebars on

deep features.

8.3 Results of Reconstructed and Fused NDT Data Measured in the Laboratory and On-Site Bridges

Author(s): Chrisoph Kohl and Doreen Streicher

Publication: Cement and Concrete Composites


Abstract/Summary: Non-destructive testing (NDT) of concrete structures plays an increasing role in civil

engineering. This paper presents the results of measurements carried out in the laboratory at BAM and

on-site at several bridges using reconstructed and fused radar and ultrasonic echo data sets. In this context

different scanning systems, developed for the on-site application of NDT-methods (e.g. reinforced

concrete bridges) are introduced. The main object was the demonstration of the improved effectiveness of

radar and ultrasonic pulse echo technique due to the automated measurements and the application of new

software for the data processing and data visualization. The results of these measurements show the high

potential of reconstruction and data fusion for the improvement and simplification of the interpretability

of large data sets measured with impulse-echo methods.

8.4 Ground Penetrating Radar for Concrete Evaluation Studies

Author(s): Michael D. Gehrig, Derek V. Morris and John T. Bryant

Publication: http://www.foundationperformance.org/pastpresentations/gehrig_paper_march2004.pdf


Abstract/Summary: Ground Penetrating Radar (GPR) is a geophysical imaging technique used for

subsurface exploration and monitoring. It is widely used within the forensic, engineering, geological,

mining and archeological communities. GPR provides an ideal technique for concrete evaluation in that it

has the highest resolution of any subsurface imaging, non-invasive method and is far safer than other

method such as x-ray technology. Recent improvements in hardware, and in particular, software

processing have contributed to the rapidly expanding popularity and usability of this technique.

Concrete evaluation studies utilizing GPR include the inspection of various foundation floor systems such

as structurally suspended slabs, post tensioned or conventionally reinforced slab-on-grade foundation

systems, retaining walls, decks, tunnels, balconies and garages. Typically, the objectives of these studies

are to accurately locate and/or delineate rebar, tension cables, grade beams, conduits, voids and slab

thickness. Several case studies will be presented where such objectives have been achieved.

8.5 Complementary Application of Radar, Impact-Echo and Ultrasonics for Testing Concrete Structures and Metallic Tendon Ducts

Author(s): Christiane Maierhofer, Martin Krause, Frank Mielentz, Doreen Streicher, Boris Milmann,

Andre Gardei, Christoph Kohl and Herbert Wiggenhauser



Abstract/Summary: Nondestructive testing of concrete structures plays an increasing role in civil

engineering, although until now the full potential of such techniques has not been tapped. For

posttensioned structures, the investigation of tendon ducts is one of the most essential testing problems.

The location of tendon ducts, the determination of concrete cover and, especially, the detection and


quantification of ungrouted areas inside the ducts are the relevant questions. Recent developments and

opportunities of radar, impact-echo, and ultrasonics for the investigation of tendon ducts are presented.

Although the obtained results on positioning and concrete cover determination are sufficient, the location

of ungrouted areas is still a matter of research. Thus, new approaches for this testing problem have to be

considered. Additionally, the combined use of complementary techniques offers a high potential to

increase the reliability of results. Data will be displayed on the combined application of acoustic and

electromagnetic impulse-echo methods and of data fusion related to the investigation of tendon ducts.

8.6 Nondestructive Evaluation of Concrete Infrastructure with Ground Penetrating Radar

Author(s): Christiane Maierhofer

Publication: Journal of Material in Civil Engineering


Abstract/Summary: In recent years the use of ground penetrating radar GPR at frequencies from 500 MHz

to 2.5 GHz has yielded very good results for inspection of concrete structures. The possibility of

performing nondestructive measurements quickly and with convenient recording of the measurement

results is particularly beneficial. The technique is well-suited for locating tendon ducts at depths down to

50 cm, detecting voids and detachments, and measuring thickness of structures that are only accessible

from one side. This paper presents the basics of GPR, its limits, and the results of laboratory

investigations and case studies. It also shows that GPR can be used for regular inspection, searching for

the cause of damage, and quality assessment of civil engineering structures.















8.8 Condition Assessment of Transportation Infrastructure Using Ground-Penetrating Radar

Author(s): Kenneth R. Maser

Publication: Journal of Infrastructure Systems


Abstract/Summary: Ground-penetrating radar (GPR) technology has been applied to the evaluation of

pavements, bridge decks, abutments, piers, and other constructed facilities to assess as-built conditions

and to evaluate damage and deterioration that develops over time. Two basic types of GPR equipment


systems are used depending on the type of antenna employed: either ground-coupled antennas or air-

coupled horn antennas. Ground-coupled equipment is most suitable for deeper penetrations and for object

detection, and where survey speed is not critical. Horn antenna equipment is most suited for driving speed

measurements, and where quantitative high resolution results are required. To evaluate the data, two types

of data processing are employed: (1) qualitative assessments and manual calculations from graphically

displayed data, which is most suited to site specific evaluations; and (2) automated processing of the raw

radar waveforms, which is most suited to production surveys such as pavements and bridge decks. In

some applications, such as the measurement of pavement thickness and depth of reinforcement, research

studies have verified the GPR findings. In other applications, such as the detection of voids under

concrete, the technique can only be reliably used on a site-specific basis, and further development and

verification is required before GPR can be considered more generally applicable. New technologies are

currently under development that offer the potential to enhance the range of applicability of the GPR

technique.

8.9 Automated NDE of PT Concrete Structures

Author(s): H. Wiggenhauser, D. Streicher and M. Friese

Publication: International Symposium of Nondestructive Testing in Civil Engineering (NDT-CE)


Abstract/Summary: Non-destructive testing (NDT) of post-tensioned concrete structures is both

challenging and complex. Locating areas in tendons without corrosion protection (un-grouted) is

technically difficult. Typically, Bridges are large structures, any testing of the complete surface with point

test methods can only be done with automated systems. At present there are acoustic and electromagnetic

methods which cover a large area of application and which can be used complementarily. But in most

cases, only one method is applied to solve a distinct problem. In this contribution, results will be

presented based on the investigation of internal structure of post-tensioned concrete bridges with three

different NDT methods. Measurements with radar, ultrasonic echo and impact-echo were carried out on

three bridges in Germany and Austria. Furthermore different scanning systems, developed at BAM, were

applied. The main object was the demonstration of the improved effectiveness of radar, impact-echo and

ultrasonic echo due to the automated measurements and the application of new software for the data

processing and data visualization. A new ultrasonic device with real-time imaging capabilities allows

detailed measurements in smaller areas without special scanning devices.


9 Remaining Prestress

9.1 Detection of Initial Yield and Onset of Failure in Bonded Post-Tensioned Concrete Beams

Author(s): Salvatore Salamone, Marc J. Veletzos, Francesco Lanza di Scalea, and José I. Restrepo

Publication: ASCE Journal of Bridge Engineering

Publication Date: November/December 2012

Abstract/Summary: This paper discusses monitoring of bonded posttensioned (PT) concrete elements

using the acoustic emission technique. In particular, a statistical pattern recognition technique based on a

multivariate outlier analysis is presented to identify initial yielding and the onset of failure. Experimental

tests on large-scale single-tendon bonded PT concrete beams, subjected to multiple load cycles, will be

presented to validate the proposed monitoring system.

9.2 Estimation of Existing Prestress Level on Bonded Strand Using Impact-Echo Test

Author(s): B.H. Kim, I.K. Lee, and S.J. Cho

Publication: 6th European Workshop on Structural Health Monitoring


Abstract/Summary: This work introduces a non-destructive way to evaluate existing prestress level on

bonded seven-wire strands embedded in a post-tensioned concrete structure. The approach utilizes the

experimental result that the longitudinal stress wave velocity varies with respect to applied stress level on

the strands. A set of prestressed concrete beam specimens with different tensile stress levels have been

prepared, and various impact-echo tests are conducted. It turns out that longitudinal elastic wave velocity

of the strands is nonlinearly increased as the applied tensile stress level increases. To investigate field

applicability and feasibility of the proposed approach, the longitudinal impact-echo tests are conducted

for two prestressed bonded tendons embedded on a nuclear power plant. The estimation results clearly

show that the existing prestress level of the tendon is close to the design value. It seems that the proposed

impact-echo technique is feasible and applicable for the unique identification of existing prestress level on

an individual strand embedded in a real post-tensioned concrete structure.

9.3 Determination of the Residual Prestress Force of In-Service Girders using Non-Destructive Testing

Author(s): Brian Kukay, Paul J. Barr, Marvin W. Halling and Kevin Womack

Publication: 2010 Structures Congress


Abstract/Summary: There continues to be a need for the accurate determination of the effective

prestress force in precast, prestressed concrete bridge girders. In general, design codes lead to

conservative estimations of in-service prestress forces which in turn can lead to permit and load posting

requirements. The focus of this research is on the development of a nondestructive method to more

accurately determine this effective prestress force.

The research results are based on the testing of eight AASHTO Type II bridge girders that were in service

for approximately 40 years. On average, the non-destructive tests results were within 94% of the results


based on cracking tests; a technique that has more traditionally been used to directly determining residual

prestress force.

The residual prestress force was also compared with values obtained according to current code

procedures. On average, the AASHTO LRFD-04 and 07 detailed methods overestimated the measured

prestress force, by 12% by way of the cracking tests and 17% by way of the non-destructive tests. On

average, the AASHTO Lump Sum Method agreed with measured residual prestress force obtained with

the cracking tests and overestimated the measured values by 6% in comparison to the non-destructive

tests. The details of the testing and proposed methodology are presented in this paper.

9.4 Non-Destructive Evaluation of the Stress Levels in Prestressed Steel Strands using Acoustoelastic Effect

Author(s): Salim Chaki and Gérard Bourse

Publication: NDTCE 2009 Conference Proceedings

Publication Date: June-July 2009

Abstract/Summary: Non destructive monitoring of prestressed civil structures such as bridges, dams,

nuclear power plants, etc. is extremely important to ensure security of users and environment. This

challenging task brings together the non destructive testing (NDT) and civil engineering (CE)

communities. As a part of this relationship, this paper deals with a non destructive procedure for stress

levels evaluation in seven-wire steel strands (T15.7) using acoustoelastic effect. For this purpose,

acoustoelastic calibration tests were performed and a small-model of seven-wire steel strands anchorage

block was built and probed to validate the proposed method. The results show the potential and suitability

of the proposed method for evaluating the service stress levels in the prestressed T15.7 seven-wire steel

strands.

9.5 Health Monitoring to Detect Failure of Prestressing (PS) Cables in Segmental Box-Girder Bridges

Author(s): Ivan Bartoli, Salvatore Salamone, Robert Phillips, Claudio Nucera, and Francesco Lanza di

Scalea

Publication: UCSD Technical Report CA 09-0938


Abstract/Summary: This project aimed at developing and demonstrating a health monitoring system for

prestressing (PS) tendons in post-tensioned concrete structures, including the popular segmental box-

girder bridges. The technique under investigation was based on ultrasonic guided waves and embedded

sensors. The goal was to provide both detectability of defects in the tendons such as corrosion and broken

wires, and measurability of applied prestress levels. The report first presents the literature review of the

techniques available for health monitoring of concrete structures, particularly cables and PS tendons. The

subsequent sections present the results obtained from both numerical simulations and experiments of

wave propagation in unloaded and loaded seven-wire strands, both free and embedded. Of particular

interest is the identification of certain linear and nonlinear ultrasonic wave properties which were

sensitive to the level of prestress applied to the strands. Those features were exploited, in combination

with acoustic emission technique, to monitor the stress level in strands and to detect and localize defects

during the testing of large scale post-tensioned concrete joints at UCSD’s Powell Laboratories. The work

in this project indicates that ultrasonic probing of the PS strands can indeed be effective for the detection

of defects as well as for the monitoring of PS forces in post-tensioned concrete structures. Based on the

findings of this research, strategies for both realtime health monitoring and routine-based inspection of


post-tensioned bridges are outlined. These strategies will need to be validated in a follow-on project

focusing on additional large-scale laboratory tests and field testing of bridges in service.

9.6 A Smart Steel Strand for the Evaluation of Prestress Loss Distribution in Post-Tensioned Concrete Structures

Author(s): Zhi Zhou, Jianping He, Genda Chen and Jinping Ou

Publication: Journal of Intelligent Material Systems and Structures


Abstract/Summary: Prestress loss adversely affects the behavior of in-service post-tensioned structures in

terms of deflection/camber, cracking, and ultimate capacity. It is thus important to determine the level of

prestressing force at various loading stages from the initial prestressing force transfer to the structure,

through different in-service loads, to the ultimate load of the structure. Prestress loss is difficult to

evaluate due to several intertwined factors such as creep, shrinkage, relaxation, geometric configuration,

distributed friction, and slippage of post-tensioned strands. Till date, there is no cost-effective and reliable

sensor and installation technique for the long-term monitoring and evaluation of prestress loss. In this

study, a smart fiber-reinforced polymer (FRP) rebar with an embedded novel optical fiber (OF) is

developed for the distributed strain of post-tensioned strands. The new OF is an integrated global and

local monitoring technology developed by combining the Brillouin optical time domain analysis/

refectory sensor and the optical fiber Bragg grating into one single fiber. The FRP rebar and six steel

wires were bundled together to form a seven-wire steel strand for the posttensioning and monitoring of

concrete structures. The performances of the smart rebar and strand were validated with static tests of a

prestressed steel frame structure and a posttensioned concrete beam. The smart steel strand can accurately

measure the prestress loss at each loading stage, which agrees well with that measured by a pressure

loading cell and predicted by a design code.

9.7 Comparison of Prestress Losses for a Prestress Concrete Bridge Made with High-Performance Concrete

Author(s): Paul J. Barr, Brian M Kukay, and Marv W. Halling

Publication: Journal of Bridge Engineering


Abstract/Summary: Five prestressed concrete girders made with high-performance concrete were

instrumented using vibrating-wire strain gages. Their behavior was monitored for three years from the

time of casting. The measured change in concrete strain at the centroid of the prestressing strands was

used to evaluate changes in prestress. The total measured prestress loss was as large as 28% of the total

jacking stress. Due to the higher stresses, this loss is larger than would be expected for a girder made with

conventional-strength concrete. The observed values of prestress losses were compared with values

calculated using the recommended AASHTO LRFD and NCHRP 18-07 procedures. The AASHTO

LRFD method overpredicted the average prestress losses for the highly stressed Span 2 girders by 20%

while the NCHRP method underpredicted the average losses by 16%. The NCHRP method was found to

be more inclusive and adaptable to regional construction. The calculated NCHRP Span 2 losses were

found to be within 10% of the average measured losses when the elastic shortening losses were calculated

based on measured data and differential shrinkage was calculated based on continuous beams.

9.8 Ultrasonic Wave Propagation in Progressively Loaded Multi-Wire Strands

Author(s): P. Rizzo

Publication: Journal of Experimental Mechanics



Abstract/Summary: In recent years methods based on guided ultrasonic waves gained increasing attention

for the nondestructive evaluation and the health monitoring of multi-wire strands used in civil structures

as prestressing tendons and stay cables. The study of wave propagation properties in such components has

been challenging due to the load-dependent inter-wire contact and the helical geometry of the peripheral

wires. The present paper addresses an experimental investigation on the ultrasonic wave propagation in

seven-wire strands loaded at different stress levels. Wafer piezoelectric sensors are employed in a through

transmission configuration for the generation and detection of stress waves. The response of the lowest-

order longitudinal mode is studied at different levels of load. Those ultrasonic features, associated with

the transmitted ultrasonic energy, sensitive to the variation of applied load are identified and discussed as

possible means of a load monitoring.

9.9 Application of a New Nondestructive Evaluation Technique to a 25-Year-Old Prestressed Concrete Girder

Author(s): Atorod Azizinamini, Bruce J. Keeler, John Rohde and Armin B. Mehrabi.

Publication: PCI Journal


Abstract: This paper summarizes the tests conducted on a 25-year-old prestressed concrete girder to

evaluate the available prestress. Two methods, including a newly developed technique, were used to

measure the available prestress in the girder. The focus of this paper is the application of the new

nondestructive technique. The proposed method is based on the stress state around a small cylindrical

hole drilled in the bottom flange of a prestressed girder. This technique overcomes some of the

shortcomings associated with previously developed strain/displacement based prestress evaluation

methods. Using the new method, the predicted effective prestress of the prestressing strands was

compared to that obtained from destructive cracking tests. Results indicate the merit and promise of the

newly developed nondestructive technique. Following the application of the proposed method, the 25-

year-old girder was tested to collapse. The ultimate capacity of the girder was also estimated using a

theoretical analysis. The actual strength and analytical prediction were in close agreement.


10 NDE Methods with Multiple Applications

10.1 Guidelines for the Thermographic Inspection of Concrete Bridge Components in Shaded Conditions

Author(s): Glenn Washer, Richard Fenwick, and Seth Nelson

Publication: 92nd

TRB Meeting Proceedings


Abstract/Summary: Infrared thermography has the potential to detect subsurface delaminations before

spalling develops, and could be used as a tool to enhance the visual inspection of concrete bridges. The

technology has traditionally been applied to bridge decks, which are exposed to radiant heating from the

sun that helps develop the necessary thermal gradients in the concrete. Thermal gradients can also be

developed from the normal diurnal temperature variations that occur. Convective heat transfer occurs to

develop the thermal gradients, although thermal gradients are of much lower magnitude than those

developed through radiant heating from the sun. This paper presents results of a study to develop thermal

imaging for detection of subsurface deterioration in the soffit areas of bridges, which are shaded and

therefore not exposed to radiant heating from the sun. Experimental studies and field testing were

conducted and are described. This paper reports on Guidelines developed for this application of the

technology that address the necessary environmental conditions to enable detection of damage in bridge

soffit areas. Specifically, the paper discusses ambient temperature rates of change necessary to ensure

subsurface damage can be detected in shaded conditions. The paper also discusses the effect of wind

speed on the detectability of subsurface damage in shaded areas of a bridge, and certain camera settings

needed to ensure temperature anomalies associated with subsurface damage can be detected by an

inspector during the inspection process. A field example is provided to illustrates the application of the

technology and highlights the needed camera settings.

10.2 Comparison of NDT Methods for Assessment of a Concrete Bridge Deck

Author(s): Taekeun Oh, Seong-Hoon Kee, Ralf W. Arndt, John S. Popovics, and Jinying Zhu

Publication: Journal of Engineering Mechanics


Abstract/Summary: The field application of three different nondestructive tests (NDTs)—air-coupled

impact echo (IE), infrared (IR) thermography, and sounding (chain drag)—are evaluated in this paper,

where an actual in-service concrete bridge deck is tested. Two different contactless IE test equipment sets

are deployed as part of an effort to develop new rapid measurement methods. The IE data are presented as

two-dimensional frequency maps, and the IR data are presented as temperature maps over the tested area.

Sounding (chain-drag) result maps are also presented. For verification of the location of near-surface

delamination damage, eight drilled core samples were extracted from the test area. The results obtained

from each of the individual NDT methods show reasonably good agreement with the drilled cores in

terms of locating near-surface delamination. Finally, the NDT methods are compared across general

performance criteria, considering accuracy, testing practicality, and costs. The analysis shows that all of

the evaluated NDT methods are comparable, and the chain-drag method is not more accurate and reliable

for detection of shallow delamination in the deck.


10.3 Use of Neutron Radiography and Tomography to Visualize the Autonomous Crack Sealing Efficiency in Cementitious Materials

Author(s): Kim Van Tittelboom, Didier Snoeck, Peter Vontobel, Folker H. Wittmann, and Nele De Belie

Publication: Materials and Structures Journal


Abstract/Summary: Penetration of moisture into building materials is at the origin of several damage

mechanisms. In the case of cement-based materials crack formation is a common problem and highly

accelerates the ingress of water and aggressive substances. Crack repair may be needed, however, repair

works are expensive and in some cases cracks are even not accessible. Therefore, in this research we aim

at autonomous crack sealing. Upon crack appearance, damage is sealed autonomously by the release of

encapsulated agents. Visualization of the water uptake by means of neutron radiography for samples with

manually and autonomously sealed cracks showed that in both cases ingress of water into the crack can be

prevented depending on the type of agent. The efficiency of three different agents was examined and it

was shown that the use of polyurethane or a water repellent agent were most promising. Neutron

tomography scans demonstrated that poor results were obtained when encapsulated methyl methacrylate

was used, since one component of the agent hardened inside the capsules before crack appearance. From

the results we can conclude that autonomous sealing of cracks is feasible and that neutron radiography

and tomography are suitable non-destructive test techniques to visualize the autonomous crack sealing

efficiency.

10.4 Commissioning of Portable 950 keV/3.95 MeV X-band Linac X-Ray Sources for On-Site Transmission Testing

Author(s): Mitsuru Ueaska, Ming Jin1,Wenjing Wu , Katsuhiro Dobashi, Takeshi Fujiwara,

Jyoichi Kusano, Naoki Nakamura, Masashi Yamamoto, Eiji Tanabe, Seiji Ohya, Yukiya Hattori,

and Itaru Miura Publication: E-Journal of Advanced Maintenance


Abstract/Summary: Development of portable 950keV/3.95MeV X-band (9.3GHz) linac X-ray sources has

been almost successfully completed. Designed X-ray intensities of 0.05, 2 Gy/min at 1m have been

achieved. Those intensities have been established with the portable three/four boxes with 182/386 kg in

total, respectively, for the first time in the world. Equivalent commercial systems using S-band

(2.856GHz) 950keV/3 MeV linac X-ray sources weighs about 1.5/1.7 tons, respectively. We have

optimized the design with respect to the X-ray intensity, compactness and weight. By using the 950 keV

system, we can get transmission views of artificial exterior wall thinning defects of petrochemical pipe of

8 mm thick and 300 mm diameter by 1 sec using the Perkin Elmer X-ray camera in the experimental

room. By using a commercial 300 keV X-ray tube, the same transmission images are obtained by several

minutes by an Imaging Plate (IP). 3.95 MeV system also enables 1 sec transmission test for 400 mm thick

PC (Prestressed Concrete) bridge samples. By using 300 keV X-ray tube, it takes about one hour to get

the similar image by IP. We have already performed the first on-site inspection using the 950 keV system

at a certain chemical plant. The targets of the 950 keV system are chemical plants, petrochemical plants,

impeller of pumps, wastaged pipes and iron bridge while those of the 3.95 MeV system are PC-, RC

(Reinforced Concrete) - bridges. Partial CT technique and new X-ray detectors having better sensitivity

for harder X-rays than 100 keV are under development.


10.5 Non-Destructive Radiographic Evaluation and Repairs to Pre-Stressed Structure Following Partial Collapse

Author(s): E. M. Reis and U. Dilek

Publication: 2012 ASCE Forensic Engineering Conference Proceedings

Publication Date: October-November 2012

Abstract/Summary: This article presents use of radiographic imaging (X-Ray) in evaluation of existing

reinforcing steel configuration of structures and development of steel retrofit and carbon fiber reinforced

polymer (CFRP) repairs. A collapse of the driving surface in a precast concrete parking deck prompted an

engineering evaluation and survey of the whole deck for damage assessment and repairs to distressed

members. Distress was identified in decking members and perimeter spandrel beams. Repairs to the

decking members involved supporting the distressed decking using supplemental steel brackets installed

through the double-tee stems containing pre-stressing tendons. The precise location of the tendons in the

stems needed to be identified to implement this repair in order not to damage the tendons during drilling.

Radiographic X-ray imaging in this application enabled locating and avoiding the tendons in the stems to

support and strengthen the decking member. The supplemental steel bracket also enabled continued

operation of an existing expansion joint in the area of repair. The same technique was also used to

identify steel reinforcement configuration in the spandrel beams exhibiting cracking at bearing locations

for evaluation of existing steel configuration and implementation of CFRP repairs.

10.6 Application of Thermal IR Imagery for Concrete Bridge Inspection

Author(s): Khatereh Vaghefi, Henrique A. de Melo e Silva, Devin K. Harris and Theresa M. Ahlborn

Publication:


Abstract/Summary: Detecting subsurface cracks and delaminations within concrete bridges has been

always a challenge for bridge inspectors and transportation authorities. This type of subsurface

deterioration can appear either on the bridge deck or girder; however, delaminated areas underneath the

bridge can be more critical as it raises safety issues for passing by traffic. Visual inspection, which is a

common practice technique for bridge condition evaluation, is not able to provide enough information of

internal defects and deteriorations. Although, recent developments in non-destructive techniques provide

bridge inspectors with advanced tools and methods for bridge inspection, most of these methods are either

expensive or difficult to apply.

Thermal Infrared imagery is a technology based on measuring the radiant temperature of an element, such

as a bridge deck. Subsurface delaminations and anomalies appear as hot spots on the thermal IR image

during the day as they interrupt the heat transfer through the concrete. In this way, delaminations can be

detected before turning to spalls on the bridge. Applying this technology can enhance the current bridge

inspection practice as well as providing useful information for maintenance and repair decision making.

The purpose of this paper is to review the recent developments in this field and to investigate the

feasibility of thermal IR application for regular bridge inspections.

10.7 Gamma-Ray Inspection of Post Tensioning Cables in a Concrete Bridge

Author(s): M. Pimentel, J. Figueiras, M. Mariscotti, P. Thieberger, L.M. Ruffolo and T. Frigerio

Publication: http://www.thasa.com/ANTECEDENTES/Edimburgo_2010.pdf



Abstract/Summary: Gamma-rays have been applied in an NDT campaign aimed at evaluating the

condition and the exact location of the internal post-tensioning cables of an existing box-girder concrete

bridge. Previous visual inspections to the bridge deck revealed systematic cracking patterns and localized

corrosion signs in the post-tensioning ducts. In this paper, the test logistics is described as well as the

main results that were obtained.

10.8 Environmental Effects on Subsurface Defect Detection in Concrete Structures Using Infrared Thermography

Author(s): Naveen Kumar Bolleni

Publication: Master Thesis – University of Missouri - Columbus

Publication Date: December 2009

Abstract/Summary: Deterioration of concrete due to corrosion of embedded steel reinforcing bars and

prestressing strands represent a significant challenge for inspection and maintenance engineers. Cracking,

delaminations and spalling that can occur as a result of corrosion of embedded reinforcing steel accelerate

bridge deterioration and lead to pot holes and even punch-through of concrete bridge decks. The typical

method for detecting delaminations is hammer sounding, which requires hands-on access to the material

under inspection. Specialized equipment and lane closures are frequently necessary to achieve the

required access. The application of infrared thermography to detect subsurface damage in concrete has the

potential to image delaminations from a distance, such that direct access to the surface of the concrete is

not required. Thermographic imaging relies on certain environmental conditions to create thermal

gradients in the concrete such that subsurface features can be detected. This thesis presents the results of

an investigation to determine necessary environmental conditions for the detection of subsurface damage

in concrete. To evaluate environmental effects, a large concrete test block has been constructed.

Embedded targets in the test block were used to model delaminations in concrete. Environmental factors

including wind speed, relative humidity, solar loading and variations in the ambient temperature have

been measured by a weather station located on-site with the block. The effects of these environmental

factors have been examined to determine their impact on the detectability of the subsurface targets.

Characteristics of optimum inspection conditions for utilizing infrared thermography in the field are

discussed.

10.9 Gamma-Ray Imaging for Void and Corrosion Assessment in PT Girders

Author(s): M.A.J Mariscotti, F. Jalinoos, T. Frigerio, M. Ruffolo and P. Thieberger

Publication: Concrete International


Abstract/Summary: As an extension of the reinforced concrete tomography technique, gamma rays have

been successfully applied for the assessment of voids and corrosion in concrete structures. In this paper,

some examples are given that illustrate the advantages of using this technique, especially in the case of PT

girders. Results obtained in the Zárate bridge in Argentina are discussed in some detail.


Author(s): Christiane Maierhofer, Gerhard Zacher, Christoph Kohl, and Jens Wöstmann

Publication: Journal of Nondestructive Evaluation




engineering. This paper presents the results of systematic measurements carried out in the laboratory at

BAM and on-site at several bridges using reconstructed and fused radar and ultrasonic echo data sets. For

investigating the influence of concrete mixture, radar and ultrasonic measurements were performed at test

specimens consisting of concrete mixtures with different pore content and distribution as well as with

steel fibres. Further, it is demonstrated how the fusion of data sets recorded with different methods at the

same structure (here: concrete bridges) enhances the information content in the fused data set. Different

approaches for data fusion algorithms are discussed. The results of these investigations show the high


of large data sets measured with impulse-echo methods. The presented results are based on the research

project FOR384 funded by the DFG (Deutsche Forschungsgemeinschaft).

10.11 Thermographic Crack Detection by Eddy Current Excitation

Author(s): G. Zenzinger, J. Bamberg, W. Satzger, and V. Carl

Publication: Nondestructive Testing and Evaluation Journal

Publication Date: June-September 2007

Abstract/Summary: Eddy current thermography is a new NDT-technique for the detection of cracks in

electroconductive materials. It combines the well established inspection techniques eddy current testing

and thermography. The advantage of this method is to use the high performance of eddy current testing

without the known problem of the edge effect. Especially for components of complex geometry this is an

important factor which may overcome the increased expense for inspection set-up. The principle of this

technique and an algorithm to increase the sensitivity for small defects are described. Some inspection

examples on aero engines parts are presented which show the potential of eddy current thermography.

10.12 Results of Reconstructed and Fused NDT Data Measured in the Laboratory and On-Site Bridges

Author(s): Chrisoph Kohl and Doreen Streicher

Publication: Cement and Concrete Composites



engineering. This paper presents the results of measurements carried out in the laboratory at BAM and

on-site at several bridges using reconstructed and fused radar and ultrasonic echo data sets. In this context

different scanning systems, developed for the on-site application of NDT-methods (e.g. reinforced

concrete bridges) are introduced. The main object was the demonstration of the improved effectiveness of

radar and ultrasonic pulse echo technique due to the automated measurements and the application of new

software for the data processing and data visualization. The results of these measurements show the high


of large data sets measured with impulse-echo methods.

10.13 Time-Domain Reflectometry to Detect Voids in Posttensioning Ducts

Author(s): Jian Li, Laura Akl, Robert Hunsperger, Wei Liu, Michael Chajes and Eric Kunz



Abstract/Summary: Because incompletely grouted posttensioned ducts result in voids, the steel strands are

vulnerable to premature corrosion. This paper describes a nondestructive evaluation (NDE) procedure that


allows bridge owners to ensure that posttensioned ducts are properly grouted (i.e., have no voids). The

NDE procedure uses time-domain reflectometry (TDR), a technique developed by electrical engineers for

locating discontinuities in transmission lines. TDR involves sending a signal created by a step-pulse

generator through a transmission line, determining whether the signal is reflected back, and, if it is

reflected back, using the elapsed time to determine the location of the discontinuity. Prior research funded

by the Delaware Department of Transportation and the National Science Foundation has shown that TDR

can be used to detect corrosion on strands and can be implemented in the field. To detect and evaluate

voids, the transmission line is placed either in or adjacent to the region where a void is suspected. The

presence of a void affects the electric field surrounding the transmission line and causes a distinct

reflection. Data are presented to show the measurement of both the relative size and the position of voids.

The effects of environmental conditions, such as moisture content, temperature, and material contained in

the void (e.g., corrosion products), also are reported.

10.14 Ultrasonic C-Scan Imaging of Post-Tensioned Concrete Bridge Structures for Detection of Corrosion and Voids

Author(s): Shivprakash Iyer, Sunil K. Sinha and Andrea J. Schokker

Publication: Journal of Computer-Aided Civil and Infrastructure Engineering


Abstract/Summary: Corrosion of the nation’s transportation infrastructure is a widespread and costly

problem. Efficiency, durability, safety, and environmental concerns have made the inspection and

structural assessment of these structures a vital issue. The current state of the art in concrete bridge

condition evaluation relies on visual inspection. However, deterioration in pre-stressing/post-tensioned

strand or tendon condition is not always reflected by distress visible on the concrete surface. Further, the

effect of deterioration of pre-stressing/post-tensioned steel is more disruptive than that of mild

reinforcement. Strand, due to its high mechanical strength and metallurgical characteristics, is smaller in

cross section than conventional reinforcing steel and is proportionally more impaired by loss of section.

Methodology for prestressed concrete bridge condition evaluation, therefore, could be revolutionized

through the development of accurate, quantitative nondestructive test methods for strand in pre-tensioned

and post-tensioned structures. This article presents a new sensing method using ultrasound C-scan

imaging for structural health monitoring of posttensioned bridges. Preliminary results from tests are

presented that show promising potential for the detection of corrosion and voids in concrete post-

tensioned bridges.

10.15 Progress in Ultrasonic Imaging of Concrete

Author(s): Martin Schickert

Publication: Materials and Structures


Abstract/Summary: Among present non-destructive methods for concrete evaluation, ultrasonic testing

uses relatively short wavelengths and therefore has particular potential for detailed assessment of

concrete. Methods like SAFT (Synthetic Aperture Focusing Technique) and tomographic reconstruction

are able to provide high-resolution images of concrete areas, which can be employed for tasks such as

area imaging, duct localization, fault detection, and thickness measurement. This contribution is intended

to give insight into some of the principles and possibilities of ultrasonic concrete imaging using SAFT

and tomographic reconstruction. It thereby reviews progress that has been achieved at the author's

institute during the last years. For SAFT reconstruction, the processing steps are explained that are

necessary to obtain an image that is easy to interpret, including the influence of transducers, their


coupling, and image noise suppression. Quantitative evaluation of ultrasonic images enables the

examination of tendon ducts for voids and the objective assessment of image quality. A field example

demonstrates the possibilities of SAFT reconstruction. In a separate section, ultrasonic tomography is

shown to have the capability to detect faults such as honeycombing in concrete pillars. Finally, the

potential of ultrasonic imaging and remaining steps necessary to open broad practical application are

described.

10.16 Recent Research in Nondestructive Evaluation of Civil Infrastructures

Author(s): Peter C. Chang and S. Chi Liu



Abstract/Summary: Assessing the condition of a structure is necessary to determine its safety and

reliability. Ideally, structural health monitoring should be similar to medical health monitoring of the

body. In medical health monitoring, the life signs such as pulse and blood pressure give an overall

indication of the overall health of the body. This is analogous to global structural health monitoring, in

which damage to the structure can be identified by measuring changes in the global properties of the

structure. Once the body signs show an anomaly, we do a battery of tests to determine the cause of the

anomaly. Analogously in structural health monitoring, nondestructive evaluation (NDE) can be used to

determine the nature of the damage. NDE methods to determine local damage are also becoming more

accepted in practice. This paper describes some of the recent and current National Science Foundation

projects in this area of research. Promising areas for NDE are identified.

10.17 Detecting Corrosion in Existing Structures Using Time Domain Reflectometry

Author(s): Robert G. Hunsperger, Jian Li, Wei Liu and Michael J. Chajes

Publication: Delaware Center for Transportation


Abstract/Summary: The effectiveness of corrosion evaluation of steel strands using time domain

reflectometry (TDR) has been established both theoretically and experimentally in our previous work. A

two-wire transmission line model has been established. The relationship between model geometry and

impedance has been under thorough investigation and corresponding experimental results have been

obtained. These results have proved its feasibility. TDR instrumentation has been successfully installed

in a newly built bridge and periodic data are being collected and studied. It has been proved that for a

new structure, if a sensor wire is applied along side the strand/rebar in the process of construction, the

future corrosion that could occur on the strand/rebar can be effectively detected and the damage to the

strand can be estimated.

However detecting corrosion in existing structures, in which sensor wires were not applied when the

structures were built, is more difficult. External detection methods must be employed instead of internal

methods. The theory of time domain reflectometry still applies, but factors such as the non-existence of

built-in sensor wires, the presence of concrete layers (which are strong dielectrics and contain non-

uniformities) and the distance from the strand to the sensor wire must be considered. They begin to exert

strong influence on the TDR results and methods of distinguishing and evaluating their effects have to be

found.

This project titled “Detecting Corrosion in Existing Structures Using Time Domain Reflectometry” has

been directed at solving these problems. Possible geometries that can be applied to externally detect steel

corrosion have been thoroughly studied during the project period.


Besides external steel strand corrosion detection, voids are another issue. It is believed that steel strands

buried in concrete structures are inclined to incur corrosion in voids where moisture is easily gathered and

thus steel is more vulnerable to corrosion. Corrosion will have less chance to happen if voids are detected

and a remedy implemented in time. In this project, void detection and the influence of voids on the

observed signal of corrosion were studied. Different void and corrosion combinations and the effect of

different void contents have been evaluated.

10.18 Ultrasonic C-scan Imaging: Preliminary Evaluation for Corrosion and Void Detection in Posttensioned Tendons

Author(s): Shivprakash Iyer, Andrea J. Schokker and Sunil K. Sinha



Abstract/Summary: Corrosion of the nation’s transportation infrastructure is a widespread and costly

problem. The most prevalent durability issue in reinforced concrete structures is chloride-induced

corrosion of the reinforcing steel. A reliable method of determining grout voids and corrosion levels in

posttensioned bridge structures is needed. Traditional techniques of corrosion monitoring (e.g., half-cell

potential and corrosion rate measurement) are problematic when used in this type of structure, as are

standard nondestructive evaluation (NDE) methods, such as impact echo. C-scan imaging, an ultrasonic

technique used primarily in the composites industry for detecting delamination, is examined as a method

of evaluating grouted posttensioned tendons. This method exhibits many promising qualities: it can be

used for internal or external tendons and on metal or plastic ducts; access to only one side of a specimen

is required; strong imaging allows easy interpretation of results; the technique poses no risk to users or the

environment; and the method has strong potential for development as a handheld field tool. The C-scan

technique may be valuable for the investigation of not only posttensioning applications but other types of

reinforced concrete structures as well. Results of preliminary investigations on lab specimens indicate that

the C-scan technique holds promise. The ultimate goal of the research is to provide a user-friendly, robust

system for the NDE of posttensioned tendons for voids, corrosion, and wire breaks. Recommendations for

optimal acquisition and processing techniques as well as for the future development of the equipment as a

field tool are proposed.

10.19 Time Domain Reflectometry for Void Detection in Grouted Posttensioned Bridges

Author(s): Michael Chajes, Robert Hunsperger, Wei Liu, Jian Li and Eric Kunz

Publication: Transportation Research Record 1845


Abstract/Summary: The presence of voids is a serious problem in grouted posttensioned bridges because

voids greatly reduce the corrosion-protective capabilities of the grout. Current methods for void detection

suffer several significant drawbacks. A new method utilizing time domain reflectometry (TDR) is

discussed. TDR is a well-developed method for detecting discontinuities in electrical transmission lines.

A recent study has indicated that TDR can be used as an effective nondestructive damage detection

method for concrete bridges. A void changes the electrical properties of transmission lines and therefore

introduces electrical discontinuities. It can be detected and analyzed by TDR. Experiments on short

specimens that are used to model grouted posttensioning ducts with built-in voids have been conducted

and demonstrate the potential of TDR as a void detection method.


10.20 Use of the Megascan Imaging Process in Inspection Systems for Post Tensioned Bridges and Other Major Structures

Author(s): Kevin Brown and John St. Leger

Publication: Proceedings NDT-CE


Abstract/Summary: The long-term safety, durability and performance of bridge structures mainly depend

upon good detail design, the quality of materials used and the standard of workmanship achieved on site.

Unfortunately, defects do occur in structures for a wide variety of reasons, ranging from poor detailing to

a lack of maintenance. The investigation and assessment of defects is a demanding task which in many

cases requires the use of experienced staff, supported where necessary by specialist inspection techniques.

One such specialist inspection technique is the MegaScanTM

Imaging Capture System, which is a capable

radiographic non-destructive testing (NDT) process. MegaScanTM

Imaging has been successfully

employed in the investigation of various components within a diverse range of structures. Major

improvement and development work carried out during the past five years have contributed to this

success by yielding significant improvements both in system safety and imaging techniques.

10.21 Non-Contact Ultrasonic Imagining for Post-Tensioned Bridges to Investigate Corrosion and Void Status

Author(s): Sunil K. Sinha, Andrea J. Schokker and Shivprakash R. Iyer

Publication: Proceedings of IEEE Sensors


Abstract/Summary: Corrosion of the nation’s transportation infrastnrcture is a widespread and costly

problem. Recent corrosion problem in post-tensioned bridge structures (Figure I ) have increased the need

for a reliable method for determining grout voids and level of corrosion in post-tensioned tendons.

Corrosion monitoring techniques such as half-cell potential and corrosion rate measurements face

problems when used in this type of structure and standard NDE (nondestructive evaluation) methods such

as impact-echo have also encountered problems. This study begins the evaluation of a method called C-

Scan ultrasonic imaging to evaluate grouted post-tensioned tendons. While this paper focuses on post-

tensioning applications, the C-Scan technique may be valuable for investigation of any type of reinforced

concrete structure

10.22 Experiments to Relate Acoustic Emission Energy to Fracture Energy of Concrete

Author(s): Eric N. Landis and Lucie Bailon

Publication: Journal of Engineering Mechanics


Abstract/Summary: Acoustic emission (AE) was used to measure energy associated with fracture of

standard concrete test specimens. The goal of the work was to identify ways in which AE could be used to

quantify damage in generic laboratory structures for the purpose of tuning damage models. A series of

mortar and concrete specimens of different compositions were tested for fracture energy Gf , while

simultaneously being monitored for acoustic emission energy release. Reasonable correlation between the

two quantities was observed for fine-grained specimens, however the relationship was not as good for

coarse-grained specimens. Toughening mechanisms such as friction are suggested as being responsible

for the poor relationship observed in the course-grained materials. It is further suggested that AE energy


release can be related to actual crack formation energy but not to friction and other internal energy

dissipation or toughening mechanisms.

10.23 Corrosion Detection of Steel Cables Using Time Domain Reflectometry

Author(s): Wei Liu, Robert G. Hunsperger, Michael J. Chajes, Kevin J. Folliard and Eric Kunz

Publication: Journal of Materials in Civil Engineering


Abstract/Summary: Corrosion of steel cables and reinforcing steel in concrete structures is a major cause

of structural deterioration. The current methods for corrosion detection suffer from several significant

drawbacks. In this paper, a nondestructive evaluation technique is developed that is capable of

determining the location and severity of corrosion of embedded or encased steel rebar and cables. This

technique utilizes time domain reflectometry (TDR), which has been traditionally used to detect electrical

discontinuities in transmission lines. By installing a sensor wire alongside the steel reinforcement, the

reinforcement can be modeled as an asymmetric, twin-conductor transmission line. Physical defects of the

reinforcement, such as abrupt pitting corrosion, general surface corrosion, and grouting voids, will change

the electromagnetic properties of the line. They can be modeled analytically, and identified using TDR.

TDR measurement results from several fabricated bridge cable sections with built-in defects are reported.

Based on the initial results, the TDR corrosion detection method has proven to be more robust than the

existing methods, because it allows one to detect, locate, and identify the extent of corrosion damage.

10.24 The Impact-Echo Method: An Overview

Author(s): Nicholas J. Carino

Publication: 2001 Structures Congress and Exposition Proceedings


Abstract/Summary: The impact-echo method is a technique for flaw detection in concrete. It is based on

monitoring the surface motion resulting from a short-duration mechanical impact. The method overcomes

many of the barriers associated with flaw detection in concrete based on ultrasonic methods. The purpose

of this paper is to provide an overview of the technique and to discuss the important parameters involved

in this type of testing. One of the key features of the method is the transformation of the recorded time

domain waveform of the surface motion into the frequency domain. The impact gives rise to modes of

vibration and the frequency of these modes is related to the geometry of the test object and the presence

of flaws. The principles involved in frequency analysis are discussed. The importance of the impact

duration in relation to flaw detection and other factors affecting the smallest flaw that can be detected are

also reviewed. The paper concludes with a summary of the ASTM standard governing the use of the

impact-echo method for measuring the thickness of plate-like structures.

10.25 Accuracy of NDE in Bridge Assessment

Author(s): Julia Martin, Michael S.A. Hardy, Asif S. Usmani and Michael C. Forde

Publication: Engineering Structures


Abstract/Summary: NDE has a key role in helping to evaluate input parameters to bridge management

systems. As the world’s bridge stock increases in age, there is considerable interest in maintaining and

extending the life of existing bridges, and hence the interest in bridge management systems. A range of

NDE methods have been identified and the accuracy of the time domain techniques has been related to

signal wavelength. Using the widely accepted criterion of a minimum resolution related to wavelength,


the techniques of ultrasonics and impact echo have been analyzed in detail. The latter has been

investigated with respect to post-tensioned concrete bridge beams. It was concluded that the minimum

resolution achievable in a dispersive medium such as concrete was a half wavelength, l/2. It was also

shown that the concept of using a null hypothesis with impact echo stating that: ‘if no defect is identified,

none exists’ —is fundamentally flawed.

10.26 Detecting Faults in Posttensioning Ducts by Electrical Time-Domain Reflectometry

Author(s): E.I. Okanla, P.A. Gaydecki, S. Manaf and F.M. Burdekin

Publication: Journal of Structural Engineering


Abstract/Summary: A series of laboratory-based experiments is described in which an electrical time

domain reflectometry (EIDR) system is used to locate, identify, and size a number of different artificial

voids and wet sections in ducts filled with sand and containing a variety of steel cables. The echo

waveforms were digitized using an EIDR instrumentation system and the signals passed to a computer for

storage and processing. Data analysis using simple regression techniques, in conjunction with simple

transmission line theory, suggests that a system based on the same principles could be used to identify

and size similar voids in posttensioning ducts used to carry prestressing cables in load-bearing concrete

structures such as bridges, retaining walls, and load-bearing platforms. Although considerably more

research into the usefulness of this technique is required, initial results suggest that EIDR sensors,

incorporated into the ductwork at the construction phase, may lead to the development of self-monitoring

or so-called smart structures.

10.27 Non-Destructive Examination of Corroded Concrete Structures using Radiography

Author(s): K. Saravanan, S. Srinivasan, V. Kapali, U. Nayak, R.H. Suresh Bapu, A. Madhavamayandi,

R.M. Kalyanasundaram, N.S. Rengaswamy, and K. Balakrishnan

Publication: Bulletin of Electrochemistry

Publication Date: January-February 1996

Abstract/Summary: Concrete deterioration and reinforcement corrosion occur due to many reasons.

These two factors weaken the structural strength and result in the premature failure of the structures

situated in aggressive environments. The condition of the concrete as well as rebar embedded in it has to

be assessed before taking up any remedial or repair strategy. Several electrochemical non-destructive

techniques are available for this purpose. Some of them lead to indirect conclusions and some lead to

partial quantification of corrosion. Radiographic technique is one of the promising non-destructive

methods for determining the porosity of concrete, voids in the cement grout, rusting of mild steel, rusting

of prestressing steel and snapping of prestressing wires. Laboratory and field experiments were carried

out on model slabs, beams and concrete structures to study the various aspects of concrete corrosion. A 6

MeV high energy X-ray equipment was used for this purpose. Concrete thickness up to 30 cm was

radiographed. Suitable safety measures were undertaken while carrying out the above work. The data

obtained during this radiographic work and the X-ray photographs were analysed as per requisities of

photogrammetric data analysis and results are presented in this paper. This pioneering work has yielded

very interesting and useful results regarding the quality of concrete and grouting, condition of prestressing

wires and other non-prestressed steel.


10.28 Using Emissivity-Corrected Thermal Maps to Locate Deep Structural Defects in Concrete Bridge Decks

Author(s): Nancy K. Del Grande and Phillip Durbin

Publication: SPIE Nondestructive Evaluation of Aging Bridges and Highways


Abstract/Summary: Dual-band infrared (DBIR) thermal imaging is a promising, non-contact,

nondestructive evaluation tool to evaluate the amount of deteriorated concrete on asphalt-covered bridge

decks. We conducted proof-of-principle demonstrations to characterize defects in concrete structures

which could be detected with DBIR thermal imaging. We constructed two identical concrete slabs with

synthetic delaminations, e.g., 1/8-in thick stryrofoam squares, implanted just above the 2-in. –deep steel

reinforcement bars. We covered one of the slabs with a 2-in layer of asphalt. We mounted the DBIR

cameras on a tower platform, to simulate the optics needed to conduct bridge-deck inspections from a

moving vehicle. We detected 4-in, implants embedded in concrete and 9-in. implants embedded in

asphalt-covered concrete. The midday (above-ambient) and predawn (below-ambient) delamination-site

temperatures correlated with the implant sizes. Using DBIR image ratios, we enhanced thermal-contrast

and removed emissivity-noise, e.g., from concrete compositional variations and clutter. Using the

LLNL/VIEW code, we removed the asphalt thermal-gradient mask, to depict the 4-in. deep, 9-in. square,

concrete implant site. We plan to image bridge deck defects, from a moving vehicle, for accurate

estimations of the amount of deteriorated concrete impairing the deck integrity. Potential longterm

benefits are affordable and reliable rehabilitation for asphalt-covered decks.

10.29 Imaging of Reinforced Concrete: State-of-the-Art Review

Author(s): Genevieve F. Pla-Rucki and Marc O. Eberhard

Publication: Journal of Infrastructure Systems


Abstract/Summary: Nondestructive evaluation plays an important role in the assessment of the nation's

infrastructure. The evaluation methods for reinforced and prestressed concrete facilities can become more

reliable if the methods incorporate imaging technology, which has been implemented widely in many

fields. Radiography, ground penetrating radar (in B-scan mode) and infrared thermography have

established themselves in civil engineering practice. They are used to nondestructively locate steel

reinforcement and concrete flaws, such as delaminations, cracks and honeycombing. This paper discusses

the principles of these established methods, as well as their advantages and disadvantages. Radioactive

computed tomography, microwave holography, microwave tomography and acoustic tomography are in

various stages of development. This paper summarizes the basis for each new method and the results of

recent research. It also discusses the potential advantages of the new methods and the barriers to their

implementation in civil engineering applications. Examples of images obtained with each technology are

provided.

10.30 Principles of Thermography and Radar for Bridge Deck Assessment

Author(s): Kenneth R. Maser and W.M. Roddis

Publication: Journal of Transportation Engineering


Abstract/Summary: Traditional methods of bridge deck condition assessment are slow, labor-intensive,

intrusive to traffic, and unreliable. Two new technologies, radar and infrared thermography, which have

recently been introduced, show promise for producing rapid and accurate condition assessment for bridge


decks. These technologies are being applied without the benefit of a firm physical understanding of their

inherent capabilities and limitations. This paper discusses the physical principles upon which these

techniques are based, and proposes simple physical models for the prediction of radar and infrared

response to various bridge deck conditions. Parameter studies are carried out using these models to

predict the radar and infrared response to moisture, chloride, delamination, and deck geometry. The

model study results show the range of sensitivity and the inherent limitations of these two techniques.

These results have led to the suggestion of a predictive technique that has been used in field studies of

repaired and rehabilitated asphalt-overlaid decks. This technique has been shown to predict the area of

deterioration to within 5% of total deck area.

10.31 Automated NDE of PT Concrete Structures

Author(s): H. Wiggenhauser, D. Streicher and M. Friese

Publication:http://www.germann.org/Publications/Sevilla/Automated%20NDE%20of%20PT%20concret

e%20structures,%20Wiggenhauser.pdf

Publication Date: Unknown

Abstract/Summary: Non-destructive testing (NDT) of post-tensioned concrete structures is both

challenging and complex. Locating areas in tendons without corrosion protection (un-grouted) is

technically difficult. Typically, Bridges are large structures, any testing of the complete surface with point

test methods can only be done with automated systems. At present there are acoustic and electromagnetic

methods which cover a large area of application and which can be used complementarily. But in most

cases, only one method is applied to solve a distinct problem. In this contribution, results will be

presented based on the investigation of internal structure of post-tensioned concrete bridges with three

different NDT methods. Measurements with radar, ultrasonic echo and impact-echo were carried out on

three bridges in Germany and Austria. Furthermore different scanning systems, developed at BAM, were

applied. The main object was the demonstration of the improved effectiveness of radar, impact-echo and

ultrasonic echo due to the automated measurements and the application of new software for the data

processing and data visualization. A new ultrasonic device with real-time imaging capabilities allows

detailed measurements in smaller areas without special scanning devices.


11 Sensor Networks and Damage Detection

11.1 Automatic Delamination Detection of Concrete Bridge Decks Using Impact Signals

Author(s): Gang Zhang, Ronald S. Harichandran, and Pradeep Ramuhalli

Publication: ASCE Journal of Bridge Engineering

Publication Date: November/December 2012

Abstract/Summary: Delamination of the concrete cover above the upper reinforcing bars is a common

problem in concrete bridge decks. Acoustic nondestructive evaluation is widely used to detect such

delamination because of its low cost, fast speed, and ease of implementation. The accuracy of traditional

acoustic approaches is dependent on the level of ambient noise, and the detection process is highly

subjective. An automatic impact-based delamination detection (AIDD) system is described in this paper.

In this system, the traffic noise is eliminated by a modified version of independent component analysis.

Mel-frequency cepstral coefficients are then used as features for detection to eliminate subjectivity. The

delamination detection is performed by a radial basis function neural network. The AIDD system was

developed using mixed-language programming in MATLAB, LabVIEW, and C11. The performance of

the system was evaluated using data from two bridges, and the results were satisfactory.

11.2 Autoregressive Statistical Pattern Recognition Algorithms for Damage Detection in Civil Structures

Author(s): Ruigen Yao and Shamim N. Pakzad

Publication: Mechanical Systems and Signal Processing

Publication Date: August 2012

Abstract/Summary: Statistical pattern recognition has recently emerged as a promising set of

complementary methods to system identification for automatic structural damage assessment. Its essence

is to use well-known concepts in statistics for boundary definition of different pattern classes, such as

those for damaged and undamaged structures. In this paper, several statistical pattern recognition

algorithms using autoregressive models, including statistical control charts and hypothesis testing, are

reviewed as potentially competitive damage detection techniques. To enhance the performance of

statistical methods, new feature extraction techniques using model spectra and residual autocorrelation,

together with resampling-based threshold construction methods, are proposed. Subsequently, simulated

acceleration data from a multi degree-of-freedom system is generated to test and compare the efficiency

of the existing and proposed algorithms. Data from laboratory experiments conducted on a truss and a

large-scale bridge slab model are then used to further validate the damage detection methods and

demonstrate the superior performance of proposed algorithms.

11.3 Procedures for Fatigue Crack Growth Monitoring and Fatigue Life Prediction Using Acoustic Emission Data and Neural Networks

Author(s): F.F. Barsoum, E.v.K. Hill, Y. Zhang, A. Korcak, and J. Suleman



Abstract/Summary: This research applied the nondestructive testing (NDT) technique of acoustic

emission (AE) to monitor fatigue cracking in steel structures and utilized artificial neural networks


(ANNs) for fatigue life prediction. The neural network classification network known as a Kohonen self

organizing map (SOM) was first used to classify the AE data into the different source mechanisms

occurring during fatigue cracking. This research developed methodologies for placing sensors, collecting

acoustic emission data and eliminating noise from the raw data. The ultimate goal of this research was to

monitor for steel fatigue cracking and accurately predict remaining fatigue lives using AE flaw growth

data and back propagation neural networks (BPNNs). The procedures for doing so are presented herein.

11.4 Time Series: Theory and Methods (2nd Edition)

Author(s): Peter J. Brockwell and Richard A. Davis

Publication: Springer



11.5 Discrete Wavelet Transform to Improve Guided-Wave-Based Health Monitoring of Tendons and Cables

Author(s): Piervincenzo Rizzo and Francesco Lanza di Scalea

Publication: Smart Structures and Materials 2004 Conference Proceedings


Abstract/Summary: Multi-wire steel strands are used in civil structures as pre-stressing tendons in

prestressed concrete and as stay-cables in cable-stayed and suspension bridges. Monitoring the structural

performance of these components is important to ensure the proper functioning and safety of the entire

structure. Among the various NDE techniques that are under investigation for monitoring tendons and

cables, the use of ultrasonic guided waves shows good promises. The main advantage of this approach is

the possibility for the simultaneous monitoring of loads and detection of defects, such as corrosion and

broken wires, by using the same ultrasonic setup. Load monitoring is achieved by measuring the travel

time of the wave across a given length of the cable. Defect detection is achieved by measuring the

reflections of the wave from the geometrical discontinuities. In this paper we present the enhancement on

defect detection by implementing the Discrete Wavelet Transform (DWT) as a data post-processing tool.

The data de-noising and data compression abilities of the DWT allow for greater sensitivity, larger ranges

and higher monitoring speed. It is shown that the implementation of the DWT in the ultrasonic guided-

wave technique becomes necessary for monitoring tendons and cables in the field.

11.6 Pattern Recognition Techniques for the Emerging Field of Bioinformatics: A Review

Author(s): Alan Wee-Chung Liew, Hong Yan and Mengsu Yang

Publication: Pattern Recognition


Abstract/Summary: The emerging field of bioinformatics has recently created much interest in the

computer science and engineering communities. With the wealth of sequence data in many public online

databases and the huge amount of data generated from the Human Genome Project, computer analysis has

become indispensable. This calls for novel algorithms and opens up new areas of applications for many

pattern recognition techniques. In this article, we review two major avenues of research in bioinformatics,

namely DNA sequence analysis and DNA microarray data analysis. In DNA sequence analysis, we focus

on the topics of sequence comparison and gene recognition. For DNA microarray data analysis, we

discuss key issues such as image analysis for gene expression data extraction, data pre-processing,


clustering analysis for pattern discovery and gene expression time series data analysis. We describe

current methods and show how computational techniques could be useful in these areas. It is our hope that

this review article could demonstrate how the pattern recognition community could have an impact on the

fascinating and challenging area of genomic research.

11.7 Introduction to Time Series and Forecasting (2nd Edition)

Author(s): Peter J. Brockwell and Richard A. Davis

Publication: Taylor & Francis US



11.8 Resampling Methods: A Practical Guide to Data Analysis

Author(s): Phillip Good




11.9 Parameter Estimation and Hypothesis Testing in Linear Models

Author(s): Karl-Rudolf Koch




11.10 Large-Scale Simulation Studies in Image Pattern Recognition

Author(s): Tin Kam Ho and Henry S. Baird

Publication: IEEE Transactions on Pattern Analysis and Machine Intelligence


Abstract/Summary: Many obstacles to progress in image pattern recognition result from the fact that per-

class distributions are often too irregular to be well-approximated by simple analytical functions.

Simulation studies offer one way to circumvent these obstacles. We present three closely related studies

of machine-printed character recognition that rely on synthetic data generated pseudorandomly in

accordance with an explicit stochastic model of document image degradations. The unusually large scale

of experiments— involving several million samples—that this methodology makes possible has allowed

us to compute sharp estimates of the intrinsic difficulty (Bayes risk) of concrete image recognition

problems, as well as the asymptotic accuracy and domain of competency of classifiers.

11.11 The Jackknife and Bootstrap

Author(s): J, Shao and D. Tu

Publication: Springer-Verlag




11.12 Document Analysis- From Pixels to Contents

Author(s): Jurgen Schurmann, Norbert Bartneck, Thomas Bayer, Jurgen Franke, Eberhard Mandler and

Matthias Oberlander

Publication: Proceedings of the IEEE


Abstract/Summary: The paper presents the conceptual framework for solving the task of document

analysis, which, in essence, consists in the conversion of the document’s pixel representation into an

equivalent knowledge network representation holding the document’s content and layout. The overall

system is structured into several levels of abstraction. Starting on the pixel level, the formation of

elementary geometric objects is described on which layout analysis as well as the definition of character

objects is based. Character recognition accomplishes the mapping from geometric object to character

meaning in ASCII representation, On the subsequent level of abstraction words are formed and verified

by contextual processing. Modeled knowledge about complete documents and about how their

constituents are related to the application form the highest level of abstraction. The various problems

arising at each stage are discussed. The dependencies between the different levels are exemplified and

technical solutions put forward.

11.13 Continuous Speech Recognition by Statistical Methods

Author(s): Frederick Jelinek

Publication: Proceedings of the IEEE


Abstract/Summary: Statistical methods useful in automatic recognition of continuous speech are

described. They concern modeling of a speaker and of an acoustic processor, extraction of the models

statistical parameters, and hypothesis search procedures and likelihood computations of linguistic

decoding. Experimental results are presented that indicate the power of the methods.

designing and detailing post tensioned bridges to accommodate

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