BRIDGE DECK DETERIORATION EVALUATION Ground-penetrating radar (GPR) is a high-speed NDT method that WaveTech—GEOVision applies to a number of engineering problems associated with aging concrete structures, including:

• identification of deteriorated—caused primarily by corrosion of reinforcement within reinforced concrete decks, and

• intrusion of chlorides into the concrete matrix. COMPARE TRADITIONAL GPR WITH NEW APPROACH

Traditional GPR Surveys. HIGH-SPEED, DUAL-POLARIZATION METHOD OF GROUND PENETRATING RADAR GPR

• Used for many years • Moderately successful, but • Mixed results! (unable to

accurately locate deterioration, estimate quantities, or define boundaries

• New technology • Highly successful • Marked improvement in the precision of location,

accuracy of quantity estimates, and boundary definition

KEY QUALITIES

• Precise survey positioning (using either a camera or

laser to maintain GPR lines at precise 2ft spacing, and surveying as close as 2ft to curbs and gutters—as shown in the adjacent figure),

• extremely dense signal sampling along each GPR

line—and a method for removing compromising influences of shallow cover, overlays, and the variable presence of longitudinal steel—

are what make this high-speed method more accurate, repeatable, reliable and versatile than the traditional single-polarization surveys that have been performed over the last decade.

0 10 20 30 40

Corrosion&

Chloride

GPR Prediction

Deterioration

Deterioration

Top plan-view map shows areas on a deck that where half-cell and corrosion potential data indicated active corrosion (dark and light orange) and regions (greens/white) where corrosion was approaching threshold or insignificant. Bottom plan-view map shows results from high-resolution ground-coupled, 1.5 GHz survey illustrating predicted zones of deterioration (all colors but blue and black,with reds being most deteriorated) on a plan view (overlay present during GPR survey).

Traditional GPR Surveys. HIGH-SPEED, DUAL-POLARIZATION METHOD

1124 Olympic Drive, Corona, CA 92881 (951) 549-1234 FAX: (951) 549-1236 www.geovision.com

OF GROUND PENETRATING RADAR GPR • Rely on single polarization • 1.5 GHz ground coupled antenna • ADVANTAGES

small size high resolution best ability to measure rebar

reflection amplitude • DISADVANTAGES

Slow speed Traffic control requirements Safety Higher cost Orientation requirement

• Dual Polarization • 1 GHz Air coupled • ADVANTAGES

Independently polarized antennas eliminate orientation issues

resolution is nearly as good as 1.5GHz fast: can be performed at 30 – 40 mph! similar ability to measure rebar reflection

amplitude lower cost

• DISADVANTAGES Higher analytical demands

& State-of-the-Practice. Continued antennas (1.0 GHz)

novel technique enabling two antennas to effectively simulate the higher-

air-coupled that one is sensitive to

is rotated 90° so that it is the signals from both antennas

survey path, allowing them to be

cted from the other, taking the

rebar reflection which is similar to the high-frequency (1.5 GHz) antenna. Removed

ence from either a thin overlay . Additionally, the negative influence of

running the somof

lly what the

influence from longitudinal bars. The result is a survey which matches the accuracy of a ground-coupled n,

0 and 40 mph. Furthermore, it makes no

r GPR method with what is commonly called “ground truth”, or most “verification

Methodology of Dual-Polarization (1.0 GHz) Horn Antenna Surveys: State-of-the-Art investigation into the use of air-coupled horn resulted in the development of a independently polarized resolution ground-coupled antenna. When two antennas, mounted in-line and deployed sotransverse steel in the deck and the othermost sensitive to longitudinal steel, are sampled from exactly the same measured and evaluated in a unique manner. The signal from one antenna is subtraoffset between the two antenna’s positions into account, and the resulting signal yields a top one measured by a

interferfrom this signal are unwantedan h(less than 2 inches), shallow concrete cover (

is randomly less th 2.5 inches), or bot

longitudinal steel—which measured al profile ong every GPR

deck’s length because the antennas are etimes d eir ctly over a bar, but more

ten are not—is removed from the top signal, which is reatransverse bar

we want to be measuring without

inspectiocan be performed at speeds between

but 3difference whether longitudinal or transverse steel is on top because the influence of the longitudinal steel—even in a continuously reinforced structure (with longitudinally-oriented steel on top)—is minimized, allowing top transverse rebar reflection to be measured and compared throughout the deck area. The resulting contour plots are virtually the same from a dual-polarization (1.0 GHz) horn antenna survey as those obtained from a 1.5 GHz ground-coupled analysis on most decks, a far better correlation than any otheand validation” using sounding and cores (bottom map on previous page shows delaminated areas detected by hammer-sounding after overlay was removed).

-5 0 5 10 15 20 25 30 35 400

2

4

6

deci

bels

(dB

)

-5 0 5 10 15 20 25 30 35 400

2

4

6

-35-30-29-28-27-26-25-24-23-22-21-20-19-18-17-16-15-14-13-12-11

-5 0 5 10 15 20 25 30 35 400

2

4

6

(a) Deteriorated aare s of bridge deck assessed using 1.5 GHz ground-coupled antenna (GSSI Model 5100)

(b) Deteriorated areas of bridge deck assessed using dual-polarized horn antennas (GSS

(c) Deteriorated areas of bridge deck mapped using hammer-sounding

I Model 4208)

1124 Olympic Drive, Corona, CA 92881 (951) 549-1234 FAX: (951) 549-1236 www.geovision.com

OTHER IMPORTANT BENEFITS In addition to mapping deterioration quantities, the high-resolution GPR survey method allows us to determine: • Very precise lateral rebar position (ground coupled method only) • Depth of concrete over rebar, or cover (ground-coupled and air-coupled surveys: more accurate when both

, engineering consultants, or government agencies.

lect Patch s

roject Maintenance Summary: Project costs were overrun due to lack of GPR information prior to budget

data.

are used, but typically more practical with air-coupled (horn) antennas unless lanes are closed) • Asphalt overlay thickness • Concrete deck thickness GEOVision has the most advanced data acquisition units, antennas and processing software available, as well as more consulting experience on this particular GPR bridge deck survey method than any company in world. We typically do not perform the traditional horn antenna surveys because we are convinced that the industry will be more responsive to dual-polarization surveys—even their higher price—once the advantages of accuracy are realized on a more widespread basis by owners

ase Study: I-89 Bridge, Connecticut River (1988)—Using GPR Survey to SeC

Location

Pdecisions. As it turned out, no one knew how reliable the GPR survey would be able to predict verifiable deterioration—or accurately map its location—once it was calibrated with some underside and chloride sample

Item 520.5519 - Quick Set Concrete (Night)

Item Unit Unit Bid Price Estimated Quantity Phase I

Actual Quantity Phases I & II

511.02 Yd2 $340.00 400.00 698.10 520.5519 Yd3 $1,245.00 50.00 77.50

1) Total cost of project = $923,266.41 The original project budget, including contingency costs = $703,778.90. Approximately $106,000.00 of the project increase was attributed to traffic control.

1124 Olympic Drive, Corona, CA 92881 (951) 549-1234 FAX: (951) 549-1236 www.geovision.com

Phase I and Phase II Maintenance (Typical of I-89 Project)

White Border: Phase I Maintenance ZonesGray (Pink, in CD-ROM version of paper) Border: Phase II Maintenance Zones

Use of GPR, based on quality of predictions, would have provided better defined regions for repair resulting in an improved capability to estimate project costs and budget appropriately. More than likely, a different maintenance strategy might have been selected (hydro-demolition and replacement of concrete, for example) if the goal was to increase service life beyond ten years. Because the structure was considered to be one that would become functionally obsolete within ten years (due to increased traffic demand)…the decision was more one of repair or replace; meaning the DOT didn’t want to repair more (or incur more expense) than absolutely necessary. More previous experience with high-resolution GPR methods (precision positioning/dense sampling) would have also reduced the amount of time spent on Phase II maintenance, the most expensive part of the work (due primarily to increased cost of traffic control). Because of the unknowns surrounding the capabilities of this method, both NHDOT and GSSI (who provided the data for this project) agreed that a more conservative approach toward identification of “patch locations” could be used, even though GSSI felt that its predicted deterioration quantities and locations (more extensive than what NHDOT used as an action level) were essentially correct. A detailed, published paper on this project (or other work) is available upon request.

SUMMARY OF BENEFITS • Accurate, specific location information for better planning • Rapid data acquisition – less inconvenience and liability for owner • Elimination of lane closures reduces cost and improves worker safety and reduces injury liability • Cost benefit due to better estimation of repair project costs, lower cost of planning, speed and safety • Better quantification of repair zones will

o extend bridge deck service life by minimizing future repairs and frequency of maintenance o improve ability to plan extent of daily maintenance activities and estimate material quantities o reduce costs associated to unplanned traffic control

Data for much of this informational brochure provided courtesy of Geophysical Survey Systems, Inc.

1124 Olympic Drive, Corona, CA 92881 (951) 549-1234 FAX: (951) 549-1236 www.geovision.com