Bond Behavior of Reinforcing Steel in High Bond Behavior of Reinforcing Steel in High Performance Fiber Reinforced Cement Performance Fiber Reinforced Cement

Composites under Monotonic and Cyclic LoadingComposites under Monotonic and Cyclic Loading

ShihShih--Ho Chao (Post Doctoral Research Fellow )Ho Chao (Post Doctoral Research Fellow )Antoine E. Naaman (Professor)Antoine E. Naaman (Professor)

Gustavo Gustavo ParraParra--MontesinosMontesinos (Associate Professor)(Associate Professor)

Presentation at ACI Convention, Denver, November 5th, 2006Presentation at ACI Convention, Denver, November 5th, 2006

Bond Deterioration (Bond Deterioration (GotoGoto, 1971), 1971)

Bond Failure Mechanism of RC ElementsBond Failure Mechanism of RC Elements

INTRODUCTION

Potential Cone-Shaped Fracture

Internal Bond Crack

TensionTension

Conventional FRC

Single Crack and

Localization

(a)I III

Softening Branch

L/2

A

B

C

strain0

STR

ESS

ccσpcσ

ccεCrack Opening

STR

ESS

ccσ

pcσ

ccεpcε

0δ

Proposed Alternative: (Tensile Strain-Hardening FRC)High-Performance Fiber Reinforced Cement Composites (HPFRCCs)

Direct Tensile TestDirect Tensile Test

Multiple Cracking in HPFRCC Specimen

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Strain up to Peak Strength(%)

0

400

800

1200

1600

2000

Tens

ileSt

ress

(psi

)

0

2

4

6

8

10

12

Tens

ileSt

ress

(MPa

)

T

T

L+ΔLcf ′= 76 MPa

fV = 2% SquareTwisted Fiber

Spectra FiberHooked Fiber

PVA Fiber

RectangularTwisted Fiber

Single Crack in Regular Concrete Specimen

Test Setup and Loading TypeP

(Load)

D (Displacement)

Monotonic Loading

Reinforcing BarReinforcing Bar

HPFRCCHPFRCCPrismPrism

Corner PlateCorner Plate

Reinforcing Bars - Performance Under Monotonic Loading

0 0.2 0.4 0.6 0.8 10.1 0.3 0.5 0.7 0.9Slip (in.)

0

4000

8000

12000

16000

20000

2000

6000

10000

14000

18000

0

300

600

900

1200

1500

150

450

750

1050

1350

Control

Specimen with No. 8 Bar2% Fiber Volume Fraction

Matrix Compressive strength = 11 ksi

Conventional FRC (Steel Hooked Fiber)

Spiral Reinforcement ( = 2%)sρ

HPFRCC (Spectra Fiber)

Load

(lbs)

Aver

age

Bond

Stre

ss(p

si)

sρ

Load

(lbs)

Aver

age

Bond

Stre

ss(p

si)

Unidirectional Force-Controlled Cyclic Loading (Typical Results)

HPFRCC (2% twisted steel fiber)

RC (2% spiral reinforcement)

Fully Reversed Force-Controlled Cyclic Loading (Typical Results)

Control

HPFRCC (2% twisted steel fiber)

RC (2% Spiral)

-0.4 0 0.4-0.6 -0.2 0.2 0.6Slip (in.)

-20000

0

20000

-15000

-10000

-5000

5000

10000

15000

Load

(lbs)

-1200

0

1200

-1500

-900

-600

-300

300

600

900

1500

Aver

age

Bond

Stre

ss(p

si)

Monotonic Curve

sρ14 cycles

26 cycles

Regular Concrete RC (2% Steel Spiral Reinforcement)

Typical Cracking Patterns

HPFRCC (2% Twisted Steel Fiber)

Reinforcing Bars - Performance Under Monotonic Loading

Unfavorable Conditions for Bond Resistance in a Beam-Column Joint

1T

Cone-shaped fracture

2SC2T

1SC

Splitting cracks along beam bars

Diagonal tension cracking

Effective anchorage length

Splitting cracks in front of lugs

Proposed Solution (Material Solution): Use of HPFRCCs in Beam-Column Joints

HPFRCC

Reinforcement:Reinforcement:

Anchorage Length = Anchorage Length = 18.7d18.7dbb

Complete Elimination of Complete Elimination of Confinement in Joint RegionConfinement in Joint Region

Conventional BeamConventional Beam--Column Column Joint (CRSI, 2003)Joint (CRSI, 2003)ACI 318 & ACIACI 318 & ACI--ASCE 352: ASCE 352:

Minimum Anchorage Length = Minimum Anchorage Length = 20d20dbb (still cannot prevent bond deterioration unless 28d28dbb is provided: Leon, 1989)

Heavy Confinement

HPFRCC BeamHPFRCC Beam--Column Joints Column Joints Evaluated in This Study:Evaluated in This Study:

Multiple Cracking of HPFRCC BeamMultiple Cracking of HPFRCC Beam--Column Connections at 6% DriftColumn Connections at 6% Drift

Typical Cracking Patterns in RCTypical Cracking Patterns in RCBeamBeam--Column Connections Column Connections

((BurakBurak and Wight, 2004)and Wight, 2004)

Cracking PatternsCracking Patterns

8 8 10.9

BBN1 BBN3 BBN4 BBN5 BBN7

1

10.9

Specimen 2

EAST WEST

-80

-60

-40

-20

0

20

40

60

80

100

Stee

lStr

ess

(ksi

)

-400

-200

0

200

400

600

Stee

lStre

ss(M

Pa)yσ

0.5% Drift1.0% Drift1.5% Drift2.0% Drift2.5% Drift3.0% Drift4.0% Drift5.0% Drift6.0% Drift

East Beam

Joint

West Beam

BBN1 BBN3 BBN4 BBN5 BBN7

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Aver

age

Bond

Stre

ss(k

si)

0

5

10

15

20

Aver

age

Bon

dSt

ress

(MP

a)

0.5% Drift1.0% Drift1.5% Drift2.0% Drift2.5% Drift3.0% Drift4.0% Drift5.0% Drift6.0% Drift

BBN1|

BBN3

BBN4|

BBN5

BBN5|

BBN7

BBN3|

BBN4

East Beam Joint West Beam

Bond Stress Distribution at Bond Stress Distribution at various drift levelsvarious drift levels

Steel Stress Distribution at Steel Stress Distribution at various drift levelsvarious drift levels

Negligible bar slippage Negligible bar slippage (less than 0.03 in.)(less than 0.03 in.)

Bond Stresses Bond Stresses (Distribution)(Distribution)

Average Bond Stress = 10 Average Bond Stress = 10 MPa (5 MPa for Typical MPa (5 MPa for Typical RC BeamRC Beam--Column Joint)Column Joint)

With same reinforcement amount (volume fraction), HPFRCCs can completely replace conventional transverse reinforcement and show much better performance, in terms of peak bond strength and cracking control. The ACI requirement for development length (assuming 2% spiral) can be reduced by 50% using HPFRCCs without any transverse reinforcement.

Complete elimination of joint transverse reinforcement while maintaining excellent bond response can be achieved by using HPFRCC materials. No bond strength degradation was observed up to a beam plastic hinge rotation of 0.04 radian (0.015 bar strain). This suggests that no repair technique, such as epoxy injection,might be needed for the restoration of bond after a major earthquake.

In SummaryIn Summary……