Bond Behavior of Reinforcing Steel in High Performance Fiber Reinforced Cement Composites under Monotonic and Cyclic Loading

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1 Bond Behavior of Reinforcing Steel in High Performance Fiber Reinforced Cement Composites under Monotonic and Cyclic Loading Shih-Ho Chao (Post Doctoral Research Fellow ) Antoine E. Naaman (Professor) Gustavo Parra-Montesinos (Associate Professor) Presentation at ACI Convention, Denver, November 5th, 26

2 INTRODUCTION Bond Failure Mechanism of RC Elements Internal Bond Crack Potential Cone-Shaped Fracture Tension Bond Deterioration (Goto( Goto,, 1971)

3 Proposed Alternative: (Tensile Strain-Hardening FRC) High-Performance Fiber Reinforced Cement Composites (HPFRCCs) STRESS I σ cc σ pc strain A B ε cc III Conventional FRC Softening Branch Crack Opening C Single Crack and Localization (a) L/2 σ pc STRESS σ cc ε pc δ ε cc

4 Direct Tensile Test Tensile Stress (psi) V f = 2% f c = 76 MPa Hooked Fiber SquareTwisted Fiber RectangularTwisted Fiber Spectra Fiber T T L+ΔL Tensile Stress (MPa) 4 2 PVA Fiber Strain up to Peak Strength(%) Multiple Cracking in HPFRCC Specimen Single Crack in Regular Concrete Specimen

5 Test Setup and Loading Type P (Load) Monotonic Loading D (Displacement) HPFRCC Prism Corner Plate Reinforcing Bar

6 Reinforcing Bars - Performance Under Monotonic Loading HPFRCC (Spectra Fiber) Control Slip (in.) Specimen with No. 8 Bar 2% Fiber Volume Fraction Matrix Compressive strength = 11 ksi Conventional FRC (Steel Hooked Fiber) Spiral Reinforcement ( ρ s = 2%)

7 Unidirectional Force-Controlled Cyclic Loading (Typical Results) ρ s RC (2% spiral reinforcement) Load (lbs) Average Bond Stress (psi) HPFRCC (2% twisted steel fiber) Load (lbs) Average Bond Stress (psi)

8 Fully Reversed Force-Controlled Cyclic Loading (Typical Results) Load (lbs) Control Slip (in.) Monotonic Curve Average Bond Stress (psi) ρ s 14 cycles RC (2% Spiral) 26 cycles HPFRCC (2% twisted steel fiber)

9 Reinforcing Bars - Performance Under Monotonic Loading Typical Cracking Patterns Regular Concrete RC (2% Steel Spiral Reinforcement) HPFRCC (2% Twisted Steel Fiber)

10 Unfavorable Conditions for Bond Resistance in a Beam-Column Joint Splitting cracks along beam bars C S1 T 1 Diagonal tension cracking T 2 C S 2 Cone-shaped fracture Effective anchorage length Splitting cracks in front of lugs

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

12 Reinforcement: Conventional Beam-Column Joint (CRSI, 23) ACI 318 & ACI-ASCE ASCE 352: Minimum Anchorage Length = 2d b (still cannot prevent bond deterioration unless 28d is b provided: Leon, 1989) Heavy Confinement HPFRCC Beam-Column Joints Evaluated in This Study: Anchorage Length = 18.7d b Complete Elimination of Confinement in Joint Region

13 Cracking Patterns Typical Cracking Patterns in RC Beam-Column Connections (Burak and Wight, 24) Multiple Cracking of HPFRCC Beam-Column Connections at 6% Drift

14 Bond Stresses (Distribution) EAST BBN1 BBN3 BBN4 BBN5 BBN7 WEST Specimen 2 Steel Stress Distribution at various drift levels Steel Stress (ksi) σ y East Beam Joint West Beam BBN1 BBN3 BBN4 BBN5 BBN7.5% Drift 1.% Drift 1.5% Drift 2.% Drift 2.5% Drift 3.% Drift 4.% Drift 5.% Drift 6.% Drift Steel Stress (MPa) Bond Stress Distribution at various drift levels Average Bond Stress = 1 MPa (5 MPa for Typical RC Beam-Column Joint) Negligible bar slippage (less than.3 in.) Average Bond Stress (ksi) East Beam Joint West Beam BBN1 BBN3 BBN3 BBN4 BBN4 BBN5 BBN5 BBN7.5% Drift 1.% Drift 1.5% Drift 2.% Drift 2.5% Drift 3.% Drift 4.% Drift 5.% Drift 6.% Drift Average Bond Stress (MPa)

15 In Summary 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 5% 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.4 radian (.15 bar strain). This suggests that no repair technique, such as epoxy injection, might be needed for the restoration of bond after a major earthquake.