Constitutive Models of Prestressed Steel-Fiber Concrete

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1 Constitutive Models of Prestressed Steel-Fiber Concrete Christopher P. Caruso Dept. of Civil & Environmental Engineering University of Houston NSF REU Program August 2007

2 Outline of Presentation Introduction Experimental Program Results Discussion Conclusions

3 Introduction

4 Purpose Investigate the behavior of prestressed steelfiber concrete (PSFC) under shear. Can steel fibers replace traditional shear stirrups? Is this a practical and economical improvement? TXDOT sponsored project

5 Prestressed Concrete High transverse load-bearing capacity Initial compressive stress Used commonly in highway bridge girders.

6 Steel Fiber Concrete Concrete with short steel wires mixed in. Known to reduce crack propagation Absorb energy released when a crack opens Energy Crack P Without Steel Fibers P Energy Fig. 3. Cracked Concrete Panel under a tensile load P. With Steel Fibers

7 Constitutive Models Relate Stress and Strain in a material. Eg. Prestressed Concrete Must be determined experimentally. Can be used to analyze indeterminate structures Consider with force equilibrium and strain compatibility Sometimes referred to as Stress-Strain Curve

8 Research Significance Earthquake load simulation Hollow Bridge Piers subjected to reverse cyclic loading (Yeh and Mo 1999) Full-scale shake-table test 1500 a b i Universal joint Load Cell Cross beam Oil jack Load Cell Hinge Actuator 64-#7 j Column Reaction Wall Dial gauge #3@ Cross beam RC Foundation Universal joint Strong Floor

9 Research Significance Constitutive Models are used to accurately predict structure behavior. Construct a Finite Element Model P 3 N 3 N 3 P 3 N 3 P 3 Rigid Beam Nonlinear BeamColumn Elements A A (a) Elevation view

10 Objective Investigate Behavior of Prestressed Steel-Fiber Concrete (PSFC). Construct PSFC panels. Test panels in sequential loading. Tension Compression Record applied loads and panel deformations. Analyze data. Determine stress strain curves for concrete and prestressing tendons. Compare to prestressed concrete panel data.

11 Experimental Program

12 Experiment Plan Fabricate two PSFC panels for testing TEF1: 0.5 % Steel Fibers by volume. TEF2: 1.0 % Steel Fibers by volume. Test panels in Universal Element Tester Tension Compression Collect load & deformation data Jack Load Sensors Linear Variable Differential Transformers (LVDT)

13 Panel Design Concrete Type 1 Portland Cement 6 ksi Compressive Strength 7 in. Slump Reinforcement 10 steel prestressing tendons. 10 steel compression bars. Dramix short hook-end steel fibers

14 t l Unit: mm

15

16 t l

17 Loaded UET Empty UET

18 Test Procedure Tensile load to 40 kips Tensile strain to 1% Tensile strain to 1.5% Tensile strain to 2% Compressive load to 30 kips Compressive strain to crushing failure

19 Results

20 Test Results TEF1 experienced premature tendon failure Most tension data was recoverable TEF2 was not tested due to malfunctioning servo control box Will be tested once box is repaired TEF1 data compared to prestressed panel data Jung Wang, Ph.D student

21 Experimental Stress vs. Strain

22 Concrete Stress vs. Strain

23 Prestressing Tendon Stress vs. Strain

24 Discussion

25 Questions Why did TEF1 experience premature tendon failure? What do the stress strain curves indicate about the panel s behavior?

26 TEF1 Failure Severe cracks formed at panel boundaries Disproportionately higher tendon loads during test. Tendon conduits not fully grouted Short lengths near panel boundaries experienced unacceptably high strain.

27 t Severe Crack Tendon Bracket Friction Severe Plate Crack Severe Crack Chuck Prestressing tendon Concrete l Ungrouted Region Flexible Metal Conduit Tendon UBracket Tendon Chuck

28

29 Conclusions

30 Conclusions TEF1 stress-strain curves appear well predicted by prestressed constitutive models. Despite premature tendon failure, results are promising for success of future tests.

31 Future Work Apply maximum compressive load through friction plates Apply high-strength grout between friction plates and panel Use tubes to pre-form bolt holes for friction plates