Constitutive Models of Prestressed Steel-Fiber Concrete

Christopher P. CarusoDept. of Civil & Environmental Engineering

University of HoustonNSF REU Program

August 2007

Outline of Presentation

• Introduction

• Experimental Program

• Results

• Discussion

• Conclusions

Introduction

Purpose

• Investigate the behavior of prestressed steel-fiber concrete (PSFC) under shear.– Can steel fibers replace traditional shear stirrups?

• Is this a practical and economical improvement?

TXDOT sponsored project

Prestressed Concrete

• High transverse load-bearing capacity – Initial compressive stress

• Used commonly in highway bridge girders.

Steel Fiber Concrete• Concrete with short steel wires mixed in.

• Known to reduce crack propagation– Absorb energy released when a crack opens

Crack

Fig. 3. Cracked Concrete Panel under a tensile load P.

P P

Without Steel Fibers

Energy

With Steel Fibers

Energy

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”

Research Significance• Earthquake load simulation

• Hollow Bridge Piers subjected to reverse cyclic loading (Yeh and Mo 1999)– Full-scale shake-table test

Actuator

Oil jack

Column Reaction

Wall

Strong Floor

RC Foundation

Load Cell

Dial gauge

Cross beam

Hinge

Cross beam

Universal joint

Load Cell Universal joint

1500

1500

900

900

64-#7

#3@200

66 168 132 168 132 168 132 168 132 168 66

300

i j b

a

Research Significance

• Constitutive Models are used to accurately predict structure behavior.

• Construct a Finite Element Model

NonlinearBeamColumnElements

Rigid Beam

AA

(a) Elevation view

N3

N3

N3

P3

P3

P3

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.

Experimental Program

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)

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

Unit: mm

t

l

t

l

Empty UETLoaded UET

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

Results

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

Experimental Stress vs. Strain

Concrete Stress vs. Strain

Prestressing Tendon Stress vs. Strain

Discussion

Questions

• Why did TEF1 experience premature tendon failure?

• What do the stress strain curves indicate about the panel’s behavior?

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.

t

l

Severe Crack

Severe Crack

Tendon Chuck

Severe Crack Tendon Bracket

Flexible Metal Conduit

Prestressing tendon

Chuck

UngroutedRegion

Concrete

Friction Plate

Tendon U-Bracket

Conclusions

Conclusions

• TEF1 stress-strain curves appear well predicted by prestressed constitutive models.

• Despite premature tendon failure, results are promising for success of future tests.

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