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