1. Civil and Environmental Engineering Departments
University of California, Berkeley Stanford University
High Performance Fiber Reinforced Concrete Composites for Bridge Columns
C.P. Ostertag and S.L. Billington
University of California, Berkeley and Stanford University
Quake Summit Meeting
October 9, 2010

2. Outline Motivation & Objective of Research
Overview of Composite Materials Being Studied
Experimental Program
Compression and Confinement Experiments
Tension-Stiffening Experiments
Future Work
Conclusions

3. Motivation for Research Ductile fiber-reinforced composites are being studied in bridge pier designs
Self-Compacted HyFRC
ECC
 Models are needed to predict structural-scale performance

4. Motivation for Research Damage reduction & enhanced performance with lower transverse reinf.
Self-compacted
HyFRC column
rv=0.37%
Conv. reinforced
concrete column
rv=0.70%
Same spalling resistance at 4% drift. SC-HyFRC had half the transverse reinforcement.
Self-compacted HyFRC column at 11.3% drift
PEER (Ostertag & Panagiotou)

5. Motivation for Research
Higher strength and ductility observed in reinforced HPFRCs
Kesner & Billington, 2004

6. Objective Additional Questions:
To conduct fundamental, small-scale experiments on unreinforced and reinforced HPFRC materials to develop analytical models and design guidelines for application to bridge pier designs.
By how much can transverse reinforcement be reduced?
How much additional strain capacity does HPFRC have when reinforced?
Additional Questions:

7. Materials Being Studied High Performance Fiber-Reinforced Composites
Deflection softening
Deflection hardening
Tension Hardening
High Performance if it achieves hardening with less than 2% fiber volume

8. Materials Being Studied High Performance Fiber-Reinforced Composites
Less than 2% by volume of (PVA) fibers
10 mm

9. Materials Being Studied High Performance Fiber-Reinforced Composites
HyFRC (1.5% fiber volume)
Can be self-compacting

10. Experimental Program Compression testing of confined HyFRC and ECC?
PI: Claudia Ostertag

11. Compression Experiments Three levels of confinement
Five mix designs: Plain concrete and HyFRC, Plain SCC and SC-HyFRC, and ECC 1” 2” 3”
v = 0.95%
v = 0.48%
v = 0.32%
#3 bars longitudinally
10-Gage wire (0.13mm) spirals

12. Compression Experiments Specimens and measurements
Unconfined 6”x 12” cylinders
Confined 6”x12” cylinders
Strain via 2 LVDTS within 8 inch section
Equipment limited to displacements < 0.4”
Confined 6”x12” cylinders w/ strain gages
Strain gages installed on spiral reinforcement
Strain averaged over entire height using 2 LVDTs
Equipment enabled strain calculations at large displacements (~ 1”)

13. Compression Experiments HyFRC compared with conventional concrete
Strain
Control (conventional) concrete
HyFRC
0.002
0.004
0.008
0.01
0.012
0.006 2 5 7
Stress (ksi) 4 6 1 3
HyFRC has stable, extended softening behavior on its own
This graph shows that the HyFRC by itself without confinement has a very stable and extended softening behavior; this is why i) it does not need confinement and ii) to get to the same softening behavior you need a very high confinement ratio in control concrete as shown in the next slides

14. Compression Experiments High confinement ratio not needed with SC-HyFRC1” 2” 3”
SCC (1.91%)
rv = 0.95%
rv = 0.49%
rv = 0.32%
Sarah, the top figure contains three sets of date: i) the SC-HyFRC and SCC at 0.95% reinforcing ratio, respectively and ii) the SCC at 1.91% reinforcing ratio. I.e. to get to the same softening behavior in concrete as in HyFRC you need a very high reinforcing ratio
SC-HyFRC
SCC
SC-HyFRC
SCC

15. Compression Experiments Confined HyFRC Results (2” spacing)
Extensive spalling
Plain
HyFRC
Delay in damage initiation and damage progression
SC-HyFRC

16. Compression Experiments No damage localization in SC-HyFRC
rv =0.95%
rv =0.95%
rv =1.91%
rv =0.95%
The important result here is that by doubling the reinforcing ratio in the control specimens the damage localization is reduced (ie. Going from 0.95 to 1.91% in the control SCC specimens; i.e. comparison of figure on the left with figure on the right in case of SCC) . But even at 1.91% the performance of the SCC is not nearly as good as in the HyFRC at half the reinforcing ratio (Figure on right).
The transverse strain were measured by strain gages attached to the steel spirals.
SC-HyFRC
Plain SCC

17. Experimental Program Tension stiffening in ECC & HyFRC
Bar in ECC
Bare bar
Bar in Concrete
Fischer & Li, 2002
Blunt & Ostertag, 2009
No uniaxial tension data
No recording beyond 0.5% strain

18. Tension-stiffening experiments Questions
What is the tension stiffening effect with HyFRC?
How does the HyFRC and ECC perform at large strains when reinforced?
Can basic material properties and geometry be used to predict the tension stiffening and reinforced response?
How does rebar size and volume of surrounding material impact tension stiffening?

19. Tension Stiffening Experiments Two specimen designs evaluated
34”
Dogbones
Prisms

20. Tension Stiffening Experiments Specimen Variables
2 geometries: prism & dogbone
3 mix designs: ECC, HyFRC, SC-HyFRC
2 reinforcing ratios: 1.25% and 1.9%
Plain specimens: (no reinforcing bar)
Material characterization tests (cylinders, beams, plates)

21. Stress concentration factor of 1.16
Tension Stiffening Experiments Specimen design and set-up validation
Dogbone specimen designed
Inserts and grips machined 6”
Stress concentration factor of 1.16

22. Dogbones - Typical Failures
ECC3-4-1
HyFRC-4-1
SC-HyFRC-4-1
DB3-4-1: multiple cracking and localization inside of the gauge region
DB3-4-2: multiple cracking and localization inside of the gauge region
SC-HyFRC-4-1: not as much cracking as the ECC & HyFRC; failure outside of the gauge region so LVDT’s data not very useful; this specimen has short steel fibers (about 1” long)
SC-HyFRC-4-2: cracking localization in three different places but final failure outside of the gauge region at the bottom; good data from LVDT’s due to major crack in the middle of the specimen; multiple cracking but less than the ECC & HyFRC
HyFRC-4-1: multiple cracking behavior (these had the long steel fibers - 2” long); failure inside of the gauge region
HyFRC-4-2: same as previous specimen, multiple cracking behavior; failure inside of the gauge region
ECC3-4-2
HyFRC-4-2
SC-HyFRC-4-2

23. Tension Stiffening Experiments ECC Dogbones – Preliminary Data
Rebar fyAs + ECC stress block
Contribution – beyond yield data – definite synergy in terms of compatibility of deformation.
Lower stiffness is from LVDT data being impacted by cracking where collar attaches.

24. Tension Stiffening Experiments HyFRC and SC-HyFRC Dogbones
Average strength of plain HyFRC dogbones ~3-4 kips

25. Future Work Experiments and Model Development
Develop modeling approaches with experimental data (additional tension/compression experiments needed)
Validate modeling on new reinforced beam and column tests, and recent and upcoming bridge pier experiments
Longer-term: Bond/pull-out testing for bond-slip characterization

26. Material Characterization for Modeling Can simple material testing be used to predict performance in reinforced components?
Plate
Beam
12”
ECC
ECC

27. Material Characterization for Modeling Can simple material testing be used to predict performance in reinforced components?
Plate
Plate
Inverse analysis of flexural response to estimate uniaxial tensile data
ECC
ECC

28. Conclusions Lower transverse steel ratios are possible with SC-HyFRC
No damage localization in compression with SC-HyFRC
Large-scale tensile dogbones loaded in curve of dogbone provide robust results
In tension, reinforced HPFRC materials can reach higher strains before forming a dominant failure crack than when they are unreinforced .

29. Acknowledgements Pacific Earthquake Engineering Research Center
Graduate Researchers Gabe Jen, Will Trono & Daniel Moreno
Headed Reinforcement Corporation