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

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

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

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)

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

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:

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

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

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

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

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

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”)

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

Compression Experiments High confinement ratio not needed with SC-HyFRC 1” 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

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

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

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

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?

Tension Stiffening Experiments Two specimen designs evaluated 34” Dogbones Prisms

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)

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

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

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.

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

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

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

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

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 .

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