Fiber Reinforcement in Prestressed Concrete Beams

Prestressed concrete I-beams are used extensively as the primary superstructure elements in Texas highway bridges. A commonly observed problem in these beams is the appearance of end zone cracking due to the prestressing forces, thermal effects of hydration, shrinkage and temperature variation. Even though a large quantity of transverse steel reinforcement is provided in the end zone, the cracking problem persists. The research described in this report was targeted to develop a workable steel fiber reinforced concrete mix that would be capable of partially or completely replacing the dense traditional reinforcement and eliminating cracking in the end zones. The research work was divided into three phases: Phase One consisted of developing Texas Department of Transportation (TxDOT) Traditional Fiber Reinforced Concrete (TTFRC) and Self-Consolidating Fiber Reinforced Concrete (SCFRC) mixes with steel fibers. Four TTFRC and three SCFRC mixes with two different types and variable amounts of hook-ended steel fibers were tested for their workability and hardened properties. Based on their performance, suitable TTFRC and SCFRC mixes with optimum fiber contents were selected to cast full-scale beams. Phase Two research dealt with the casting and end zone monitoring of seven 25-ft.-long [American Association of State Highway and Transportation Officials (AASHTO) Type-A] I-beams using optimized TTFRC and SCFRC mixes. Conventionally used equipment and techniques were applied for mixing, transporting, placing and steam curing the beams at the precast plant. Strain gauges and temperature loggers installed inside the beams measured strains and temperatures, respectively, during steam curing and release of prestressing force. This instrumentation was aimed at finding the influence of steel fibers on controlling/eliminating the end zone cracks. Phase Three research consisted of load testing the seven beams to failure to determine the effects of steel fibers on the structural performance of the beams. Both ends of the simply supported beams were tested to failure using four hydraulic actuators with strain controlled capability. For the first time, descending branches of load-deformation curves were obtained for the end zones of prestressed concrete beams to assess the ductility. The research findings proved that the end zone cracking would be eliminated by completely or partially replacing the traditional transverse steel reinforcement by steel fibers. Additionally, steel fibers enhanced the ductility and crack resistance of the prestressed TxDOT I-beams. This report also provides design guidelines and recommendations for producing, testing and casting steel fiber reinforced concrete mixes for successful application in the end zones of prestressed concrete I-beams.

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