Unbonded Pre-tensioned Bridge Columns with Hybrid Fiber-Reinforced Concrete Shells
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Many bridges in the United States are getting old and will need to be replaced in the near future. If these bridges are constructed with conventional cast-in-place methods, this construction will cause traffic congestion, which is a costly problem. Furthermore, these cast-in-place systems are susceptible to earthquake-induced damage, such as bar buckling, bar fracture and residual displacements. A new pre-tensioned precast bent system has been developed to meet these challenges. The system consists of precast technology to accelerate the bridge construction, unbonded pre-tensioning to minimize residual displacements, and high-performance materials that extend the bridge durability. Davis et al. (2012) tested the new system using only conventional concrete. They found out that pre-tensioning improves system's re-centering capabilities, but it results in earlier bar buckling and bar fracture than in previously tested reinforced concrete columns (Pang et al. 2008, Haraldsson et al. 2012). Two columns were designed and tested in the University of Washington Structural Laboratory. In the plastic-hinge region of the columns a very ductile concrete shell was added. The shell was made of a hybrid fiber reinforced concrete (HyFRC, developed by Prof. Ostertag at U.C. Berkeley) containing both polymer and steel fibers. The main goal of adding the shell was to delay spalling and buckling of the longitudinal reinforcement bars. One of the columns was the same as one of the columns tested by Davis with only the addition of HyFRC shell. The other column had a HyFRC shell in the plastic hinge region and stainless steel reinforcement bars as longitudinal reinforcement instead of regular black steel rebars. The addition of the stainless steel rebar was expected to increase the ductility of the system and minimize the corrosion susceptibility. The tests showed that the HyFRC delayed the concrete spalling, and to a limited extent, the buckling of the longitudinal bars. The main benefits of having the HyFRC shell was that the columns kept 80% of its strength at 10% drift ratio, which was much higher than the conventional concrete specimens tested by Davis et al. (2012). The response of the stainless steel column was comparable to the black steel column, the main difference being that the stainless steel column was stronger, because the stainless steel was stronger than the black steel.
- Civil engineering