Analytical Modeling of Fiber Reinforced Post-Tensioned Concrete Anchorage Zones

The use of post-tensioning in bridge girders causes tensile bursting stresses to occur some distance ahead of the anchorage device in a region known as the general zone. Large amounts of mild steel reinforcement are placed in this area of the bridge girder in order to resist these highly tensile stresses. This causes congestion in the area of the steel and poses difficulty during concrete placement. The objectives of this study were to determine the feasibility of reducing the mild steel reinforcement by adding fibers to the general zone and to determine the impacts of doing so. Fiber reinforced concrete (FRC) improves the mechanical properties of non-fibrous concrete. So it is expected to support the proposed reduction of mild steel reinforcement in the post-tensioned anchorage zone. The first phase of the study involved researching past studies on the use of FRC in order to determine the material and mechanical properties pertaining to the fibers. Steel fiber was deemed to be the most useful for enhancement of non-fibrous concrete properties. The second phase of the study was to determine a realistic and reasonable specimen for FRC application. The pier segment of a currently used bridge in Florida was chosen. This selection was based on having common and less complex geometry. After selection, information was gathered about the segment such as the volume of concrete, mild steel reinforcement details, and post-tensioning system details. In the final phase of this study, a finite element model was developed for the segment using design required mild steel reinforcement. Using the initial model, duplicate models were analyzed with varying steel fiber volumes. The theoretical results indicate that a maximum reduction of 65% of the design mild steel reinforcement can occur when replaced by 0.50% steel fiber to the concrete volume of the general zone. However, it is recommended that a mild steel reinforcement reduction of 50% be replaced by 0.50% steel fiber in order to stay conservative and safe. It was also observed that higher volumes of steel fiber could increase stresses in the general zone. Therefore, it is recommended that experimental testing of these procedures be done for complete verification.