Prestress losses in pretensioned high-strength concrete bridge girders

The use of high-strength concrete for pretensioned concrete bridge girders has become an acceptable practice by a number of state highway agencies because of its technical and economic advantages. It provides designers the flexibility to design bridges with longer spans and wider girder spacing. The design of pretensioned concrete girders requires accurate estimates of losses in the tensile stress of prestressing strands. These prestress losses are affected by factors such as level of prestress, concrete compressive strength, material properties, mix design, curing, and service exposure conditions.; Recent researches have indicated that the current AASHTO Specifications, developed for normal-strength concrete, may not provide reliable estimates for high-strength concrete bridge girders. Thus, research is needed to evaluate the applicability of the current provisions and to develop guidelines for estimating prestress losses for high-strength concrete.; Available literature, design specifications, research findings, and current domestic and foreign practices were collected and reviewed. Based on the information gathered, the range of concrete conditions for which the present AASHTO Specifications are applicable is between 4 and 6 kips per square inch.; A detailed work plan for the experimental program was developed to cover material properties and other factors that affect prestress losses in pretensioned high-strength concrete girders. The investigation also included full-scale girder testing that consisted of instrumenting and monitoring prestress losses in seven full-scale girders at four different states. The measurements were taken at various stages of the girder and the composite section service life.; This research revealed many promising areas that were researched and the theoretical analysis developed a closed-form solution that accounted for interaction between concrete and prestressing strands, and included the composite action effect.; A Detailed method was developed for estimating prestress losses in a format suitable for adoption by AASHTO. In addition, an approximate method was also introduced for easy use by bridge designers.