Transfer Length in Bulb-Tee Girders Constructed with Self-Consolidating Concrete

Abstract

This thesis is focused on the transfer bond behavior of self-consolidating concrete (SCC) relative to conventionally vibrated concrete (CVC) in full-scale, plant-cast, prestressed concrete bulb-tee girders for implementation in Alabama bridges. The study involved twelve girders: six BT-54 girders—three SCC and three CVC—and six BT-72 girders—three SCC and three CVC. All of the concrete used in the study was considered high-strength with values of approximately 8,300 psi at the time of prestress transfer. Unlike the previous phases, for the first time, this study also considered partially debonded strands and their performance in SCC. Two different standard strand diameters were investigated. It was found that the results from the girders cast with SCC produced longer transfer lengths than those cast with CVC. The larger cross-sections and less violent methods of prestress release resulted in shorter transfer lengths. Transfer lengths grew over time. Transfer length expressions given by both ACI and AASHTO standards greatly overestimated the transfer lengths in comparison to the measured values. In general the predicted values were roughly twice those measured in the study primarily because of the high-strength concretes used. In addition, the expressions did not take into account several of the parameters that can have a significant effect on the transfer length, particularly concrete strength. Consequently, an expression proposed by Levy (2007) produced the best overall representation of the trends seen in these girders and in prior related studies.