Effects of Increasing the Allowable Compressive Stress at Release of Presstressed Concrete Girders

In this report, the feasibility of increasing the allowable compressive stress limit at prestress transfer of 0.60f'ci is evaluated. For this purpose, the live-load performance and initial camber of pretensioned beams was investigated. Static-load tests were performed on 24 scaled and 12 full-scale specimens that were subjected to a maximum compressive stress at release ranging from 0.46f'ci to 0.91f'ci. From the comparison of the measured and predicted cracking loads of these test specimens, an increase of the allowable compressive stress to 0.65f'ci was justified. For the specimens subjected to release stresses exceeding 0.65f'ci, premature cracking in flexure was detected due to nonlinear deformation at release and associated microcracking. In addition, an initial camber database of information from 223 pretensioned girders was compiled. The data suggested that increasing 0.60f'ci did not affect the ability to accurately estimate initial camber. However, for a given section, higher cambers were detected as the compressive stress at release increased. Lastly, a simple technique for improving the initial camber estimates of conventional girders was presented as a feasibility study for future work.