Estimation of Long-Term Prestress Losses in Post-Tensioned Girders

Post-tensioned concrete girders are commonly used in bridge structures. Long-term prestress losses due to the creep and shrinkage of concrete in post-tensioned girders are smaller than those in pretensioned girders because of the higher amount of non-prestressed steel present in the former, and also the fact that post-tensioning can take place a long while after a girder has been cast and when the concrete has reached a mature age. Two methods that can be used to estimate long-term prestressed losses in post-tensioned girders are presented in this paper. One is a refined method and the other is a simplified method The accuracy of the methods has been verified by field data collected from two bridge structures monitored for long-term losses. A numerical parametric study conducted with these methods has shown that the concrete strength, the amount of non-prestressed steel, the relative humidity, and the age of concrete at post-tensioning are equally important for the assessment of prestress losses. Because the post-tensioning of a bridge girder in a long multi-span bridge can take place at a concrete age between 100 and 200 days, when an appreciable amount of shrinkage has already occurred, an accurate account of the incremental shrinkage strain from the time of post-tensioning is as important as that of the ultimate shrinkage strain. The calculation of this incremental strain relies on an accurate equation to predict the increase of shrinkage strain with time over the entire shrinkage period.