A Comparison of Time Dependent Prestress Losses in a Two-Span, Prestressed Concrete Bridge: Fid-Bau Portal

A Comparison of Time Dependent Prestress Losses in a Two-Span, Prestressed Concrete Bridge

Kukay, Brian / Barr, Paul J. / Halling, Marvin W.

This paper describes the measured behavior of a two span, live-load continuous bridge made with precast, prestressed, self-consolidated concrete girders. The lengths of each span are 27.2 m (89 ft 3 in.). The self consolidated concrete was considered high-performance, because of its high early compressive strength of 69.5 MPa (10.1 ksi) at release and 76.2 MPa (11.1 ksi) at 28 days. By using this high strength, self-consolidating concrete, the bridge designer is able to reduce the number of girder lines. These girders are among the first to be constructed in state of Utah using self-consolidating concrete. In order to monitor the behavior of the bridge, instrumentation was embedded in four of the twelve girders. Two interior girders in each span were chosen as representative girders. The instrumentation consists of vibrating wire strain gages with integral thermistors. The gages were placed at the centroid of the prestressing strands, girder centroid and composite girder centroid. Each of the gages has been monitored continuously since the time of casting. Data collected from each girder encompass the following phases: casting, de-stressing, curing, and deck placement. The measured changes in strain have been used to determine prestress loss values for each of the instrumented girders. These measured values are compared with predictive values using a newly proposed NCHRP method. The differences between the measured and predicted prestress losses are compared and recommendations for designers are provided.