High-Strength Concrete Prestressed Bridge Girders: Long Term and Flexural Behavior


Theresa Ahlborn, Catherine French, Carol Shield

November 2000

Report no. MnDOT 2000-32

This project involved the construction of two long-span, high-strength composite prestressed bridge girders to investigate their structural behavior and the adequacy of American Association of State Highway and Transportation Officials (AASHTO) 1993 provisions for their design. The scope of the research included examining prestress losses, transfer length, cyclic load response, and ultimate flexural strength. The research revealed that prestress losses could not be determined solely from strain gage instrumentation. Foil strain gages attached to the strand cannot measure losses caused by relaxation and drift over time. Vibrating wire strain gages embedded in the concrete cannot account for losses in the prestressing strand before the concrete hardens. Researchers used vibrating wire gage data to measure the prestress losses incurred since the time of strand release. To back-calculate the losses that occur before release, researchers used total prestress losses determined from flexural cracking and crack reopening loads. The measured prestress losses were found to be much higher than those predicted by analytical methods. Prestress losses predicted by AASHTO not only ignore concrete stress before release but also overestimate the high-strength concrete modulus, leading to lower initial losses, and overpredict the creep and shrinkage, leading to higher long-term losses.

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