High strength prestressed concrete bridge girders

Two long-span high-strength composite prestressed bridge girders were constructed 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 this research included investigation of prestress losses, transfer length, cyclic load response and ultimate flexural strength. The non-composite span to depth ratio was 35 for the 45 in. deep girders. Twenty-eight day concrete compressive strengths exceeded 11,000 psi for the girders and 4,500 psi for the composite decks.; It was found that prestress losses could not be determined solely from strain gage instrumentation. Foil strain gages attached to the strand cannot measure losses due to 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. The vibrating wire gage data were used to measure the prestress losses incurred since the time of strand release. To back-calculate the losses incurred prior to release, total prestress losses determined from flexural cracking and crack reopening loads were used. The measured prestress losses were found to be much higher than those predicted by analytical methods. The major source of the difference was attributed to the higher experimentally-determined initial losses. The analytical methods did not account for the existence of tensile stresses in the concrete prior to release.; Prestress losses predicted by AASHTO not only ignore concrete stress prior to 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).; The AASHTO relationship for transfer length was found to be conservative for large diameter prestressing strands (0.6 in.) spaced at 2 in. on center. Load-deflection responses to simulated cyclic HS25 truck load and overload levels were predictable, and no stiffness degradation was noted after more than 2.4 million cycles. Ultimate flexural strength, controlled by the normal strength deck, was conservatively predicted by AASHTO design specifications.