Effect of Allowable Compressive Stress at Release on Prestress Losses and on the Performance of Precast, Prestressed Concrete Bridge Girders

摘要

align="left">This paper presents the results of a research program that examined prestress losses in high-performance concrete bridge girders and the effects, if any, of compressive stresses at release exceeding the? current allowable stress limit of 0.60 f '_ci. Four I-shaped girders were cast and tested with compressive release stresses ranging from 0.57f '_ci to? 0.82f '_ci. Two ofthe four girders were made with air-entrained concrete; prestress losses were measured on these as well and compared with losses in girders made with non-air-entrained concrete. The measured prestress losses were then compared with prestress? losses estimated using: (1) the 2004 American Association of Highway Transportation Officials load-resistant factor design (AASHTO LRFD) Bridge Design Specifications (refined method), (2) the PC Design Handbook method described by Zia et al., and (3) the method proposed in the National Cooperative Highway Research Program (NCHRP) Report 496 (detailed method). Ofthe three methods to estimate losses, the NCHRP Report 496’s “Detailed Method for Estimating Prestress Losses” most accurately predicted the measured losses, followed by the Zia et al. equations, and then the 2004 AASHTO LRFD equations. A second objective of the research was to provide additional data measuring? prestress losses for cases where the actual compressive stresses atrelease exceed the allowable? compressive stresses at release. The research results support increasing the allowable compressive stress at release from 0.60f '_ci to 0.70 f '_ci.