Effects of Increasing the Allowable Compressive Stress at Release on the Shear Strength of Prestressed Concrete Girders

In recent years, several research projects have been conducted to study the feasibility of increasing the allowable compressive stress in concrete at prestress transfer, currently defined as 0.60f'ci in the AASHTO LRFD Bridge Design Specifications. Increasing the limit would result in many economical and design benefits for the precast concrete industry, such as increased span lengths and faster turnover of beams in stressing beds. This research study focuses on the effects of increasing the allowable compressive stress at release on the shear strength of prestressed concrete members, a topic which has not yet been explored by past research projects. The current experimental work is funded under TxDOT Project 5197, which initiated in 2004 at the University of Texas at Austin.

In the shear performance evaluation, 18 shear tests were performed. In the shear tests, the beams were loaded to fail in web-shear, with a shear span to depth ratio of 2.22. The diagonal cracking shears and shear capacities were experimentally measured for all specimens tested. All test specimens were TxDOT Type-C highway bridge girders (40-inch deep pretensioned I-beams) and were fabricated by three different precast plants in Texas. The compressive stress at release for the test specimens ranged from 0.57f'ci to 0.77f'ci. The measured cracking shears and shear capacities were compared to the estimated cracking shears and shear capacities, as calculated using ACI 318-08 and AASHTO LRFD (2007), and the effects of higher release stresses on shear strength were evaluated by examining the conservativeness and accuracy of the predictions. Based on the experimental results reported in this thesis, an increase in the allowable maximum compressive stress in concrete in the end regions of prestressed concrete beams at prestress transfer to 0.65f'ci or 0.70f'ci can be justified.