Shear Capacity of High Strength Prestressed Concrete Girders

Recent studies have shown that it is commercially feasible to produce prestressed concrete girders utilizing concrete strengths in the 12,000 psi range. However current codes and specification provisions for important structural parameters such as shear strength are largely empirical and are based on tests using concrete strengths less than 6000 psi. This program was undertaken to evaluate the adequacy of current design provisions for shear capacity when applied to high strength concrete girders.

This report summarizes the results of the shear testing of ten pretensioned girder specimens made from concrete with compressive strengths ranging from 10,800 psi to 13,160 psi. Both monolithically cast slabs of high strength concrete and compositely cast slabs of 3300 psi and 5350 psi concrete were utilized. Web reinforcement ratios varied from unreinforced webs and very lightly reinforced webs near current minimum web reinforcement ratios to very heavily reinforced webs near current minimum web reinforcement substantially above current maximum shear capacity limits. The tests indicated that the current maximum shear reinforcement limits could be substantially increased.

In addition to the laboratory tests performed, a comprehensive evaluation of shear tests in high strength concrete girders reported in American literature was carried out. All of the test results were evaluated in comparisons with the current AASHTO/ACI provision, the compression field theory recommendations of the Canadian Code, and the variable inclination truss models proposed in Study 248. All three methods gave generally conservative results for both reinforced and prestressed high strength concrete members. These design methods are acceptable for concrete strengths ranging to at least 12,000 psi. All three design procedures slowed little change in conservatism as a function of concrete strengths. The tests indicated that the current maximum shear reinforcement limits could be substantially increased.