Title

SHEAR STRENGTH OF HIGH STRENGTH CONCRETE BEAMS

Date of Completion

January 1984

Keywords

Engineering, Civil

Degree

Ph.D.

Abstract

Structures have been built using concretes with compressive strengths up to 11,000 psi, even though little data is available on the properties of such concrete. Engineers have designed these structures by assuming that the design rules and procedures developed for low strength concrete are also satisfactory for the new, higher strengths. This work investigated the applicability of the current ACI shear design provisions to concrete strengths in excess of 10,000 psi.^ Thirty-nine reinforced concrete beams with and without stirrups were tested to determine their diagonal cracking and ultimate shear capacities. In beams without stirrups the shear span/depth ratio was held constant at 3.6, 2.5, or 1.5 while the concrete strength was varied from 3000 psi to 15000 psi in otherwise identical beams. Concrete strength was also varied in beams with shear span/depth ratios of 3.6 and having stirrups with a shear capacity (V(,s)/bd) of 50, 100, or 150 psi. To study the influence of concrete strength on the shear transferred by aggregate interlock two additional series of beams with stirrups were cast with thin plates inserted along the expected inclined crack path to produce a smooth crack surface. Design equations based on the test data from this study are presented. Based on the results of the experimental study, it was concluded that the ACI equations for predicting shear capacity of slender beams with or without stirrups and deep beams without stirrups are conservative throughout the entire range of the concrete strengths tested although the factor of safety against shear failures for slender beams decreases as the concrete strength increases. The ultimate shear capacity of beams increases with the concrete strength, stirrup strength, and decreasing a/d ratios. The shear contribution due to aggregate interlock was significantly affected by the concrete strength. Its contribution varied from about 50% of the total concrete contribution at 3,000 psi to 0% at 9,000 psi and higher. The dowel action contribution increased significantly from about 30% at 3,000 psi to 75% at 13,000 psi. The contribution from the compression zone stayed fairly constant at about 25% with increasing compressive strength. ^