Shear Strength of Composite Beams with Dual Concrete Strengths

Abstract

In the design of precast concrete (PC) composite members, PC is often used in combination with cast-in-place (CIP) concrete of a different concrete strength. This present study investigates the vertical shear strength of such composite beams with two different concrete strengths (24 and 60 MPa [3.5 and 8.7 ksi]). The test variables were the ratio of the cross-sectional areas of two concretes, the flexural reinforcement ratio, and the shear span-depth ratio. Shear reinforcement was not used. When the average concrete strength estimated from the area ratio of the two forms of concrete was used, ACI 318-11 safely estimated the vertical shear strengths of all the specimens, except for monolithic and composite beams having high-strength concrete and a low flexural bar ratio. On the other hand, ACI 318-11 significantly underestimated horizontal shear strengths in the interfaces between the two concrete strengths. These test results were used to investigate the effects of design parameters, particularly the vertical shear strength contributions of the tension zone and compression zone.