Shear Behavior of Ultrahigh-Performance Concretes

Ultrahigh-performance concretes (UHPCs) possess significantly higher shear strength compared to conventional concrete. However, current design standards/recommendations do not account for this gain in properties. Additionally, existing studies in the literature for estimating the direct shear properties of UHPC disregard the effect of the fiber material and the matrix properties. In this study, push-off tests were performed on six different UHPC materials to investigate the effects of material properties and constituents (matrix compositions, fiber materials, and volume fractions) and specimen size on their direct shear behaviors. A direct relationship was obtained between the shear cracking strength of UHPC composite and the shear fracture strength of the UHPC matrix, independent of the fiber type and volume fraction. The postcracking shear capacity of UHPC depended mainly on the crack bridging by the fibers through tension and dowel action, and therefore, related strongly with the fiber volume fraction and fiber material. An empirical model was proposed to estimate the direct shear strength of UHPC with steel fibers based on the shear cracking strength of the matrix and the postcracking contribution from the fibers.