Direct shear strength of UHPC considering size effect: Theoretical model and experimental verification

The direct shear performance of UHPC is of great importance for UHPC members in some practical cases, such as shear key and corbels. However, there are seldom available theoretical methods in existing studies for determining the direct shear strength of UHPC. A theoretical model was developed in this study to predict the direct shear strength of UHPC which was supposed to be the superposition of those contributions of UHPC matrix and fibers. In this model, the shear contribution of fibers was theoretically determined using the average contribution rate of fibers in the force direction, i.e. the fiber effective factor Kf. This coefficient comprehensively revealed the effects of fiber orientation distribution, fiber efficiency and effective pull-out length. The fiber effective factors Kf in zones constrained by no, one-side and two-side moulds are 0.283, 0.408 and 0.641, respectively. The size effect caused by the wall effect of formwork was also considered in the determination of shear contribution of fibers based on Kf. The direct shear test on 14 UHPC Z-shape specimens was performed to verify the proposed model. Test results indicated that both the fiber content and the specimen size had certain influence on the direct shear strength of UHPC. The comparison with the predicted values showed that the proposed model provided relatively good predictions of the direct shear strength of UHPC.