AbstractA total of 65 Z-shaped ultrahigh-performance concrete (UHPC) specimens with monolithic interfaces, flat-wet-joint interfaces (roughened with a high-pressure water jet), and keyed-wet-joint interfaces were tested under the classical push-off test setup. The influences of steel fiber properties, keyed-joint shapes, and confining stress on the shear strength of the UHPC specimens are discussed. A high-precision equation for predicting the shear capacity of the UHPC specimens with monolithic interfaces, flat-wet-joint interfaces, and keyed-wet-joint interfaces is proposed and verified by experimental results. The test results indicate that steel fibers had a significantly positive effect on improving the shear strength of the UHPC specimens. For the flat-wet-joint specimens, the shear strength increased approximately linearly as the fiber content increased. Using long and hooked-end fibers improved its shear strength. For the keyed-wet-joint specimens, the shear strength improved almost linearly with confining stress, whereas the keyed-joint shape had little influence. The strength reduction factor (the ratio of the ultimate shear strength of the flat-wet-joint interface to that of the monolithic interface) increased with the fiber volume fraction. A relationship between the strength reduction factor and the fiber characteristic parameter is proposed.