Shear mechanical characteristics of monolithic and segmented cast UHPC interfaces: Roles of confining stress and shear reinforcement

The direct shear behavior and governing mechanisms of ultra‐high performance concrete (UHPC) members under the coupled effects of shear reinforcement and confining stress remain poorly characterized. To address this gap, direct shear push‐off tests were conducted on 32 UHPC specimens to evaluate the influence of joint type, reinforcement ratio, and confining stress. Results indicate that confining stress effectively mitigates interface debonding by enhancing interfacial friction. Notably, shear capacity increases exceeded 20% for both flat and keyed wet‐joint specimens, compared to approximately 15% for monolithic cast specimens. The most significant enhancement was observed within the 2–5 MPa confining stress range, with increases of 142.8% and 34.6% for flat and keyed wet‐joint specimens, respectively. Conversely, an excessively high reinforcement ratio (2.1%) reduces the effective shear area, restricting the strength gain to less than 10% and impeding the full synergistic interaction between reinforcement and the UHPC matrix. In contrast, the combination of an optimal reinforcement ratio and moderate confining stress effectively alleviates brittle failure in keyed joints, leading to substantial improvements in both ductility and shear capacity. Furthermore, shear reinforcement effectively sustains and transfers shear forces after cracking in wet‐joint specimens, significantly enhancing the post‐failure residual shear capacity. Residual shear stress is increased by over 69% and 11% for flat and keyed wet‐joint specimens, respectively. Finally, based on the experimental results, a predictive formula for the direct shear capacity of UHPC wet‐joints was developed, incorporating the combined effects of confining stress and shear reinforcement.