Estimation of shear behaviour between ultra-high-performance concrete and normal-strength concrete under direct shear load

This study investigated shear-transfer behaviour through direct shear tests on 72 Z-shaped composite specimens. The key parameters included the compressive strength of ultra-high-performance concrete (UHPC; 82–166 MPa), the volume fraction of steel fibres (0–3%) and interface treatment methods (chipping and grooving). The importance of influencing factors was identified through correlation analysis of the test results: interface roughness was the most dominant, followed by the steel fibre volume fraction, with the UHPC compressive strength being the least influential among the three. Based on the test results, a novel predictive model for interfacial shear strength was developed, which innovatively incorporated an influence coefficient that accounted for the groove width in grooved interfaces. The proposed model demonstrated superior accuracy compared to several existing codes and models when validated against an extensive database. This work provides engineers with a validated tool for optimising the design of UHPC-strengthened structures, emphasising the cost-effectiveness of interface treatment and steel fibre utilisation over merely increasing concrete strength.