Bond behavior and multi-scale enhancement mechanisms of UHPC–NC composite interfaces

Interfacial bond performance between ultra-high-performance concrete (UHPC) overlays and normal concrete (NC) substrates is critical to concrete repair and strengthening. This study investigates the bond behavior and enhancement mechanisms of UHPC–NC interfaces considering UHPC placement method, overlay thickness, and interface roughness. UHPC–NC bonded specimens with cast or sprayed overlays were tested using slant shear and splitting tensile tests, while digital image correlation was used to monitor deformation and damage evolution. Finite element analysis was conducted to interpret thickness-induced load transfer and strain localization under slant shear loading, and SEM/EDS, nanoindentation, and X-ray computed tomography were used to reveal multi-scale mechanisms. Compared with cast overlays, sprayed overlays increased the slant shear and splitting tensile bond strengths by 12.9%–32.1% and 20.0%–35.5%, respectively, under comparable substrate and interface conditions. This improvement is mainly attributed to impact-induced compaction and enhanced paste penetration during spraying, as supported by reduced interfacial defects, improved micromechanical continuity, and lower interfacial zone porosity. Increasing the overlay thickness from 75 mm to 150 mm improved the slant shear bond strength by up to 27.8%, mainly by broadening the load-transfer path and delaying localized strain development near the interface. Increasing interface roughness further enhanced mechanical interlocking and promoted the transition from interfacial debonding to mixed interfacial–substrate or substrate-involved failure. These findings provide multi-scale evidence for understanding and optimizing UHPC–NC interfacial bond performance in concrete repair and strengthening.