Multiscale investigation of interface bonding properties between ultra-high-performance concrete and normal-strength concrete: Effects of substrate moisture content and substrate strength

Ultra-high-performance concrete (UHPC) is a highly promising repair material for deteriorated concrete structures owing to its superior mechanical properties and durability. However, the combined effects of substrate moisture content and strength on the interfacial bond behavior between UHPC and normal-strength concrete (NSC) remain insufficiently investigated systematically. This study comprehensively examines the influences of these two critical factors on the macroscopic mechanical properties, microstructure, and phase composition of the UHPC-NSC interface, and proposes an interfacial bonding mechanism. The results demonstrate that interfacial bond strength exhibits a consistent trend of initial increase followed by subsequent decrease with increasing substrate moisture content, with an ideal moisture content range that significantly enhances interfacial bond performance. Increasing substrate strength consistently improves bond behavior. Digital image processing was employed to quantitatively characterize failure surface roughness, which is confirmed as an effective indicator of interfacial bond strength, representing an objective, practical, and convenient characterization approach. Grey correlation analysis reveals that the moisture content of the substrate exerts a more sensitive influence on the interfacial bonding strength. Furthermore, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses indicate optimal moisture content and higher substrate strength promote hydration in the overlay transition zone (OTZ). Abundant hydration products fill interfacial voids, forming robust mechanical interlocking and chemical bonding. For UHPC-based repair of NSC structures, the optimal substrate moisture content is recommended to be maintained at 50~75%, and the repair effectiveness improves progressively with increasing substrate strength.