Influence of various factors on the bonding properties of interface between LUHPC and NC based on cohesion and friction coefficient

Owing to its superior material properties, lightweight UHPC (LUHPC) exhibits great potential as a repair and reinforcement material for old normal concrete (NC). However, research on the bonding properties of the interface between LUHPC and NC remains in its infancy. In particular, the influence of various important factors on its cohesion and friction coefficient is still unclear. Therefore, this study conducted direct shear and multi-angle slant shear on LUHPC-NC interface with varying interfacial parameters. The Mohr-Coulomb equation was decoupled based on the results to determine the interfacial cohesion and friction coefficient under varying interfacial parameters. The results indicated that the interfacial cohesion could be enhanced by increasing the water saturation of the old concrete, appropriately increasing the interfacial roughness (not exceeding 2.77 mm), or moderately increasing the lightweight aggregate (LWA) content (not exceeding 75 %). The corresponding improvement reached 32.0 %, 108.4 % and 8.0 %, respectively. The interfacial friction coefficient could be improved by reducing the water saturation of the old concrete, increasing the interfacial roughness, or appropriately elevating the LWA content (not exceeding 75 %). The corresponding improvement reached 5.8 %, 26.0 % and 11.0 %, respectively. Based on these findings, equations for calculating interfacial cohesion and friction coefficient considering these three factors were developed. Moreover, BSE and EDS tests further revealed that integrity of the interfacial microstructure and the proportion of C-S-H and CH distribution zone in the overlay transition zone were also affected by the interfacial parameters, which was the micro-mechanisms of evolution of interfacial cohesion and friction coefficient.