Influence of interfacial parameters and testing methods on UHPC–NSC bond strength: Slant shear vs. direct tensile testing

The bond strength between ultra-high-performance concrete (UHPC) and normal-strength concrete (NSC) is difficult to measure owing to the strong bond performance in a rough repair surface, particularly in the substrate failure mode. It is worth developing a relatively simple testing method to measure the bond strength of UHPC–NSC. A modified direct tensile test was conducted to measure the bond strength in this study. Moreover, the effects of interfacial parameters and testing methods on bond strength between UHPC and NSC were investigated. A total of 135 composite specimens were fabricated to measure the bond strength. Slant shear and direct tensile tests were performed to obtain the UHPC–NSC bond strength. Three types of substrate surfaces were obtained using a high-pressure water-treated substrate at different times. Moreover, the effectiveness of the modified direct tensile test method to measure the UHPC–NSC bond strength was verified. The results suggested that the bond failure mode of the NSC substrate failed for a rough surface (average sand-filling depth equal to or larger than 0.63 mm). The results of all the methods to evaluate the surface roughness of the substrate were consistent. Additionally, the direct tensile bond strength had a higher coefficient of variation than that of the modified direct tensile test method. Thus, the modified direct tensile test method, rather than the direct tensile bond strength, could reflect the effect of roughness on bond performance, specifically on rough surfaces.