The influence of roughness and bond area size on bond shear performance between UHPC and normal strength concrete

This study investigates bond shear strength and enhancement mechanism between ultra-high performance concrete (UHPC) and normal strength concrete (NSC). A bi-surface shear test was adopted to evaluate bond strength. The factors of bond area and surface roughness were considered. The interface porosity, crack width, and microhardness were performed to reveal the bond enhancement mechanism of UHPC-NSC. The results indicated that UHPC-NSC has high bond strength and causes substrate failure. The peak load is used to calculate bond strength, which is conservative. The substrate becomes relatively stronger in the whole system to measure the real bond strength by the method of decreasing bond area. UHPC-NSC bond strength could increase by 28.3 % using the bi-surface test method of decreasing bond area. Roughness could enhance bond strength by 66.4 %-105.1 %. The bond strength of UHPC-NSC is higher than that of NSC-NSC by 63.9 %-92.8 %. The overlays transition zone (OTZ) porosity of UHPC-NSC is 69.7-86.9 % lower than that of NSC-NSC. The OTZ microhardness of the UHPC-NSC is higher than that of NSC-NSC by 64.0 %-92.8 %. It means the hydration products of the UHPC-NSC interface are dense. There is a limit to improving the interface bonding properties by roughness due to the substrate near repair surface failure. It is feasible to disperse the stress at the interface to increase the bearing capacity of the repair specimen, such as planting rebar at the repair interface.