Review of factors affecting bond strength between ultra-high-performance concrete and normal strength substrate

This paper presents a state-of-the-art review and key information on bond strength between ultra-highperformance concrete (UHPC) and normal strength substrate (NSC). The reason for the bond as a weak area in the repair system was analyzed. Several major factors that affect the bond strength between concrete layers such as surface treatment, bond agent, and repair materials types were considered in the discussion. The test methods developed to determine UHPC-NSC bond strength under various load combinations are also described. The findings show that different test methods have different results on the interface bonding properties under the same stress state. The concentration of calcium hydroxide in the interfacial transition zone hurts the bond strength. From the perspective of three parts of the repair system and two stages of interface failure, the mechanical bite force at the macro and micro scales of the interface is improved to enhance bond strength. UHPC combined with NSC can overcome the difference in physical parameters to obtain high interface performance. To promote the research and application of UHPC repair NSC, the internal stress generated by UHPC-NSC in alternating environments and its evaluation of long-term collaborative service performance need to be further studied.