Interface bonding performance and mechanical properties of UHPC-NC composites

Ultra-high-performance concrete (UHPC) possesses outstanding mechanical properties, making it highly promising for use in prefabricated UHPC–normal concrete (UHPC–NC) composite components, as well as in the repair and strengthening of existing concrete structures. This study investigated three UHPC–NC composite specimens with different casting intervals and evaluated their interfacial bonding performance, along with their static and dynamic mechanical properties. The results showed that the casting interval had a pronounced influence on interfacial bonding. Specimens cast within 30 min exhibited markedly superior bonding performance compared with those produced at longer intervals. The mechanical properties of UHPC–NC composites were positively correlated with interfacial bonding strength. Stronger bonding corresponded to better mechanical performance. Both interfacial bonding strength and static/dynamic tensile–compressive strength demonstrated a significant strain rate effect. Compared with NC, UHPC–NC composites showed substantially enhanced mechanical performance, with improvements becoming more pronounced as interfacial bonding strengthened. These findings suggest that applying UHPC to prefabricated components and in the repair or reinforcement of existing structures can effectively preserve and even enhance structural performance. This study addresses a research gap concerning the interfacial behavior and mechanical properties of UHPC–NC composites under dynamic loading and provides both theoretical insights and experimental evidence to support the practical application of UHPC in engineering. • Microscopic observations and macroscopic mechanical performance experiments both indicate that among the three casting methods, the best pouring interval time of UHPC and NC two-phase interface is 30 min (Method I). • Both compressive strengths and cleavage strength of different types of UHPC-NC composites demonstrated the strain-rate reinforcement effect. • Superior interfacial bonding forms enhances UHPC-NC composite's static/impact mechanical properties. • Performance of UHPC-NC better than NC, verifying the feasibility of prefabrication/retrofitting.