Interface bonding performance and failure mechanism of UHPC-NSC composites with variable grooved substrates

This study investigated the interfacial bonding performance between ultra-high-performance concrete (UHPC) and normal-strength concrete (NSC) under the bi-surface shear test. Composite specimens were fabricated by casting UHPC, reinforced with either steel (ST) or polyoxymethylene (POM) fibers, onto NSC substrates with prefabricated grooves of varying sizes. The coupling effects of fiber type and keyway geometry formed by grooving on the mechanical response were examined, with the failure process concurrently monitored using digital image correlation (DIC) and acoustic emission (AE) techniques. The results indicated that the peak load increased with larger keyways on the interface. Compared to the consistent adhesive failure observed in the POM group, specimens with steel fibers primarily exhibited a mixed failure mode. An asynchronous crack development in the bonding area in the ST group was observed, which was consistent with the fluctuation of the load-displacement curve. Moreover, the cracks along the periphery of the keyways led to stress concentration and further weakened the keyways' availability, thereby compromising the interfacial shear capacity. The findings highlighted that optimizing the synergy between fiber type in UHPC and keyway parameters of substrate is vital for enhancing the bonding performance of the repair system.