Push-out tests and theoretical models of steel-UHPC composite members considering interfacial bonding strength

Recently, ultra-high performance concrete (UHPC) has been incorporated into steel-concrete (SC) composite structures, where the mechanical behaviors at their interfaces play a crucial role in the overall stability. Previous research often overlooks the importance of bonding strength between steel and concrete. To address this gap, this paper introduces a novel test method to measure bonding strength. Twelve push-out specimens were tested to evaluate their mechanical behaviors and failure modes. Three distinct failure modes were identified: concrete cracking failure, connector fracture failure, and a combination of both. The results show that UHPC significantly alters failure modes, shifting from concrete cracking to connector fracture. It also improves shear capacity, increases ductility, and reduces the initial slip modulus. Notably, the bonding strength at the interface contributes to 30 %-40 % of the shear capacity. Finite element models were developed and validated against experimental results. This combination of testing and simulation reveals the influence of UHPC, interfacial bonding strength, and different shear connectors on the structural behavior. Additionally, a parametric study was performed to determine the damage length of concrete near shear connectors, aiding in the development of a theoretical model. Consequently, theoretical models for predicting shear capacity at the steel-concrete interface were developed, specifically considering the interfacial bonding strength. The accuracy of these models has been validated, confirming the applicability in practical engineering design.