Influence of surface treatments and test methods on tensile strength of UHPC-NC interface bond

Accurately determining the tensile strength of interface bond in cementitious bi-layer hybrid materials and establishing the relationship between splitting tensile strength and direct tensile strength under different interface treatments are crucial in interface characterization and engineering application. However, research on the relationship between interface splitting tensile strength and direct tensile strength is limited. In this study, based on an improved direct tension test apparatus, the influence and bond failure mechanism of different treatment methods (i.e., smooth, high-pressure water jet, sandblasting, and chiseling) for ultra-high performance concrete (UHPC) and normal strength concrete (NC) interface bond are investigated through the splitting tension and direct tension tests. The results show that the improved direct tension test apparatus avoids the initial damage to the interface which may be caused by core sampling and is easy to operate with low variation of test results. The roughness of substrate surface is directly proportional to the tensile bond strength. For both the splitting tensile and direct tensile bond strengths, the order from the highest to lowest is: chiseling > sandblasting > high-pressure water jet > smooth surface, primarily due to the increased roughness, thus enhancing the effective contact area at the interface and strengthening the chemical bonding and mechanical interlocking at the interface. The tensile bond strength alone cannot manifest the failure mode, and the failure mode is jointly determined by the specimen stress level, crack morphology, and microstructure. In addition, the prediction models of splitting tensile and direct tensile strength of UHPC-NC interfaces are proposed, from which their relationship is established by considering the Weibull distribution with good reliability and accuracy. The present study provides viable test methods and models for tensile strength characterization of hybrid material interface bond.