Bonding performance between high-performance concrete and normal strength concrete based on splitting-tensile test：Effect of size and substrate surface roughness

The study investigates the influence of substrate surface roughness and size effect on the bonding performance of high-performance concrete (HPC) and normal strength concrete (NSC). The bonding performance of the composite specimens was evaluated by splitting tensile strength and DIC technology was employed to monitor crack development. Furthermore, the size effect mechanism of tensile bonding strength considering the influence of surface roughness was revealed combined with microscopic experiments. Results show that the combination of 3D scanning and digital image processing methods enabled a comprehensive evaluation of substrate roughness from both local and overall feature dimensions. The increase in substrate roughness will only significantly improve the repair effectiveness within a certain range. The optimal repair effectiveness can reach up to 194.08 %. Specimens with smooth surface showed single crack propagation path with gradual strain evolution, whereas specimens with rough surfaces exhibited tortuous crack paths with abrupt strain surges. In addition, the size effect of ordinary concrete was dramatically smaller than that of composite specimen. As substrate roughness increased, the size effect of composite specimens first intensified and then weakened. Specifically, the size effect parameters of type B and type C specimens were 2.12 and 1.14 times those of type A specimens, respectively. The substrate roughness influenced the size effect by altering the characteristic structures of the bonding regions. Based on the Weibull statistical size effect theory and Bažant energy size effect theory, the calculation formula for the size effect law parameters of the splitting tensile strength of composite specimens was established.