Study on the influence of roughness and coarse aggregate area on UHPC-NC interface bonding performance

The aim of this study is to clarify the effect of roughening the surface of normal concrete (NC) substrates on the interfacial bonding performance, and the effect of substrate surface roughness and coarse aggregate area on the interfacial bonding performance was investigated by digital image technology. The results showed that the shear damage model of the roughened ultra-high performance concrete (UHPC) and NC composite specimens was divided into linear elasticity, yielding, and damage stages; the failure loads of the gouge interface group and the high-pressure water jet interface group were elevated by 18.3–33.9% and 43.0–140.0%, respectively, compared with those of the smooth interface group. In addition, the chiselled interface group and high-pressure water jetting interface group obtained an overall failure load of 44.2–50.0% and 53.4–90.0%, respectively; the exposed area of coarse aggregate on the substrate surface and the roughness of the concrete surface of the substrate showed a linear correlation with the interfacial shear strength, and the presence of coarse aggregate provided more mechanical anchorage points at the interface and increased the contact area with the UHPC. Substrate surface roughness increased the probability of steel fibre insertion into the groove of the substrate, and the formation of pin effect and bridge effect at the interface improved the shear slip capacity and bond strength at interface failure; substrate concrete surface roughness is a key parameter affecting the secondary damage of the UHPC-NC interface, and the degree of interface damage increased linearly with the increase of roughness.