Influence of substrate properties and interfacial roughness on static and dynamic tensile behaviour of rock-shotcrete interface from macro and micro views

The instability of shotcreted structures, e.g., the underground tunnels after shotcreting, often results from the tensile failure of the rock-concrete interface. However, the understanding of the properties, e.g., the failure mode and the failure mechanism, of the rock-concrete interface against tension, in particular dynamic tension, is still at its infancy. In this study, static and dynamic Brazilian disc tests were conducted to investigate the effect of rock type, concrete strength, concrete admixture and interface roughness on the adhesive properties of rock-concrete specimens from macro and micro views with the combination of high-speed photography, digital image correlation, laser scanning and SEM techniques. The results showed that rock type, concrete strength and admixture determine the nominal tensile strength of rock-concrete interface through changing the adhesive force between rock and concrete. Interfacial adhesive properties are not dependent on rock strength but on rock physical properties, e.g., constituent grains. Increasing concrete strength as well as adding concrete admixture could improve the adhesive force between rock and concrete except for adding basalt fibre under dynamic tension. Interface roughness determines the bearing capacity of rock-concrete specimen through influencing the mechanical interaction of the interface. The nominal tensile strength of rock-concrete specimen increases with interface roughness degree. The failure mode of rock-concrete specimen depends on the interface type and sawtooth dip angle, and the bearing capacity of rock-concrete specimens varies with failure mode. In addition, from a micro-view, the compactness degree of the failure surface and the content of adhesive materials such as calcium silicate hydrate and hydroxide carbon determine the adhesive behaviour between rock and concrete. The findings in this study could facilitate better understanding of the adhesive behaviour of rock-concrete structures and are of great engineering significance to the maintenance and reinforcement of shotcreted structures, in particular tunnels after shotcreting.