Assessment of the influence of roughness, concrete adhesion and scale effect on the shear strength of concrete-gneiss rock interfaces

The concrete-rock interface is considered one of the most fragile regions of a concrete dam. Therefore, the reliable determination of the shear strength of this region is of paramount importance. However, due to uncertainties and the need for more in-depth studies to obtain reliable parameters representative of field conditions, conservative parameters are often adopted in projects, as they usually do not consider the influence of rock roughness, concrete adhesion and scale effect. Specially, roughness is very important for the real determination of concrete-rock shear strength. However, as obtaining is a difficult task, it is common practice to carry out tests considering smooth surfaces or to use residual resistance, disregarding the roughness effect of the surface on strength. On this context, the present study aimed to evaluate the behavior of connected and supported concrete-gneiss interfaces, considering these different variables. The evaluation was carried out based on direct shear tests performed on prismatic samples of 50 and 100 mm side-length, with different roughness (second order) and concrete traces. In general, significant differences were observed between the results in the peak and residual conditions for each type of interface studied. Regardless of surface roughness, cohesion values in the order of 500 kPa were found for the 50 mm samples, indicating that concrete adhesion had a significant contribution to the cohesive behavior. Roughness had also shown a directly proportional relationship with the friction angle. Furthermore, the sample size directly influenced the definition of strength parameters in the peak condition of bonded concrete-rock interfaces, and the use of samples smaller than 100 mm to define peak strength parameters may result in overestimated values.