The design of rock foundations may need to evaluate shear strength of rock-concrete interface by performing laboratory direct shear tests. A series of tests were carried out on specimens for different values of the spacing free of encapsulating material about the shear plane and different values of normal stress. Different kinds of failure were observed and not necessarily at the rock-concrete interface. The spacing free of encapsulating material appeared to be of significant influence on the test results. A mechanical analysis is presented which illustrate the influence of the loading conditions in the direct shear box on the failure mechanism.
The design of rock foundations subjected to significant lateral loads requires defining the value of the shear strength of the interface between rock and cast in place concrete. Verifying the stability against sliding of strip footings subjected to inclined loads (SETRA, 2009) or concrete gravity dams (CFBR, 2013) are common examples where design modelling approaches imply the shear strength, sometimes the deformation behavior, of the rock-concrete interface, more or less assumed as a failure plane.
Defining the behavior of rock-concrete interfaces relates to tribology. Many studies have been conducted to improve the representation of this behavior (e.g., Johnston & Lam, 1989; Kodikara & Johnston, 1994; Gu, Seidel & Haberfield, 2003; Saiang, Malmgren & Nordlund, 2005; Fishman, 2008; Eltervaag, 2013; Gutierrez, 2013), raising many questions. The behavior appeared to be influenced by many factors such as the geometry of the interface which is not planar, the roughness of the natural rock, the rock mineralogy and the concrete formulation. In addition, the study of the shear strength of rock-concrete interfaces involves considering the two different deformation behaviors of both rock and concrete. Finally, the observed failure mechanism is strongly influenced by the initial cohesion due to adhesion between the two materials when concrete is directly cast onto rock.
As a consequence, the various criteria proposed to define failure along discontinuities in rock masses (Patton, 1966; Ladanyi & Archambault, 1969; Barton & Choubey, 1977; Kulatilake, Shou, Huang & Morgan, 1995; Maksimovic, 1996; Zhao, 1997; Grasselli, 2001; Johansson, 2009; Xia, Tang, Xiao & Song, 2014) are not well suited to represent failure of rock-concrete interfaces. Recent works by Moradian (2011), Eltervaag (2013) and Gutierrez (2013) have suggested considering the strength of the different materials and the effect of initial cohesion between materials in the studies of the shear strength of rock-concrete interfaces.
