Experimental and numerical study of the dependency of interface fracture in concrete–rock specimens on mode mixity

• Four-point shear tests are used to investigate fracture of concrete–rock interface.
• Fracture parameters are computed using the scaled boundary finite element method.
• The mode mixity angle influences the failure mode at the concrete–rock interface.
• The fracture toughness increases with increase in the magnitude of mode mixity.
• Strong nonlinearity is observed at the interface when the mode mixity angle is small.

The interface between the concrete and the rock is usually considered the weakest link in concrete structures built on rock foundations. The fracture behaviour at the concrete–rock interface is influenced by many factors e.g. the material properties of the individual constituents, the fracture process zone at the interface and the mode mixity ratio. This paper investigates the dependency of the fracture behaviour of concrete–rock interfaces on the mode mixity ratio using experimental and numerical methods. The experimental program involves four-point-shearing of concrete–rock composite beams. It is designed to test a wide range of mode mixity ratio. Using linear elastic fracture mechanics theory, the fracture toughness and the fracture energy are first quantified in terms of the mode mixity ratio. The scaled boundary finite element method, which is known for its accuracy in modelling fracture, is used to compute the fracture toughness and fracture energy. Next, the crack propagation process of the concrete–rock composite beam is modelled using nonlinear fracture mechanics theory. The scaled boundary finite element method is coupled with interface elements to model the fracture process zone, which is a characteristic of fracture in quasi-brittle materials such as concrete and rock. A revised scaled boundary finite element method formulation using generalized coordinates is used to model the cohesive tractions. The cohesive crack at the interface is assumed to propagate when either the Mode 1 or the Mode 2 stress intensity factors change sign. A simple remeshing algorithm is used to propagate the crack at the interface. The numerical simulations are validated by the experimental measurements. The simulated crack propagation processes are described in terms of the mode mixity ratio.