Mode I fracture behaviour of concrete under biaxial loading

Most concrete structures are biaxially loaded when cracking occurs and propagates. A test equipment was developed to evaluate fracture mechanic parameters of concrete, based on the principle of wedge splitting. Notched cubic specimens are tested under stable crack propagation. An additional compressive load application device simulates a homogeneous biaxial state of stress. A force-crack opening displacement diagram is obtained from which the specific fracture energy is calculated. The strain softening behaviour is then evaluated by means of numerical modelling. The approach was applied for biaxially loaded concrete samples with 8, 16 and 32 mm maximum size aggregate (MSA). Based on the experimental data a model is developed and discussed. It is found that the fracture energy changes non-uniformly with increasing compressive stress level, and that interaction of microcracking and aggregate interlocking influences the fracture mechanism.