MIXED-MODE FRACTURE OF CONCRETE

Strain-softening and strain-localization in cementitious and ceramic materials can be described by using a cohesive crack model, with closing forces at the crack tip, which represent plasticity, inclusion interlocking, fibre bridging and any kind of non-linear behaviour. In the present paper, the cohesive crack model is extended to mixed mode propagation and an experimental confirmation is provided by testing four-point shear specimens of concrete. A constant crack mouth sliding displacement rate is imposed, so that is is possible to control and detect the snap-back load vs deflection branches. The behaviour of the larger specimens is found to be the brittler. On the other hand, the brittleness in the numerical simulations is controlled through the crack length, which is certainly a monotonic increasing function during the irreversible fracture process. The experimental load vs deflection diagrams as well as the experimental fracture trajectories are caputured satisfactorily by the numerical model. The mixed mode fracture energy results tend to be of the same order of magnitude as the Mode I fracture energy G(F), each elementary crack growth step being produced by an opening mechanism along the curvilinear trajectory. This is particularly true for the larger specimens, where energy dissipation due to friction and interlocking is negligible if compared with the energy dissipated by separation.