The paper presents a model, based on nonlinear fracture mechanics, for analysing crack propagation in quasi-brittle materials, such as concrete. The work is limited to two-dimensions, and therefore, the fracture modes of interest are mode I (pure tension) and mode II (pure shear). The constitutive model has been implemented in the context of the finite element method using interface elements. The fracture is simulated through a discrete crack represented by the interface with a cohesive crack stress-separation relation derived from the model, which is based on a fracture criterion, together with a flow rule and a softening law. The model is used for simulating results from an experimental study on beams with centric and eccentric notches of high and normal strength concretes, and explaining other test results available in the literature.

The fracture response of brittle materials like concrete can be characterised using modified linear elastic fracture mechanics models, such as the rising resistance curve (R-curve). In this study, the R-curve, determined by utilising the experimental response in the notched beam test with mode I fracture, is proposed as ameasure of the toughness of fibre-reinforced concretes (FRCs). The unambiguity of the R-curve obtained is assessed by comparing the predicted flexural response of specimens of different geometries with experimental data. The variation in the R-curves with the dosage of steel and polymer fibres is also discussed.