| Fracture behavior of quasi-brittle materials is governed by the development of a fracture process zone at the crack tip. The size of the fracture process zone is related to the microstructure of the material, and is not proportional to the size of the structure. This is the source of fracture size effect, which affects ductility, strength and other properties, investigated in the present dissertation. First, a numerical study shows how ductility decreases with structure size and slenderness. Ductility is defined as the ratio of the deflection at stability loss under displacement control to the elastic part of the deflection at maximum load. Second, the size effect on compression fracture of unidirectional kink band propagation is demonstrated, using the equivalent linear elastic fracture mechanics approach. The validity of the model is confirmed by comparison with existing Soutis et al.'s tests. Third, for the aforementioned analyses to be made, fracture energy values need to be predicted. A new formulation of the fracture energy of concrete in terms of the basic material parameters is developed. The fracture energy obtained by the work-of-fracture method is considered separately from the fracture energy obtained by the size effect or other related methods. The statistical analysis of the prediction formulations shows a much lower stastistical scatter of the fracture energy evaluated by the size effect and related methods. This validates the size effect method as a more reliable test procedure for the determination of fracture energy. Fourth, a new analysis of diagonal shear of reinforced concrete beams is presented. The fracture mechanics energy criterion is applied to the classical truss model, leading again to size effect. A linear regression analysis of a large set of size effect tests shows an excellent agreement between the proposed formula and the experimental data. The results are used to calibrate the parameters of the model. Comparison of the predicted shear strength with 643 tests from the literature confirms that the model can predict diagonal shear strength of reinforced concrete beams. |
