Fracture mechanics of concrete and its role in explaining structural behaviour

Major steps forward in the history of the development of fracture mechanics, as a tool for describing or predicting the behaviour of concrete structures, have been the fictitious crack model, introduced by Hillerborg, and the compressive damage zone model, introduced by Markeset. In structural elements, however, often the conditions are less clear than in concentric tension or compression tests. The behaviour of concrete beams subjected to shear and slabs subjected to punching are illustrating examples, especially when size effects are concerned. Another phenomenon that required further study is the rotation capacity of slabs at intermediate supports. Since mostly the ultimate rotation is reached at failure of the concrete in the compression area, the question is justified whether rotation capacity is subject to size effects as well. Since concrete is a brittle material, it is necessary to design structures in such a way that the structural behaviour becomes ductile nevertheless. One way is to adequately reinforce the structure. Another way is to provide the material concrete itself with ductility. This can be done by adding steel (or other) fibers to the concrete mixture. Indeed the fracture toughness of the concrete is considerably enhanced: however further questions can be raised with regard to the influence of the production process of fiber reinforced structural members on the mechanical properties and the most appropriate method of determining the mechanical properties. An essential question is furthermore whether improving the fracture toughness of concrete by adding fibers by definition leads to an improved behaviour of the structural members.