A Simple Fracture Mechanical Interpretation of Size Effects in Concrete Fracture Toughness Tests

Concrete is a so called quasibrittle material which, despite predominantly elastic material response, exhibits in tension loading a stable non-linear fracture response, when tested under displacement control. The reason for the non-linearity is the development of a fracture process zone, in front of the crack, due to micro-cracking and crack bridging. The effect of the fracture process zone is to make the specimen sense the crack as being longer than a0+a. The fracture process zone causes thus an effective increase in the crack driving force and apparent fracture resistance. Here, a novel LEFM based estimate of the effective stress intensity factor and the effective crack growth at maximum load in a fracture mechanics test is used to obtain a simple power law approximation of the effective K-R curve. It is shown that it is applicable to the description of not only different size specimens, but also specimens with varying geometry. The method is based on a new theoretical estimate of the effective crack growth corresponding to maximum load.