Fracture toughness of plain concrete from three-point bend specimens

A simple method is proposed for determining the fracture toughness of plain concrete from three-point bend specimens, based on the concept of effective notch depth to account for the non-linear pre-peak load-deflection behaviour. The fracture toughness so determined is shown not to depend on the specimen size. The method improves an earlier version of the effective crack model in several ways. First, it is no longer necessary to linearize the pre-peak non-linearity, thereby eliminating the inaccurate task of establishing the limit of elastic response. Secondly, regression expressions for determining the effective notch depth should be far more accurate because they are based on an analysis of not only the authors’ test data but that of several researchers around the world. Thirdly, these expressions do not depend on the size of the test specimen. It is shown that the predictions of the effective crack model are in good agreement with two other non-linear process zone models, as far as three-point notched beam specimens are concerned.