Determination of fracture parameters for crack propagation in concrete using an energy approach

Double-G fracture model, a new analytical model describing fracture behaviour on cracked concrete, was recently proposed by Xu and Zhao based on the conception of energy release rate. This model couples the Griffith brittle fracture theory with the bridging softening property of concrete, and it is an extension of double-K fracture model proposed by Xu and Reinhardt. In this model, two fracture parameters, i.e., the initiation fracture energy release G(Ic)(ini) and the unstable fracture energy release G(Ic)(un), are termed to distinguish the different crack propagation stages undergoing during the whole fracture process in concrete. The difference between the two parameters, written as G(Ic)(un), is assumed to come from the contribution by aggregate bridging interlock, which results in the presence of fracture process zone. In our present work, firstly, the new model is elaborately introduced. Then, fracture tests are conducted, where besides three-point bending beams, a new testing method, called wedge-splitting on compact tension is adopted. In the experiments, electrical strain gauges are used to measure initial cracking load. Based on recorded load-crack mouth opening displacement curve (P-CMOD) or load-displacement curve (P-delta) and load-strain curve (P-epsilon), double-G fracture parameters are experimentally determined. Further more, based on the assumed three parameters relationship among G(Ic)(ini), G(Ic)(un) and G(Ic)(c), unstable fracture energy release are calculated. A comparable result between the measured G(Ic)(un-e) and the calculated G(Ic)(un-c) confirms this assumption. In order to verify the feasibility of this new model, the effective double-K fracture parameters converted by double-G fracture parameters using K = root EG are compared with the double-K fracture parameters calculated by double-K fracture model. It is found that there is a good agreement. Another two series of different initial crack-depth ratios three-point bending beams carried out by Refai and Swartz are also collected to provide more experimental verification. It shows that the results obtained from the double-G fracture model agree well with those of double-K fracture model. (C) 2008 Elsevier Ltd. All rights reserved.