13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Determination of Residual Fracture Toughness of Post-fire Concrete Using Analytical and Weight Function Method YU Kequan1,*, LU Zhoudao, YU jiangtao 1 Department of civil engineering, Tongji University, Shanghai 200092, China * Corresponding author: zjzjykq@163.com Abstract: Determination of double-K fracture parameter using both analytical and weight function method is carried out in present research. In calculating the cohesive fracture toughness, two situations are divided at critical load. Wedge-splitting tests with ten temperatures varying from 20ºC to 600ºC are implemented. The complete load-crack opening displacement curves are obtained from which the initial and critical fracture toughness could be calculated experimentally. The validation of double-K fracture model to the post-fire concrete specimens is proved. Meanwhile the weight function method agrees well with the analytical method. Keywords: double-K fracture parameter, analytical method, weight function method, post-fire concrete 1. Introduction It was established that linear elastic fracture mechanics could be only applicable to large-mass concrete structures and could not be applicable to medium and small-scale concrete structures. Since late 1970s, many nonlinear fracture models have been proposed by various groups of researchers to study the behavior of crack propagation in quasi-brittle materials like concrete [1-6]. Experimental results show that the fracture process of concrete structures undergoes three main stages: (i) crack initiation, (ii) stable crack propagation, and (iii) unstable fracture. Accordingly, the double-K fracture criterion initially introduced by Xu and Reinhardt [6] shows the crack initiation, crack propagation and failure during a fracture process until the maximum load is reached. And the two size-independent parameters, initial cracking toughness, KI ini and unstable fracture toughness, KI un can be used to study the crack propagation of concrete. In order to determine the double-K fracture parameters analytically [7, 8] the value of cohesion toughness, KI c due to cohesive stress distribution in the fictitious fracture zone is computed using method proposed by Jenq and Shah [9]. In this method, the determination of KI c is done using a special numerical technique because of existence of singularity problem at the integral boundary. Under such circumstances, the use of universal form of weight function will provide a closed form expression for determining the value of KI c. And it has proven its accuracy in determining the double-K fracture parameter compared to analytical method [10]. The influences of geometrical parameter [11], specimen geometry [12, 13] and size-effect [7, 8, and 14] on fracture toughness were studied by various researchers. It was found that the influence of ao/D ratio and shape of test specimen are relatively less than the size- effect on the values of fracture parameters. The influence of temperature on the fracture parameters was also considered by several researchers, but mainly on the fracture energy and material brittleness [15-19], relatively fewer discussion on the fracture toughness [20-21].s Considering there exist many structures subjected to fire or high temperatures, the influence of temperature on the fracture properties needs further studied.
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