Experimental Studies On Model â…  Fracture Of Fully-graded Concrete And Numerical Simulation On Complete Process Of Cracking

Concrete is a kind of three-phase composite material composed of the coarse aggregate, cement paste and the bonding interface between them. The fully-graded concrete with a maximum aggregate size of150mm is widely used in actual hydraulic structures. The proportion of coarse aggregate and cement paste in fully-graded concrete share a large difference with ordinary concrete. The content of coarse aggregate in fully-graded concrete is generally up to60%～70%, while only30%to40%in ordinary concrete. The fracture indexes measured under laboratory conditions with ordinary concrete or wet-screened concrete cannot truly represent and fully reflect the actual fracture properties of fully-graded concrete. To more accurately reflect the fracture properties and crack propagation mechanism of fully-graded concrete, several aspects of work is mainly done in this dissertation as follows:1、Fully-graded concrete wedge-splitting specimens made of raw materials of the Xiluodu dam, whose maximum aggregate size is150mm, geometric dimension is800×830×450mm and initial crack height ratio a0/D=0.4, were used in the fracture test. The initial cracking loads are specified by the retraction of strain gauges at the crack tip. Supporting conditions of the wedge-splitting specimen are adapted to abate the influence of additional vertical load induced moment on the stress field and the calculation of fracture parameters around the crack tip. With the software ANSYS. the initial fracture toughness and unstable fracture toughness (double-K fracture parameters) of non-standard wedge-splitting specimens in the test are calculated, which are compared with the fracture parameters calculated on the formulae specified by the <Norm for fracture test of hydraulic concrete> for standard specimens and on the Fictitious Crack Model respectively. The results indicate that fully-graded concrete has a higher brittleness, empirical parameters in bilinear softening curves of ordinary concrete are inapplicable to fully-graded concrete. The method proposed by the norm can also be applied to the calculation of non-standard large-size specimens.2、The initial fracture toughness was taken to be as the crack propagation criterion. The crack propagation of fully-graded concrete was simulated by using a software ANSYS, which was based on the theory of macroscopic fracture mechanics cohesive force model and asymptotic superposition assumption, and was combined with the specific fracture parameters (such as fracture energy, initial fracture toughness etc.) of fully-graded concrete. Complete P-CMOD curves, critical crack lengths and double-K fracture parameters of the wedge-splitting specimens were calculated respectively. The simulated results show a good agreement with the fracture test data of the Xiluodu dam. It is concluded that once the elastic modulus, the uniaxial tensile strength, the uniaxial compressive strength, and the initial fracture toughness of the fully-graded concrete are measured, the double-K fracture parameters and the complete process of crack propagation can be calculated by the proposed numerical method.3、With the comparison of mode I crack propagation criterion and KR crack extension resistance curve, it is found that driving force curves expressed by the stress intensity factor KIP caused by applied loads are coincident with KR crack extension resistance curves caused by the resistance force during the crack propagation. Accordingly, the KR crack extension resistance curves of fully-graded concrete were calculated.