Experimental measurement of double-K fracture parameters of concrete with small-size aggregates

The double-K fracture model can well describe the development of cracks undergoing during the entire fracture process in concrete. Therefore, it has been selected as the theoretical basis of the “Norm for fracture test of hydraulic concrete” (China) and the double-K fracture parameters proposed in this model are regarded as the essential fracture parameters needed to be measured. Therefore, the primary objection of this paper is to present an overall report on the determination of the double-K fracture parameters of concrete involving the fundamental theory, calculation formulas, experimental arrangement and identification of initial cracking load in order to provide a reference for the specification committee. The fracture tests are carried out on the total of 43 concrete specimens where the small-size aggregates with the maximum size of 10 mm are chosen. Two typical widely-used geometries, i.e. three-points bending beam and wedge splitting specimen are adopted. The initial cracking load is determined by means of graph method or electrical resistance strain gauge method. Then, the initial fracture toughness and unstable fracture toughness are measured. It is found that the ratio of initial cracking load to maximum load is 0.67–0.71 and the ratio of initial fracture toughness to unstable fracture toughness is 0.45–0.50. It is also found that, for the initial fracture toughness and unstable fracture toughness, the values are approximately close to the constants when the depth of specimens is large than 200 and 400 mm for three-points bending beams and wedge splitting specimens respectively. Besides, the critical crack tip opening displacement of each specimen is calculated, too. The big difference between two geometries is noticed.