Fractures in the rock due to the applied load lead to the crack initiation and propagation and eventually the formation of separation plates in the rock. Fracture in rock is usually done under three modes of tensile, shear and mixed. In fracture mechanics, the critical condition of a cracked rock is determined by the stress intensity at the crack tip. In order to analyze the stress intensity at the crack tip, the main fracture parameters K_I and K_II, which are the stress intensity factors in mode I and mode II, respectively, determine the strength of the material against crack growth and propagation. Rock fracture toughness is used in many fields including rock foundations, hydraulic fractures and underground space design. In this study, the fracture toughness was determined by considering the grain size and using a three-point bending test on concrete specimens. Toughness values have been determined and compared with different fracture criteria. The effect of grain size on fracture parameters and crack propagation path has been eva‎luated. For this purpose, at least three replications of the fracture toughness test were performed on SCB specimens. In this research, the maximum tangential strain fracture criterion and the extended maximum tangential strain criterion along with conventional fracture criteria have been used to predict the crack initiation angle and fracture toughness values. With increasing grain size, the amount of fracture toughness increased under modes I, II and mixed mode (I-II). Samples with a grain size of 0-19 mm had the highest fracture load and samples with a grain size of 0-5 mm had the lowest fracture load. This indicates that the amount of fracture load increases with increasing grain size and also with increasing sample size in proportion to grain size. With increasing grain size, the crack propagation angle decreased and for samples with a grain size of 0-19 mm had the lowest value and samples with a grain size of 0-9.5 mm showed the highest amount of crack propagation angle. With increasing crack inclination angle, the ratio of secondary crack length to primary crack length has increased, which has the highest value in samples with a grain size of 0-9.5 mm and in samples with a grain size of 0-12.5 mm, the lowest ratio of secondary crack length to initial crack length has Shown.

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