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In this study, the aggregate information in concrete was extracted by Digital Image Processing (DIP) technology, and the two-dimension three-phase (coarse aggregate, mortar, and interface transition zone) meso-numerical model corresponding to the realistic aggregate distribution was established. Based on the model, considering the effects of coarse aggregate distribution, volume fraction and size on the fracture behavior, the fracture test and numerical simulation of four-point shear beams with notches under mixed-mode I-II stress were carried out. The results showed that the concrete meso-numerical model based on real aggregate can accurately reflect the meso-scale fracture process of concrete, and cracks always develop along the weak interface transition zone. It is the aggregate in the fracture trend zone that makes the fracture path tortuous, and the bending and shearing stress around the aggregate will change substantially. Moreover, the external force work of numerical model increases with the increase of aggregate volume fraction and maximum aggregate size, and the dissipated energy of coarse aggregate with small particle size increases with the rise in its proportion in gradation. Especially, the spatial distribution of the coarse aggregate determines the meso-mechanical properties of concrete and then affects the macro-mechanical behavior. To reduce the cracking risk of concrete, the volume fraction and gradation of aggregate should be kept within an appropriate range.

The fracture and damage behaviors of concrete under I–II mixed-mode loading are of great importance in determining the failure mode of concrete material and structure. In this paper, the four-point bending beams with different aggregate volume contents have been fabricated and tested to obtain the limited loading capacities, and the Digital Image Correlation (DIC) method was utilized for monitoring the local displacement above the notch during the loading procedure. Furthermore, based on the aggregate occupation and translation algorithm, a modified random aggregate model has been proposed for generating the four-point bending beam with precast notch, and the numerical simulations for the four-point bending beam with different aggregate volume contents and depths have been proceed.

The size of the fracture process zone (FPZ) has significance for studying the fracture mechanism and fracture characteristics of concrete. This paper presents the method of assessing the FPZ of Mixed-Mode I-II for quasi-static four-point shearing concrete beams with pre-notched by Lagrangian strain profiles from digital image correlation (DIC). Additionally, it explores the influences of volume rates of the coarse aggregate of 0%, 28%, 48%, and 68%, and the specific surface areas of 0.12 m2/kg, 0.15 m2/kg, and 0.26 m2/kg on the size of the FPZ. It shows that the size of FPZ in four-point shearing concrete beam can be characterized by the displacement field and strain field using DIC. The size of FPZ conforms to linear positive correlation with the volume rate of coarse aggregate, and linear negative correlation with the specific surface area of coarse aggregate. It presents that the crack initiation of the four-point shearing beam with the pre notch is dominated by mode I load, and the propagation and fracture of Mixed-Mode I-II cracks are caused by the combined effect of Mode I and Mode II loading.

This paper presents the result of an experimental and numerical investigation on I–II mixed-mode meso-scale fracture behavior of concrete by the Monte Carlo Simulation (MCS) random aggregate numerical simulation method, involving the tests of 6 four-point shear notched concrete beams with different coarse aggregate. Simultaneously, the effects of volume fraction and specific surface area (SSA) of coarse aggregate on fracture characteristics and peak load were discussed at the meso-level. The results showed that the crack growth patterns of four-point bending concrete beams with separate coarse aggregate volume fraction and SSA were basically consistent under the same loading condition. The actual path of fracture propagation is determined by the difference of the aggregate distribution, and with an increase in the volume fraction of coarse aggregate, the fracture surface is rougher, and I–II mixed-mode fracture of concrete required more fracture energy, which is corresponded to a higher peak load. Also, with an increase in the proportion of small size coarse aggregate, the contact surface area between aggregate and mortar increases, and consequently, peak load increased by the proportion of small size coarse aggregate increase.

This paper presents the mesolevel numerical simulation and experimental verification of mixed mode I-II fracture for the four-point shearing beam with a single-side notch based on the concrete damaged plasticity (CDP) method and the Monte Carlo simulation (MCS) random aggregate method. And it discusses the influence of volume rate and maximum size of the coarse aggregate on the fracture path and the initiation angle of concrete mixed mode I-II fracture. The results show that the coarse aggregate has a significant influence on the distribution width, average length, and the dispersion degree of the fracture path, the distribution of which can be fitted by a normal distribution function. Also, the distribution width of the fracture path has a quadratic function relationship with the height along the failure direction. The distribution of coarse aggregates at the top of the reserved notch also has a significant influence on the initiation angle, which determines the initiation crack direction of mixed mode I-II fracture in concrete. The initiation angle distributes near the failure direction and follows a normal distribution function.