In order to study the mesomechanical properties of recycled aggregate concrete (RAC) under uniaxial tension, a numerical model of RAC with two different aggregate shapes (circular and convex) and five different replacement ratios of recycled aggregate (0, 30, 50, 70, and 100%) was established. A new finite element method-base force element method (BFEM) was used to derive the element strain and the element stiffness matrix with an explicit expression without Gauss integral. The two-dimensional numerical model of the RAC was simulated to study the effect of aggregate shape, replacement ratio of recycled aggregate, aggregate distribution and interfacial transition zone (ITZ) properties on mesomechanical properties of RAC. Simulation results demonstrated that once the first crack appeared, the peak stress and peak strain were reached. The first crack appeared in old ITZ, which was located in whether the upper part or the lower part of the large-size recycled aggregate. The continuous cracks were mainly around the recycled aggregate and the aggregate concentrated area. Comparing with natural concrete, when the replacement ratio of recycled aggregate was 100%, the elastic modulus decreased by 16~25%, the peak stress decreased by 12~15%, and the peak strain changed slightly. The ITZ had a significant influence on the mechanical properties of RAC and must be considered in the analysis.

Since recycled aggregate concrete (RAC) is valuable for green concrete and global sustainability, it has attracted many numerical research studies as a five‐phase material in mesolevel. In this study, the digital image processing (DIP) technique is used to obtain the real aggregate and mortar distribution of recycled concrete specimens. By applying the DIP MATLAB model, the real mesoheterogeneity of RAC is well represented. Based on the base force element method (BFEM), a mesomechanical model is constructed to simulate the uniaxial tension and compression tests of recycled concrete specimens subjected to displacement loads. The damage and the cracking mechanisms of RAC are analyzed in the mesolevel. Moreover, the failure process and the failure mode are investigated. Results indicate the nonlinearity deformation, stress redistribution, and cracks propagation of RAC. The stress–strain curve, fracture energy, and fracture distribution are also figured. The comparative analysis between the simulation and the experimental results of in situ RAC samples shows the applicability of the model; therefore, the failure mechanism is revealed. It also extends the application of the BFEM in the field of failure mechanism analysis of RAC as a heterogeneous composite material.

Summary The finite element method (FEM) is an effective approach for exploring the failure mechanism of heterogeneous materials. According to the complementary energy principle, the use of FEM might suffer from several difficulties in terms of keeping the elements and their boundaries balanced, as well as finding interpolation functions. In this study, we introduced an efficient approach to researching the failure mechanism of the material, named base force element method (BFEM), according to complementary energy principle. Specifically, the element compliance matrix of an arbitrary quadrilateral element with four mid‐edge nodes was expressed based on the complementary energy principle. Then, the node displacement was obtained by the governing equation using the Lagrange multiplier method. In addition, both the compliance matrix and the node displacement were represented as explicit expressions without the use of Gaussian integration. A numerical model of the recycled aggregate concrete (RAC) was established according to the Monte Carlo method. A comparative sample of the digital image model was also established using digital image technology. The influences of substituting recycled aggregate and the relative mechanical properties of adhered mortar to those of new mortar on the failure mechanism of RAC were studied. The simulation results indicated that the BFEM is an effective approach to researching the damage mechanism of heterogeneous materials.

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