Based on micromechanics, an elastic-plastic-brittle damage model of concrete is developed by considering the aggregate gradation curve algorithms and the heterogeneity of concrete. In the model, the concrete is taken as a three-phase composite material that consists of the mortar matrix, aggregates and bonds between matrix and aggregates. The numerical experiment of the uniaxial compression shows that the model can clearly simulate microscopic plastic yield, and the initiation and extension of crack. The relevant physical simulation is used to validate the model. In the numerical experiment of bending failure of concrete beam strengthened by bonded steel plate, the strength of the steel plate is relatively lower, and it firstly yields. The bending stress born on the steel plate is transferred to the concrete beam, which results in that the inner cracks of concrete beam increase rapidly and coalesce until the failure of the whole specimen. The final failure mode of concrete beam strengthened by bonded steel plate is the ductile bending failure.
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