Three-dimensional microstructural simulation of the thermodynamic behavior of recycled concrete under freeze-thaw action

A thermal-water-mechanical coupling model was utilized to simulate pore water crystallization and melting heat transfer processes in recycled concrete. Based on the random spherical microstructure of recycled concrete, The characteristics of aggregates, new mortar, old mortar, and the interface transition zones (ITZ1, ITZ2, and ITZ3) between natural aggregates and new mortar, old aggregates, and mortar, as well as new and old mortar, were clearly described. The proposed model was validated by comparing it with experimental data available in the literature. Threedimensional microstructural finite element simulations were performed to assess the influence of the water-to-cement ratio of new mortar, the water-to-cement ratio of old mortar, aggregate gradation, replacement ratio of recycled aggregates, aggregate volume fraction, and temperature difference on the performance of recycled concrete. The research results demonstrated that the aggregate gradation, replacement ratio of recycled aggregates, and aggregate volume fraction significantly affected recycled concrete's freeze-thaw characteristics. In contrast, the influence of the water-to-cement proportions of fresh and old mortar was relatively small. The impact of environmental temperature differences should also be given due attention.