Novel non-convex aggregate model for interfacial transition zone (ITZ) mesostructure and diffusivity of concrete

•A non-convex aggregate is generated by contractions on the contour of an ellipse.•A packing of non-convex aggregates is obtained by discrete element method.•Interfacial transition zone fraction is obtained numerically and theoretically.•Effects of non-convex aggregates on concrete diffusivity are analyzed. This work aims to establish a numerical framework for mesostructure characterization and diffusivity estimation of concrete with non-convex aggregates. Firstly, a new two-dimensional (2D) non-convex aggregate model is proposed by applying contraction deformation to the contour of an ellipse. Then, a 2D three-phase mesostructure of concrete is constructed, which includes simulating a random packing model of non-convex aggregates by the discrete element method (DEM) and forming interfacial transition zones (ITZs) surrounding aggregates based on the hard core-soft shell (HCSS) model. Subsequently, the ITZ area fraction of the 2D three-phase concrete mesostructure is numerically estimated through a Monte Carlo random point sampling method. In addition, a theoretical model of ITZ area fraction incorporating aggregate properties is derived. Lastly, the effective diffusivity of the concrete mesostructure is evaluated by a developed Dual-probability-Brownian motion (DP-BM) scheme. The results show that the effective diffusivity of concrete depends on the competition of the decreasing effect by more tortuous diffusion paths and the increasing effect by a higher ITZ fraction, which can be resulted from a higher packing fraction, a smaller sphericity, and a finer gradation of aggregates. This work has the potential to serve as a foundation for estimating and optimizing the properties of concrete.