Concrete is non-homogeneous and is composed of three main constituent phases from a mesoscopic viewpoint, namely aggregates, mortar matrix, and interface transition zone (ITZ).A mesoscale model with explicit representation of the three distinctive phases is needed for investigation into the damage processes underlying the macroscopic behaviour of the composite material. This paper presents a full 3-D mesoscale finite element model for concrete.On top of the conventional take-and-place method, an additional process of creating supplementary aggregates is developed to overcome the low packing density problem associated with the take-and-place procedure. An advanced FE meshing solver is employed to mesh the highly unstructured domains. 3D mesoscale numerical simulation is then conducted for concrete specimen under different loading conditions, including dynamic loading with high strain rate. The results demonstrate that detailed mesoscopic damage processes can be realistically captured by the 3D mesoscale model while the macroscopic behaviour compares well with experimental observations under various stress conditions. The well-known inertial confinement effect under dynamic compression can be fully represented with the 3D mesoscale model and the trend of dynamic strength increase with strain rate from the 3D mesoscale analysis agrees well with the experimental data.

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Shock and impact loading often generates stress waves in the responding structural object with a drastic time and spatial variation. Consequently the transient response of the material involves not only high strain rate, but also a highly non-homogeneous stress field with respect to the mesoscale heterogeneity of the concrete-like materials. To comprehensively capture the material response and the underlying mechanisms under such loading conditions, the use of a mesoscale model is desirable. This paper discusses the development of a mesoscale model that describes reasonably the random mesoscopic structure of concrete materials and at the same time allows for the simulation of response under complex dynamic loads. Due to the complexity in the generation and meshing of the random aggregate structure of concrete, at the present stage the random mesoscale geometry is generated within a 2D plane. The use of a 2D representation of a concrete is common and may be acceptable in lower loading rate applications, however, under high rate loading a 2D model for concrete in compression could introduce noticeable errors due to an inability to fully capture the crucial lateral inertia confining effect. This special phenomenon is investigated in this paper, and a remedial modelling scheme is proposed to minimize the inaccuracy in a 2D mesoscale model for high strain rate loading. Finally, the experimentally observed strain rate increase factor is discussed in light of the mesoscale numerical modeling results.

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