Issue 45

F. Qui et alii, Frattura ed Integrità Strutturale, 45 (2018) 1-13; DOI: 10.3221/IGF-ESIS.45.01 3 aggregate was meshed into grids, the ITZ was generated by unit reconfiguration, and a 3D micro-mechanical model was created for concrete. The established model enjoys high computing efficiency. In this paper, the dynamic concrete features are simulated at different loading speeds, aggregate volume contents and aggregate particle sizes, using the 3D micro-mechanical model proposed by Xu et al., and the influence rule of different factors on the dynamic mechanical properties of concrete are discussed in details. The mortar, aggregate and the ITZ were built on the material constitutive relation provided by Xu and Wen [29] and embedded in the business software ANSYS LS- DYNA for numerical simulation. E STABLISHMENT OF THE MESO - MECHANICAL MODEL s mentioned above, the ANSYS LS-DYNA dynamic analysis software was adopted to simulate the response of concrete with randomly distributed 3D aggregate under dynamic load. As shown in Fig. 1, the test specimens are 100mm, 150mm or 200mm in side length. The upper and lower rigid plates respectively act as an actuator and a support, while the part between them is the concrete sample. The load was imposed onto the upper rigid plate to simulate concrete damage. Figure 1: 3D meso-mechanical model. Grid size Both mortar and aggregate were simulated with the 3D entity unit of the same shape. The grid size was set to 2mm according to the side lengths of the cubic test specimens. Compared to mortar, the ITZ is a weak, porous, nonuniform thin layer wrapped in the outer surface of aggregate particles. The typical thickness of the ITZ is merely 10~50  m , which limits the minimum grid size. To improve the computing efficiency, 3D shell units were introduced to simulate the ITZ. Here, it is assumed that the ITZ consists of homogeneous materials and its thickness is the average of the maximum and minimum values: 30  m . Distribution of aggregate particle size Considering the effect of aggregate grading on the mechanical behaviour of concrete, Fuller’s grading curve was adopted in our model to optimize concrete compactness and strength. The curve can be expressed as [30]:   max n d P d d        (1) where P ( d ) is the cumulative percentage of aggregates passing a sieve with aperture diameter d ; d max is the maximum size of aggregate particle; n is the shape parameter of the gradation curve and ranges from 0.45 to 0.7. In this paper, n is taken as a common value of 0.5. A