A numerical framework for the ITZ percolation, effective fraction and diffusivity of concrete systems considering the nonuniform ITZ

Abstract

The critical percolation and effective fraction of interfacial transition zone (ITZ) around the aggregates are generally considered as two important structural factors in the prediction of macroscopically transport properties in concrete, and their solutions are very susceptible to the geometric features of both aggregate particles and ITZs. Understanding the influence of aggregate features and ITZ thickness on the variation of these ITZ configurations and their effects on the macroscopic performance of materials would have great significance for the optimization and adjustment of concrete materials. However, in most existing numerical studies, both the coarse and fine aggregates are generally simplified as the hard particles of the same morphology and the ITZ thickness around them is also set to be uniform, which don’t reflect the reality perfectly and may lead to the great deviation. To clarify the variation of ITZ features and ionic diffusivity with the ITZ effect in a more realistic way, the polydispersity of aggregate shapes is taken into consideration by assuming coarse and fine aggregates as polygons and ovals, respectively. Moreover, a differential ITZ thickness is also assigned for the aggregates according to their sizes and W/C ratio. Based on the above premises, a numerical framework for the prediction of the effect of individual phases on the ITZ percolation, effective fraction and diffusivity of concrete is developed sequentially in this paper. The whole framework can be divided roughly into three parts: (1) the numerical simulation and quantification of the critical percolation of nonuniform ITZs; (2) the statistical evaluation and quantitative analysis of the effective ITZ fraction by statistical geometry; (3) the continuum percolation-based effective medium prediction for the relative diffusivity of polyphase concrete systems. According to the systematically numerical investigation, it can be found that the oversimplification of both ITZ thickness and aggregate shape may result in the serious deviation for the evaluation of ITZ percolation, effective fraction and macroscopic properties, especially the ITZ thickness. The numerical results show that when the ITZ thickness is assumed to be uniform for all the aggregates, the ITZ percolation threshold ϕagg,c, its effective fraction ϕITZ and ionic diffusivity Dcon of concrete system would generally be overestimated, underestimated and underestimated compared with the ones with the nonuniform ITZs, respectively. The proposed models here may give more realistic results than the previous prediction models in 2D case.