Multiscale Modeling of Interactive Diffusion Processes in Concrete

Deterioration of concrete is governed by coupled diffusion processes such as heat conduction, moisture transfer, and chloride ion penetration. The diffusion processes can be characterized by multiscale modeling. At the macroscopic level, a thermodynamic approach is used in the present study as a framework to derive coupled diffusion equations and constitutive equations. In the case of hygrothermal coupling, it is found that there are additional coupling effects in the stress-strain relationships aside from the conventional thermal expansion and drying shrinkage. At the mesoscopic level, composite theories are used to develop the material models between effective transport properties (macroscopic) and transport properties of constituents (mesoscopic). The results shows that, even for noncoupled processes, the effective properties are not simply the volumetric average over the constituent phases. An analytical multiscale prediction model for chloride diffusion is developed based on an assumption that both chloride diffusivity and binding capacity are independent of free chloride concentration. The analytical solution is used as an example to compare with available test results, showing that the model is quite accurate for regular concrete cured 28 days or longer. The multiscale model can accurately predict the effect of mesoscopic parameters (e.g., aggregate content) and microscopic parameters (e.g., water-to-cement ratio) on chloride penetration.