Determining effective diffusion properties of concrete through mesoscale analysis

Paper in proceeding, 2012

Diffusivity of both moisture and chloride ions in concrete are determined here by the use of computational homogenization. Concrete is modelled on the mesoscale as a heterogeneous three-phase material within a representative volume element (RVE), consisting of the pure cement, gravel and interfacial transition zone (ITZ). By imposing moisture and chloride ion gradients over the RVE, its response is solved for and homogenized through finite element (FE) analysis and computational homogenization. By this method, the diffusivity of concrete is determined as a function of the ballast content within the RVE. The numerical results correspond well with an analytical model used for determining the influence of ballast on concrete. The model proposed here allows for the inclusion and study of the ITZ and its influence on the diffusivity. In practise, the ITZ is included in the FE model as lattice elements placed in between the cement paste and ballast. In this manner, it was possible to determine the diffusivity coefficient of ITZ by calibrating the model to experimental data of concrete diffusivity. Furthermore, a multiscale (FE2) framework, for coupled chloride ion and moisture transfer in concrete is also presented. The mass transfer problem is set up as an initial boundary value problem on the macroscale where the mesoscale heterogeneities are introduced via RVEs in the Gauss-points. The macroscale quantities in terms of mass fluxes are obtained through suitable homogenization of the RVE response in a nested fashion during the computations. In this manner, the RVE problem serves as a constitutive model for the macroscale. The overall goal is to better simulate chloride ion and moisture transfer in concrete by including the strong heterogeneities of the material in the model.