Numerical analysis of multiple ion species diffusion and Alkali-Silica Reaction in concrete

This paper presents a macroscopic model for the prognosis of the initiation phase and the swelling process leading to ASR induced damage in concrete. Concrete is assumed to be a fully saturated deformable porous media with interacting constituents such as the solid skeleton and ions in the pore fluid. Ion transport is assumed to be governed by the Nernst-Planck-Poisson (NPP) system of equations. The Nernst-Planck equation describes the transport of multiple ion species and the Poisson equation describes the variation of the electric potential according to the spatial distribution of the electric charges. Diffusing ions in the pore fluid such as the Alkalis and hydroxyl ions break the silanol and siloxane bonds in reactive aggregates in the solid skeleton, forming an alkali-silica gel. Once the volume of the gel becomes greater than the available volume of the pore space, stresses and swelling occur, leading to microcrack nucleation and micro-crack propagation in and around the aggregates causing overall deterioration of the material. The reaction process is modeled as a sink term in the Nernst-Planck equation. To take into account the influence of the topology of the pore space and the presence of oriented micro-cracks on ion diffusion and fluid transport, a novel continuum micromechanics homogenization model is used. The strain induced by the ASR gel is assumed to be a function of moles of gel created, which is itself a function of the moles of ions involved in the reaction process.