A model describing water and salt migration in concrete during wetting/drying cycles

In order to predict the life span of concrete structures, models describing the migration of chloride are needed. In this paper, a start is made with a simple, theoretical model describing water and chloride transport in a concrete sample. First, transport of water in concrete is considered with Fick’s second law of diffusion, which describes the time-evolution of the water content. Chloride can be transported through concrete by advection with water and by diffusion due to a difference in the chloride concentration of the pore solution. Furthermore, chloride can be bound to the surface of the pore walls. This bound chloride is immobile, thus unavailable for transport. The result is a partial differential equation for the time-evolution of the chloride concentration, coupled to the partial differential equation for water transport. This partial differential equation is solved using the Crank-Nicolson implicit method for a situation in which chloride enters, and in a certain extent also leaves the sample. Because the goal is to describe the migration behavior in the sample during wetting and drying cycles, the sample gets wet during one half of a day and dries during the other half.