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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.

Chloride penetration into concrete structures reduces their service life. When chloride reaches the steel rebar, it may corrode which eventually leads to concrete deterioration. Hence, it is important to test concrete on chloride penetration resistance. Such testing often takes a long time, because of the slow process of chloride penetration. Therefore, using an accelerated test is an obvious choice. The RCM test is one of the most commonly used techniques to measure the chloride diffusion coefficient in concrete. One of the disadvantages of the RCM test is that it requires a saturated sample, while structures in practical conditions are mostly unsaturated. In this paper, the RCM test is performed for unsaturated mortar. Standard mortar samples with various air content of 0%, 2%, 4%, 6%, 8% and 10% are investigated. The air content in the sample is obtained by first water-saturating the sample under vacuum and than by placing the sample in a climate chamber with a relative humidity of 10% and a temperature of 50°C to induce drying, until the desired air content of the sample is reached. After the RCM test, the chloride penetration depth is measured with colorimetric and titration methods. The results show that the more air content in the samples is, the deeper chloride penetration in the samples will be.