Mathematical Modeling of a Concrete Surface “Skin Effect” on Diffusion in Chloride Contaminated Media

Concrete skin is considered the closest zone to the surface of concrete cover of reinforcements. It usually has a different composition than the internal concrete due to phenomena such as contact with molds or segregation of aggregates. In addition, environmental actions induce a gradient of moisture along the cover depth. These circumstances sometimes produce an irregular chloride profile in the cover, which either exhibits a maximum of chloride content some millimeters inside the outer surface or sometimes shows an anomalously high chloride concentration right at the concrete surface. In the present paper, analytical modeling of chloride diffusion is used to study the relative influence of the skin thickness. This theoretical analysis aims to show that there are cases where, if the diffusivity of the skin and the bulk concrete are very different, an error is introduced when the “skin effect” is not accounted for. The clarification of this error may contribute to understanding the differences found between laboratory experiments and the long-term record of chloride profiles in the same concrete.