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The “Skin Effect” Assessment of Chloride Ingress into Concrete Based on the Identification of Effective and Apparent Diffusivity

The method of assessment of the “skin effect” for chloride ingress into concrete has been proposed, based on the inverse problem for the identification of at-surface variability of chloride diffusivity under fully saturated conditions. For this purpose, experimental results of 180-day diffusion tests of five types of concrete were used, which allowed the calculation of their chloride apparent diffusivity (taking into account the chloride binding by the cement matrix) and effective diffusivity (relating to the transport of free chloride ions in the pore liquid). The tested concrete samples with a water to cement ratio of 0.5 differed only in the type of cement (high early strength Portland, low-alkali normal early strength Portland, ash Portland, blast furnace, and pozzolanic). In order to effectively describe the chloride binding isotherm, a first-degree non-uniform spline function was used. Finally, the “skin effect” depth at the untreated outer surface of the concrete samples was estimated up to about 5 mm when analyzing space variability of apparent chloride diffusivity for four types of concrete with low-alkali normal early strength Portland, ash Portland, blast furnace, and pozzolanic cement. In this respect, the “skin effect” on the concrete with high early strength Portland cement was not detected.