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

Zbigniew Perkowski,Zofia Szweda

doi:10.3390/app12031730

Zbigniew Perkowski, Zofia Szweda

Open Access

https://doi.org/10.3390/app12031730

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Abstract

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.

Highlights

The penetration of chloride ions into concrete structures is one of the main causes of their steel reinforcement’s corrosion, especially in facilities located in the coastal zone, as well as those treated with de-icing salts
The most important results achieved in the work can be formulated in the following points: (1) Using a spline function to a description of chloride binding isotherm for 180-day diffusion experiments enables an assessment of the “skin effect” in terms of depth of its occurrence and change in apparent chloride diffusivity, which results from a different concrete composition and structure and different chloride binding properties in subsurface layers compared to a bulk part of material
(2) From five types of concretes tested in the study, concrete C1 containing high early strength Portland cement (CEM I 42.5 R) showed the best protective properties in terms of chloride corrosion due to the lowest effective diffusion coefficient of order of 10−13 m2/s (Table 5)

Summary

Introduction

The penetration of chloride ions into concrete structures is one of the main causes of their steel reinforcement’s corrosion, especially in facilities located in the coastal zone, as well as those treated with de-icing salts. Taking into account the facts discussed above, the authors, to the best of their knowledge, did not find in the literature review an attempt to identify the binding isotherm parameters together with the diffusion coefficient for chlorides in concrete based on the knowledge of their profiles from diffusion tests, which will lead to a quantitative assessment of the “skin effect” including different types of cement.

Experimental Results

C2 C3 C4 C5

Mathematical Model

Results of the Calculations

C3 C4 C5

Conclusions