Combination of the Wenner resistivimeter and Torrent permeameter methods for assessing carbonation depth and saturation level of concrete
Résumé
Assessing carbonation depth is of great interest for the diagnosis of reinforced concrete structures because carbonation is one of the origins of steel corrosion. The assessment of carbonation depth is usually performed by a simple and reliable semi-destructive test consisting in spraying a colored indicator on a sample extracted from the structure. When the structure is large, this test must be reproduced many times if an assessment of the variability of carbonation depth is required. In this case, the extraction of multiple samples can be prohibitive from the technical and economic points of view. So, in this case, a non-destructive testing (NDT) method could be relevant. However, even when NDT methods can be used, there is a need to improve the interpretation of their results. In this study, the use of two usual NDT methods is proposed: resistivity measurement by a Wenner probe and surface permeability assessment by a Torrent permeameter. Both techniques are implemented on carbonated slabs having different carbonated depths and at different saturation degrees. The results show that the two techniques are sensitive to moisture and carbonation. For a given saturation level, resistivity increases when the carbonated depth increases and resistivity decreases when the saturation level increases. Torrent permeability decreases when the saturation degree and carbonated depth increase. Good repeatability is observed for resistivity measurements while a larger scatter is obtained for Torrent permeability. Empirical laws are built for the relationships between resistivity or permeability and saturation degree and carbonated depth. For resistivity, either a linear model between resistivity and saturation degree is used or Archie’s law is adapted to take the effect of carbonation into account. Following the same idea, a linear law between Torrent permeability and saturation degree is adapted to the effect of carbonation. Because resistivity can be measured only if the saturation degree is rather high, i.e. if there is a continuity of the interstitial solution, and because permeability assessment is impossible if the concrete is fully saturated, these laws are limited to saturation degrees in the 40–83% range.
Then, these laws are used to predict carbonation depth and saturation degree on a wall designed with the same concrete but stored in different conditions. The results show that resistivity and Torrent permeability can be used for the combined assessment of carbonation depth and saturation degree in laboratory conditions.
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