Moisture and salt ingress monitoring in concrete by resistivity and evanescent field dielectrometry techniques

The main corrosion mechanism affecting ancient concrete is rebars corrosion, either induced by carbonation or chlorides pollution (Marie-Victoire 2006, Neff 2007). Therefore, in terms of maintenance strategy, corrosion monitoring is a key issue. Standard on site corrosion monitoring techniques such as potential mapping or polarization resistance measurements, have proven their efficiency. Nevertheless, those electrochemical tools, as the corrosion process itself, can be affected by variations in the climatic conditions (Andrade 2003, Bouteiller 2012, Marie-Victoire 2012) and salts contamination. More disconcerting, conservation treatments (realkalization, migrating corrosion inhibitors), or even measuring device (Sehgal 1992, Marie-Victoire 2012) can disturb the results. As a consequence, accurate corrosion diagnosis can be complex. Corrosion process needing water, being able to monitor water ingress in concrete could be an alternative way to identify corrosion risks. In concrete, the presence of water is generally approached by resistivity measurements, either measured by the Wenner or the Disc methods (Feliu 1996, Polder 2000). But resistivity is interlinked with both moisture content and salt presence (Polder 2000). In the field of wall painting, a new technique, the SUSI© system, based on evanescent field dielectrometry (Olmi 2006, Olmi 2008, Di Tullio, 2010), was developed. The advantage of the SUSI© system is to give separate quantitative information about moisture and salt contents. The aim of the study presented in this paper, was to compare the performances of standard resistivity techniques versus the SUSI© system, on specifically designed specimen and under controlled measuring conditions.