Measurement of water content and salinity index in concrete by evanescent field dielectrometry

Abstract Water and chloride ions are key parameters in the corrosion processes of reinforced concrete. Consequently, rebar corrosion is the first cause of decay in historical monuments made of concrete. Thus, developing new techniques to be able to detect or quantify water and chloride ions seems to be an interesting approach in the diagnosis of historical reinforced-concrete buildings, especially if these new techniques are non-destructive. SUSI-R © is a non-invasive microwave system based on the evanescent field dielectrometry technique. The SUSI system is equipped with a resonant probe able to analyze the tested material down to 2 cm in depth. The water content and salinity index are calculated from the resonance properties of the probe, which are a function of the material permittivity. Initially developed to quantify the moisture content and to detect the presence of salts in mural paintings, SUSI-R © was considered in this study for the analysis of concrete. In a first phase, the SUSI-R © technique was tested to follow the moisture and salt content in reinforced concrete slabs, made with CEM I or CEM III cements, after their semi-immersion in water or in a sodium chloride solution. Several series of measurements were then carried out before and during the rise of the capillary absorption front. In the second phase, the calibration of SUSI-R © was undertaken in a laboratory on prisms made with the same concretes as those used for the slabs, and which had undergone the same curing conditions. The resonance parameters were correlated to the moisture content contained in these samples, with various saturation rates, in plain water and salted water. The first results of this study showed that the technique makes it possible to follow the hydric changes in the material but also to differentiate between the two types of capillary absorption solution: plain water or salted water. Distinct evolutions of the salinity index were also distinguished according to the type of cement. The calibration in laboratory allowed the resonance parameters of the probe to be correlated with the moisture content values, and thus to quantify the moisture content of the concrete slabs investigated during the first phase of the study.