Développement de méthodes RF pour la caractérisation hydrique et l’évaluation non destructive de matériaux cimentaires et d’éco-matériaux cimentaires

This research work focuses on the development of RF electromagnetic methods with a view to characterize the hydric properties of some cementitious materials, in particular eco-materials. A goal related to issues regarding building and civil engineering applications. Indeed, the performance and the mechanical properties of such materials depend on their hydric features. Hence, it is important to measure them, either for material characterization purposes, or for in situ applications, such as structural health monitoring (SHM).In such a context, RF dielectric measurements have the advantage of being sensitive to moisture, thanks to the strong contrast between the dielectric permittivity of water and that of the other constituents of the material. Besides, non-destructive or non-invasive RF sensing techniques can be envisaged.The first part of this work focuses on hemp concrete, an eco-material featuring a high porosity which is used in buildings for moisture and temperature regulation.Firstly, we have characterized the dielectric spectrum of hemp concrete samples coupled to their sorption cycle using RF measurements in an waveguide cell operating in the [1.72 - 2.61] GHz band. We have studied the measurement errors, which were found to be low; as well as the variability of the dielectric features of several hemp concrete samples.Secondly, we have used the measured dielectric data to design non-invasive hydric sensors operating at 2.4 GHz consisting in patch antennas used as reflectometry probes. At such a frequency, the patch surface allows the sensor to probe the representative elementary volume of hemp concrete. Several sensors have been manufactured and characterized. They feature good linearity and sensitivity of the resonant frequency to the material water content.In addition to these non-invasive sensors, we have designed and manufactured a microstrip two ring resonator device, with localized moisture measurements inside a hemp concrete wall in view. The simulation and the sensor characterization results are encouraging.In the last part of this work, we deal with a more conventional cementitious material, namely a mortar: i.e. a mixture of cement, water and sand, for which we propose an RF approach for monitoring hydration (chemical reaction between cement and water). The latter is based on the measurement of the dielectric permittivity of the mortar by means of an open-ended coaxial probe and on a dielectric model based on mixing laws, which combine the volume fractions (which change during hydration) and the dielectric permittivity of the mortar constituents. The experimental study, carried out for a given mortar sample, shows a good sensitivity of the RF measurements to the ongoing hydration, with results consistent with the phenomena at stake.