Characterization of Water Gradients in Concrete by Complementary NDT Methods
Résumé
Several NDT methods (ultrasonic surface waves, capacitive method, ground penetrating radar and electrical resistivity probes or tomography (ERT)) were used in similar conditions for the evaluation of the moisture gradients inside concrete. After a long exposure in laboratory conditions to obtain a stabilization of the hygrometry, one face of the tested concrete slabs is put in contact with water. By capillary absorption, water penetrates gradually, and at different time intervals of the process, NDT measurements are performed both on the opposite face and on the face in contact with water. This protocol allows the formation of different water gradient profiles in the concrete slabs during water absorption. At each measurement step, the water gradient profiles are controlled by embedded relative humidity sensors but also by gammadensimetry on a specific sample. The results show that the embedded sensors are able to monitor the water front evolution during water absorption. Phase velocity of ultrasonic surface waves, even-though not significantly affected by moisture variations, can follow up relative modifications within the first concrete centimeters. GPR, electrical resistivity and capacitive measurements are sensitive to the evolution of the water front during absorption. Finally, the ERT provides interesting results for the monitoring of the water front.
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