Ultrasonic Tomographic Monitoring of Water Content in a Concrete Wall Undergoing Unidirectional Drying

As part of the ScaNING project, funded by the French National Research Agency, embedded sensors (thermo-hygrometers, resistivity, capacitive, and ultrasonic) were integrated into a concrete wall (1 m x 1 m x 0.3 m) before casting in March 2023, to continuously monitor water saturation gradients. At the end of October 2024, the wall was placed in a climatic chamber at 45°C to accelerate drying. This paper presents the results of ultrasonic monitoring using 8 sources and 8 receivers. A moisture gradient was induced by leaving one side of the wall exposed. The source-receiver pairs were positioned at varying depths, reaching up to 22 cm below the surface. The sensors, specifically designed for this study, operate at a central frequency of 130 kHz, and ultrasonic signals were recorded for transmission tomography. We first analyze the evolution of compression wave velocity using the first arrival times during the drying process, considering the thermal and moisture gradients, with calibration from similar concrete samples. A differential tomography algorithm is then applied to the experimental data to monitor several months of drying through imag-ing. Finally, the potential of using P-wave amplitude for further analysis is discussed.