Non-destructive evaluation of concrete properties in depth

Concrete can evolve over time due to internal swelling reactions as well as aging linked to mechanical and environmental stress conditions. These reactions depend on the transport of reactive elements (chlorine, water, and carbon) from the surface as well as the initial and local conditions when the concrete sets and after during the structure life. They therefore vary with depth and involve aging gradients. The evaluation of concrete according to depth is possible by Non-Destructive Testing. Usual NDT often leads to data either limited to a zone close to the surface or integrating properties over the entire depth tested. We propose two ultrasonic methods. Their objectives are, for one, the measurement of water content gradients as a function of depth, and the other, the localization of damage resulting from delayed reactions at the concrete core. For the first, sensor pairs are installed at different depths to measure the variations of the velocity of ultrasonic pressure waves linked to water content. These measurements are developed on a concrete plate 30 cm thick. For the second, measurements are done in the depth of the concrete block of 2*2*1 m3 to detect the concrete's nonlinear behaviour. That enables us to identify and even localize a zone of internal swelling reactions. We present the two principles and devices developed in these works. We detailed some measurement results of these NDTs. These different ways allow us to have perspective on the continuous control of the concrete. To conclude, we propose a strategy for the continuous monitoring of concrete in depth, based on these two approaches.