The use of impulse and stepped-frequency radar to characterize the hydric behaviour of a porous pavement structure
This study focusses on examining the behaviour of an innovative asphalt pavement which is created as a solar energy collector, using non-destructive testing involving ground-penetrating radar. The concept of heat exchanger is based on the use of drainage asphalt in the bonding layer through which a heated fluid flows via gravity to de-ice the roadway. In order to develop hydrothermal models for a test site repre-senting such a pavement, the saturated porous layer assumption was required when the water level in each tank (up- and downstream of the structure) was same as the top of the porous layer. Two ground-penetrating radar techniques were tested at this test site: a ground-coupled impulse radar and an air-coupled stepped-frequency radar. The impulse ground-penetrating radar, a high-efficiency non-destructive testing technique which is widely used in civil engineering, provides accurate geometric information, especially for pavement investigation. In the second innovative approach, air-coupled stepped-frequency radar was combined with full-waveform inversion to obtain quan-titative information, while retrieving the electromagnetic properties of the successive pavement layers. We concentrated on the early-stage water imbibition in the pavement structure using the impulse ground-penetrating radar to estimate the fluid transfer velocity and both ground-penetrating radar techniques to verify the saturated porous layer assumption in the steady state. Ground-coupled radar enabled us to follow the water front and to capture different water-transfer behaviours in the porous asphalt layer. Our observation could be explained by the vertical topology of the upper watertight interface. Air-coupled stepped-frequency ground-penetrating radar presented simi-lar results to ground-coupled ground-penetrating radar but provided a quantitative estimate of the changes within the porous layer during the test.