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 representing 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 quantitative 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 similar results to ground‐coupled ground‐penetrating radar but provided a quantitative estimate of the changes within the porous layer during the test.