Full-waveform inversion using the step frequency radar in the GPR band to characterize the tack coat in hot-mix asphalt (HMA) layers of flexible pavements

The use of tack coats in the construction of road structures is a technique that ensures pavement layer bonding. Unfortunately, existing techniques commonly used in France to study tack coats require coring of structures for a punctual characterization in a laboratory. This paper presents an innovative non-destructive approach to characterize the change of geometric and dielectric properties of the tack coats present inside the hot-mix asphalt (HMA) layers of flexible pavements, with the use of a step frequency radar with mono-static off-ground ultra wide band Vivaldi antennas. The principle of this method is to consider the flexible pavement as a multilayered medium with changing properties (thicknesses, dielectric susceptibilities, and dispersion parameters). Then we use a full waveform inversion that allows the extraction of the dielectric and geometric parameters of each layer composing the structure. Following numerical validations, we have validated the multilayered forward model (Green's function model integrating 2p variant of Jonscher's model) experimentally using calibration media (until three layered media). Then, our discussion focuses on the link between the dielectric and the geometric parameters of the tack coats and the emulsion quantity (bonding) used in the laboratory experimental design. The inversion results are compared to the semi-destructive gammadensimetry results and they present a good correspondence.