A capacitance sounding method has been proposed and developed for evaluation of the actual thickness and dielectric permittivity of asphalt pavement. The method is based on the continuous measurements of the electrical capacitance between two electrodes in real time. One of them is grounded within the soil immediately adjacent to the side of the road. Another one (sensing electrode) is mounted on a motor-driven positioner to obtain the capacitance measurements at multiple locations along the direction perpendicular to asphalt surface. The principle of the method is to vary the clearance of the sensing electrode to sound the 1D structure of the pavement beneath that electrode. A distinguished feature of the proposed technology is that the measured signal depends only on the thickness and the permittivity of the asphalt layer. All underlying layers do not affect the capacitance readings. A set of capacitance values versus sensing electrode positions is considered as a sounding curve. Software tools were created for solving forward and inverse problems of the capacitance sounding. An unknown thickness and permittivity are derived from a real-time inversion of the data obtained. Upon completing the inversion, an operator can move the assembling with the second electrode across the asphalt surface to the next sounding site. The capacitance sounding method proved to have a good lateral resolution: the dielectric permittivity values and the asphalt layer thickness are accurately detected over distances of 40 cm from a measuring point. An extensive feasibility study of capacitance sounding using both mathematical modeling and field measurements has been carried out. As a result, portable, low-frequency equipment has been designed and created. Asphalt layer thickness and the dielectric permittivity measurements were carried out on a high-traffic highway. The comparison of field trial results with core sampling analysis demonstrated the efficiency of the proposed method.
