Volumetric water content evaluation in structures, substructures, soils, and subsurface in general is a crucial issue in a wide range of applications. The main weaknesses of subsurface moisture sensing techniques are usually related both to the lack of cost-effectiveness of measurements, and to unsuitable support scales with respect to the extension of the surface to be investigated. In this regard, ground-penetrating radar (GPR) is an increasingly used non-destructive tool specifically suited for characterization and imaging. Several GPR techniques have been developed for different application purposes. Moisture evaluation in concrete is important for diagnosing structures at early stages of deterioration, as water contributes to the transfer of degrading and corrosive agents e.g., chloride. Traditionally, research efforts have been focused on the processing of GPR signal in time domain, although more recent studies are being increasingly addressed towards frequency domain analysis, providing additional information on moisture content in concrete. Concerning the evaluation of subsurface soil water content, different models ranging from empirical to theoretical are used for converting permittivity values into moisture. In this regard, two main GPR approaches are commonly employed for permittivity evaluation in time-domain measurements, namely, the ground wave method and the reflection method. Furthermore, the use of borehole transmission measurements, traditional off-ground methods, and of an inverse modelling approach allowing for a full waveform inversion of radar signals have been developed in the past decade. More recently, a self-consistent approach based on the Rayleigh scattering theory has also allowed the direct evaluation of moisture content from frequency spectra analysis.@en

Clay content is one of the primary causes of pavement damages, such as subgrade failures, cracks, and pavement rutting, thereby playing a crucial role in road safety issues as an indirect cause of accidents. In this paper, several ground-penetrating radar methods and analysis techniques were used to nondestructively investigate the electromagnetic behaviour of sub-asphalt compacted clayey layers and subgrade soils in unsaturated conditions. Typical road materials employed for load-bearing layers construction, classified as A1, A2, and A3 by the American Association of State Highway and Transportation Officials soil classification system, were used for the laboratory tests. Clay-free and clay-rich soil samples were manufactured and adequately compacted in electrically and hydraulically isolated formworks. The samples were tested at different moisture conditions from dry to saturated. Measurements were carried out for each water content using a vector network analyser spanning the 1 GHz–3 GHz frequency range, and a pulsed radar system with ground-coupled antennas, with 500-MHz centre frequency. Different theoretically based methods were used for data processing. Promising insights are shown to single out the influence of clay in load-bearing layers and subgrade soils, and its impact on their electromagnetic response at variable moisture conditions. @en