The non-destructive evaluation with the aim of characterizing objects before or after treatment has taken place, and the monitoring of long-term performance is analyzed in this thesis. Generally, these test methods measure material properties or changes in these properties that decision makers are interested in. There is a variety of non-destructive testing (NDT) methods to choose from, depending on the aim of the analysis. Generally speaking, different non-destructive testing methods can be used for investigating the inner structure of materials. It is worth noticing that there is not one single analytical method able to provide all the necessary information. Therefore, a combined test series capable of providing complementary information is usually adopted. Most of the methods used in NDT generate pictures of the object interior that can help locate structural flaws. Even though not common, these methods can give quantitative results. The use of electromagnetic waves within the radio frequency bandwidth, in particular, can be valuable in detecting defects, for assessing the deterioration and the success of refurbishment activities. A particular radar technology was used in this study as a tool for assessing the ability to characterize materials in specific built environments. Numerical and laboratory studies were carried out to evaluate the feasibility of the proposed methodology for the mentioned applications. A secondary objective was to provide a contribution to the road safety, preventing the risk of severe damage of pavement, induced by clay content in sub-asphalt layers, and to improve the operations of rehabilitation and maintenance through an effective inspection. In Chapter 2 the electric properties of multiphase aggregate mixtures were evaluated for a given mineralogic composition at frequencies between 300 kHz and 3 GHz. Two measurement techniques were employed: a coaxial transmission line and a monostatic stepped-frequency ground-penetrating radar. The propagation matrices analytical method was used to retrieve the electrical permittivity and conductivity of the mixtures from the measured scattering parameters in the coaxial transmission line. The effect of increasing water content was analyzed in several sand-clay mixtures. For the end-member case of maximum clay (25% by weight) and increasing water content, investigations were compared between the two measurement techniques. The electrical properties of materials are influenced by the amount of water, but clay affects the frequency dependency of soils showing distinctive features regardless of the mineralogy. The microwave attenuation, expressed by the quality factor Q, is partly dependent on frequency and on water content. The performance of one empirical and one volumetric mixing model was evaluated to assess the capability of indirectly retrieving the volumetric water content for a known mixture. The results obtained were encouraging for applications in the field of pavement engineering with the aim of clay detection. The models used show similar behaviors, but measured data were better modeled using third order polynomial equations. High-frequency, ultra-wideband penetrating radar has the potential to be used as a non-invasive inspection technique for buildings, providing high-resolution images of structures and possible fractures affecting constructions. To test this possibility, in Chapter 3 numerical and laboratory experiments were conducted using a proximal, stepped-frequency continuous-wave radar system operating in zero-offset mode, spanning the 3-8 GHz frequency range. The reconstruction of material electrical properties is achieved by resorting to full-waveform inverse modeling. Numerical experiments showed that for typical electric permittivity and electrical conductivity values of concrete and plaster, it is possible to retrieve the physical properties of the material and to detect fractures less than 1 mm thick. Laboratory experiments were conducted on non-reinforced concrete and plaster test slabs in different configurations. The results showed the good potential of this method: (1) to provide a thorough fracture response model in buildings or artworks and (2) to non-invasively characterize the samples in terms of their electromagnetic properties. The characterization of the subsurface can be performed by full-waveform inversion of electromagnetic data relating to a particular model. The modeling process relies on the ability of retrieving the scattered field Green’s function from the measured data. This is achieved using sets of antenna characteristic global reflection and transmission coefficients to describe the media in terms of their scattered field impulse response. As described in Chapter 4, crucial for a successful implementation of this technique is the understanding of uncertainties involved in the acquisition of the antenna calibration and survey measurements, and how these propagate in the parameter estimation results. It was found that averaging a number of possible Green’s functions obtained from one measurement with several antenna characteristic coefficients sets works remarkably well in reducing the uncertainties. The accuracy of the inversions improved using characteristic coefficients acquired as close as possible to the measurement conditions. Moreover, a clear relation between dynamic range and system resolution was highlighted, based on the number of effective bits contained in the data. The presence of cohesive soils as bearing courses of road pavement frequently causes damages and defects (e.g., transversal and longitudinal cracks, deformations and ruts). In Appendix A, different ground-penetrating radar (GPR) methods and techniques were used to non-destructively investigate the clay content in sub-asphalt compacted soils. The experimental layout provided the use of typical road materials, employed for road bearing courses construction. Three types of soils classified as A1, A2, A3 by the American Association of State Highway and Transportation Officials (AASHTO) were used and adequately compacted in electrically and hydraulically isolated test boxes. Analyses were carried out for each clay content using two different GPR instruments. A pulse radar with ground-coupled antennae at 500 MHz center frequency and a vector network analyzer (VNA) with a 1–3 GHz bandwidth were used. Signals were processed in both time and frequency domains, and the consistency of results was validated by the Rayleigh scattering method, the full-waveform inversion and the signal picking techniques. Promising results were obtained for the detection of clay content or cohesive soils affecting the bearing capacity of sub-asphalt layers.