This article presents an optimized microwave sensor for the non-contact measurement of complex permittivity and material thickness. The layout of the proposed sensor comprises the parallel combination of an interdigital capacitor (IDC) loaded at the center of the symmetrical differential bridge-type inductor fabricated on an RF-35 substrate (ε_r = 3.5 and tanδ = 0.0018). The bridge-type differential inductor is introduced to obtain a maximum inductance value with high quality (Q) factor and low tunable resonant frequency. The central IDC structure is configured as a spur-line structure to create a high-intensity coupled electric field (e-field) zone, which significantly interacts with the materials under test (MUTs), resulting in an increased sensitivity. The proposed sensor prototype with optimized parameters generates a resonant frequency at 1.38 GHz for measuring the complex permittivity and material thickness. The experimental results indicated that the resonant frequency of the designed sensor revealed high sensitivities of 41 MHz/mm for thickness with a linear response (r² = 0.91567), and 53 MHz/Δε_r for permittivity with a linear response (r² = 0.98903). The maximum error ratio for measuring MUTs with a high gap of 0.3 mm between the testing sample and resonator is 6.52%. The presented performance of the proposed sensor authenticates its application in the non-contact measurement of samples based on complex permittivity and thickness.

Asphalt pavements expose early damage during their service life. The early damage grows internally until it accumulates and transforms into macro-damage that leads to structural failure. To promote real-time monitoring of internal damage evolution in asphalt pavements, this study proposes an electrical resistance change (ERC) method that exploits the piezoresistive properties of conductive asphalt concrete as a nondestructive testing technique capable of detecting microstructural defects. For this purpose, the traditional asphalt mixtures were first made electrically conductive by adding carbon fibers and iron tailing aggregates as conductive aggregates to satisfy the piezoresistivity requirements. Secondly, piezoresistivity experiments were performed by applying cyclic tensile force in fatigue and progressively increasing mode. The self-monitoring ability of damage evolution was assessed based on the correlation between the fractional change in electrical resistance (FCR), displacement, and damage degree for each mode of the force application. It was observed that under fatigue mode, the FCR and displacement increase upon loading and decrease reversibly upon unloading in each cycle. Compared with the displacement, the FCR exhibits partial reversibility upon unloading, indicating damage initiation in each cycle. The damage grows with the number of loading cycles until it accumulates in the elastic regime and gradually affects the load-carrying capacity of the mixture. Under a progressively increasing mode, the mixture exhibits a good correlation between the FCR and force in the elastic regime, ensuring real-time monitoring of internal micro-damage. However, the FCR develops a nonlinear correlation with the force in the inelastic regime that reflects damage to the conductive network and eventually structural failure. In summary, the ERC method provides real-time monitoring of microstructural defects that manifest as early damage and gradually transform into macro-damage under repeated traffic loads.