Dielectric measurement by open-ended coaxial line for hot-mix asphalt roads: From laboratory test to on-site investigation

Dielectric permittivity of road material is an important parameter to calculate the velocity of electromagnetic waves when evaluating the geometry of pavements by ground penetrating radar (GPR). Conventional methods need to prepare core samples for the measurement, which is not convenient nor representative. In this work, we present a non-destructive open-ended coaxial probe to measure the frequency-dependent dielectric permittivity and the results include laboratory tests and on-site investigation. In the laboratory part, we test the slab samples and compare the results with a validated test (coaxial/cylindrical transition line). Due to the limited size, the dielectric results obtained by the open-ended probe are a bit lower than those by the validated test. However, the thickness problem can be ameliorated in the on-site investigation because of the infinitive scale of the road. In the on-site part, the tested dielectric constant of the road can well agree with the calculated value by the Maxwell-Garnett law based on the volumetric fractions of the bitumen, aggregates, and air. Moreover, the stable dielectric permittivity in the tested frequency by open-ended probe can be used to fit the dielectric constant at higher frequency by Debye model. This can provide a dielectric constant at high frequency for estimating the geometry of the multi-layered structure in the surface course. The estimated results are close to the tested lengths of core samples.