Aggregates largely influence a road's dielectric properties, which is an important physical parameter when using non-destructive methods such as ground penetrating radar to evaluate a road's geometry. Due to the irregular shapes, the direct measurement of an aggregate piece by piece in the microwave domain is not easy. To address this problem, an epoxy-based composite is used to estimate the dielectric constant of the aggregates by resonant test and mixing law calculation. As a water-free binder, the epoxy composite eliminates the water effect on the dielectric properties. Meanwhile, the distribution of each component can be studied by X-ray computed tomography, which allows knowing the composite's morphology. Moreover, the composite has a smooth surface that avoids the texture effects on the dielectric measurement. In addition, the impedance analysis of individual aggregates at low frequencies was used to evaluate the results obtained with composites and mixing laws.