Measurement of Complex Permittivity of Asphalt Pavement Materials

Abstract

To facilitate applications of road penetration radar and other nondestructive testing technologies on roads, an apparatus was developed for the measurement of complex permittivity of construction materials. The system consists of an automatic network analyzer, a sample holder, coaxial cable connections, and data processing software. The sample holder can accommodate either laboratory-prepared or field-drilled asphalt and concrete samples with maximum aggregate sizes up to 19 mm and lengths up to 400 mm. A computer program is used to convert the parameters measured by the automatic network analyzer into the real and imaginary parts of the complex permittivity. The dielectric constants of 43 laboratory-prepared asphalt samples of fairly consistent bulk densities were measured over the frequency domain of 0.1 MHz to 1.5 GHz. The average dielectric constants were 6.0 ± 0.15 for dry specimens and 6.52 ± 0.99 for soaked samples over the frequency range of 8–900 MHz. A linear relationship was observed between the gravimetric moisture content and dielectric constant up to a moisture content of 1%, with dielectric constant increasing by 0.62 for every 1% increase of moisture content. The results indicate that the asphalt content and mix type did not significantly affect the dielectric constants, whereas the moisture content was identified as the predominant factor. The results provide much needed information on the dielectric properties of asphalt pavement materials.