Volume 20, Issue 1 (January 1992)
Using Microwave Measurements to Detect Moisture in Asphaltic Concrete
An experimental technique based upon dielectric properties was developed to measure the volumetric moisture content in hot-mix asphalt concrete. The dielectric properties were calculated from the measured reflection coefficients and phase angles. The measurements were obtained over a band of frequencies in the microwave range of 12.4 to 18.0 GHz. The test program included different aggregate types, aggregate gradations, asphalt types, asphalt contents, air void contents, specimen thickness, and surface smoothness. Microwave reflection measurements were obtained for the hot-mix asphalt concrete specimens and their components in a sweep mode over the above frequency band. These measurements were analyzed, and the magnitude of the dielectric constant and the loss factor were calculated. The dielectric properties for these mixes were obtained at the dry state and at two levels of moisture content. The dielectric properties were correlated to the mix characteristics. Regression models were developed to predict the volumetric moisture content of hotmix asphalt concrete. A model developed from a theoretical approach was found to give the best correlation.