Figure 27: A prototype of MnDOT remotely operated rolling asphalt density meter Specifically, within the DPS system, GOR sensors send and receive the EM waves while recording the amplitudes and frequency of the signals. The signal is processed using a concentration box to determine the necessary amplitudes and the on-board computer uses the amplitudes to compute the dielectric constant of the pavement in real time. Dielectric constant is a measure of a material’s ability to store electrical energy. In the case of pavement engineering, this constant can be directly related to air void content and therefore density. A low dielectric value means the pavement will have a higher air void content and therefore a lower density. As a reference, the dielectric constants of air and water are 1 and 81, respectively, meaning EM waves travel much slower underwater. Asphalt is typically between 4 – 8, depending on the air content. Ensuring accurate dielectric constants is crucial to predicting the air void percentage across the pavement. A slight change in dielectric constant can lead to a large change in air content. The AC surface reflection method is a popular way to determine the bulk dielectric constant of the HMA. It utilizes the ratio of the amplitude of the GPR signal reflection from air to the HMA surface, A0, to the incident amplitude (represented by the reflection from the metal plate), Ai. The dielectric constant of the surface is determined, using the Equation 1 [76]:
41
