reflection amplitudes and two-way travel time of the received EM waves are used to calculate and analyze dielectric constants that can be correlated with percent air voids [63, 71]. GPR technology comes in two main forms: ground-coupled and air-coupled. Air-coupled systems holds the antennas certain height off the ground instead of in direct contact making it the more commonly used method for calculating compaction of HMA [71, 73]. Figure 25 presents a diagram of a Pavement Density Profiler (PDP) process which uses an air coupled GPR system but a differing data processing method.
Figure 25: PDP instrument background principle of operation [73]. A Density Profiling System (DPS) is a recent GPR technology used for determining asphalt compaction in the field. A DPS is a non-contact, air-coupled GPR that has shown promise to supplement, and possibly eventually replace, most field coring activities [74]. DPS method is also beneficial for longitudinal joint density evaluation because it allows for continuous, real-time measurements. DPS operators have also suggested potential for this application to thicker layers [75]. The device is equipped with antennas that are suspended a short distance above the pavement. This device has been used attached to a human operated vehicle (Figure 26) or behind trucks and rollers. The continued innovations of this technology will be explored in this report as DPSs become remote controlled devices.
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