VAASPHALT.ORG 09 Achieving adequate and uniform density is critical to obtaining the desired performance of a newly placed asphalt pavement layer. The mix compaction process is critical, as the aggregate particles are reoriented and the volume of air within the mixture is reduced, and many studies have shown that inadequate compaction results in a decreased durability of the asphalt mixture. The degree of compaction is usually expressed as a percentage of a reference density. However, the reference density may be a theoretical maximum density (the density if all the air voids were removed), the density of a field-produced control strip or a laboratory value derived during mix design. Current tests for quality control and acceptance of asphalt mixtures placed on Virginia Department of Transportation (VDOT) projects relies on nuclear density gauge testing and collection and testing of sawn plugs, respectively. Neither of these processes are optimal. Use of the nuclear density gauge requires stringent use and storage requirements due to the Cesium source material. Collecting and testing sawn plugs is a destructive and time-consuming process. In addition, these tests only assess a small portion of the pavement mat (about 0.003%). Instead, the use of a density profiling system (DPS) could allow more rapid assessment over a larger sampling area and allow contractors to have real-time feedback of achieved density during the paving operation. A DPS device operates by correlating a measured material property to pavement density. In its current form, DPS uses ground penetrating radar (GPR) technology, measuring the dielectric constant of the pavement surface. The dielectric constant of a material—or the relative permittivity—represents the ratio of the electrical permittivity of a material to the permittivity of a vacuum. For use in pavement surveys, GPR operates by transmitting pulsed electromagnetic energy into a pavement and collecting the reflected energy. A portion of the incident pulses are reflected at boundaries between two materials with differing dielectric constant values. Specifically, in the case of measuring the surface layer of pavements, air and the pavement are the two boundary materials. The dielectric constant of the pavement surface layer is calculated based on the ratio of the amplitude of the signal reflected at the air-to-pavement interface to the incident amplitude, obtained by measuring the reflection from a metal plate. Figure 1 shows a DPS device that was used by the Virginia Transportation Research Council (VTRC) on four asphalt paving projects during the 2021 construction season. The DPS device, manufactured by Geophysical Survey Systems, Inc. (GSSI), consists of three GPR antennas operating at a central frequency of 2.0 GHz, a data recording and processing computer, and a global positioning system (GPS) sensor mounted on a push cart. The four projects assessed in 2021 were located in the Culpeper, Lynchburg, Richmond and Northern Virginia districts. The projects included an asphalt overlay placed over either a milled or existing asphalt pavement surface. Testing was conducted to evaluate the DPS technology first by determining the correlation between the field-measured dielectric constant and the lab-measured density from cores for different asphalt pavement mixtures, and then by assessing the density longitudinally and transversely. Testing at each project was completed by scanning an approximately 500-foot section from which nine calibration cores were collected. Next, the DPS device was used to scan selected portions of each project site. Within each lane, the DPS was operated in Percent Maximum Density Dielectric Constant 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 100 98 96 94 92 90 88 86 84 82 80 y=9.8978x + 41.73 R2 = 0.9612 Figure 2. Correlation between dielectric constant and percent maximum density for one project two passes, with three antenna passes covering the left and right side of each lane, for a total of six antenna passes per lane. After field testing, the density (expressed as a percentage of the theoretical maximum) was measured for each calibration core. These data were used to establish a correlation between the DPS device output and the pavement density. Using this correlation, the density was assessed for the scanned areas of each project. Results: Density Correlation and Continuous Pavement Density The dielectric properties of an asphalt mixture are dependent on the dielectric properties of the mixture components that can vary from project to project. Calibration cores were collected from each project, covering a range of dielectric constant values, to establish a relationship between percent maximum density and dielectric values. The study found that the dielectric constant values from the DPS were well correlated with the percent maximum density (with R2 values ranging from 0.82 to 0.98). Figure 2 shows an example of this relationship. The results of testing using the DPS were evaluated with respect to the longitudinal and transverse directions. For each project, relatively lower density values were found along certain longitudinal joints and unsupported edges. As expected, lower density variability was found on primary routes while higher density variability was found on pavements located in subdivisions. ASPHALT PAVEMENT DENSITY PROFILING continues on page 10 △
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