of asphalt mixture uniformity. The research was done by the Texas Transportation Institute (TTI), which worked with GSSI to develop the technology into the PaveScan RDM asphalt density assessment system. The non-contact PaveScan RDM technology uses a sensor that typically outputs a measurement each half-foot along the lane traveled, so a mile’s worth of data includes roughly 10,000 measurements for each sensor used. This system is ideal for uncovering inconsistencies that occur during the paving process, including poor uniformity and significant variations in density. To arrive at pavement density, PaveScan RDM measures the dielectric properties of the asphalt surface. The dielectric constant is the abil- ity of a substance to store electrical energy in an electric field. For example, air dielectric is 1.00059; asphalt aggregate is about 3 to 6; while the dielectric of water is 80. With new pavement, the mixture is uniform; dielectric variation occurs primarily due to the percentage of air voids – which directly correlates to density. The measurement is based on the ratio of reflection from the asphalt surface to the reflection from a metal plate. On the road in Maine Maine is one of several states that have conducted pilot studies using PaveScan RDM equipment. The PaveScan RDM uses one or three 2 gigahertz (GHz) sensors mounted on a portable push cart that can scan a of up to a 6-foot width. Each antenna collects a continuous line of dielectric/density. An onboard computer captures dielectric values, which can be correlated to core densities. Operators scan a pavement section and the device identifies high, low, and median density locations. They take a static reading directly over each location, obtain cores at each location, and then test the cores, entering the results in the software. Correlation accuracy de- pends on obtaining core densities over the entire range of measured dielectric values. Figure 1 shows a typical density profile. In this scan, three sensors map anomalies of newly laid pavement. Areas in red identify high compaction areas, while green represent uniformity, and blue indicate low compaction areas. Figure 2, provided by Maine DOT, compares the calibration of dielec- tric to air voids for several pilot study locations. Figure 3 (left) shows the distribution of dielectric values collected at one location, showing curves with a desirable uniformity. By contrast, the graphic on the right shows less uniform curves, which is not what Maine DOT was looking for. To date, Maine DOT has collected data on about a dozen recent paving projects and found that the data aligns well with what they have seen in the field. Says Peabody, “We were looking for a better way to ensure we’re achieving desired density ranges, since studies show that good density leads to improved service life. PaveScan RDM gives a much larger sample of the HMA mat and can be used as a QC/QA tool. The results to date have been really positive.”
a non-nuclear asphalt density gauge for QC purposes and cores are col- lected sent to a lab to determine voids/density acceptance. “The down side of cores is limited sampling and not having real time values.” National highway research solutions include use of GPR The issue of pavement density has been extensively researched as part of the Federal Highway Administration’s Strategic Highway Research Program (SHRP2).[3] SHRP2 eventually came up with two non-destructive techniques for evaluating asphalt pavements during construction: infrared thermal scanning and the use of ground-pen- etrating radar (GPR), which uses electromagnetic wave reflection to “see” through materials. According to SHRP2 studies, GPR can be used to measure uniformity and potential defect areas in asphalt pavements during construction. Significantly, it offers real-time testing of potentially 100 percent of the pavement area. Compare that to current density tests, in which typical random sampling measures only about 0.003 percent of pavement area. Widely used for many applications, GPR is a common tool for utility location, measuring pavement thickness and bridge deck deterioration. For decades, researchers have been investigating whether it could be used for measuring pavement density, but they could never achieve the level of accuracy that would warrant recommending its widespread use. In addition, the use of GPR technique previously required special- ized equipment, a great deal of data interpretation, and a number of manual steps. In 2013, SHRP2 funded work aimed at developing a streamlined and operator-friendly GPR device that would provide real time profiling PaveScan RDM set up with three 2 gigahertz (GHz) sensors mounted on a portable push cart
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october 2020
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