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31 ment from a moving antenna. These waves travel through the pavement structure and echoes are created at boundaries of dissimilar materials. An air-coupled horn antenna attached to the back of a small SUV (see Figure 11) was used in the field evaluation of NCHRP Project 1065 to evaluate HMA, unbound aggregate base, and embankment soils. The speed of data collection is one of the biggest advan- tages of GPR technology. There should be no impact to the contractor's operation, because this system collects the same information regardless of material temperature and is capa- ble of taking measurements at speeds of up to 40 miles per hour. Higher speeds have been used on more recent projects through enhancements made to the equipment and data acquisition systems. The disadvantages of the technology are the interpretation of the dielectric values that are measured and personnel requirements for calibrating and maintaining the equipment and data interpretation software. The system is simple to operate and provides results imme- diately, at least in terms of dielectric values. The results are in the form of a "picture" of the pavement system, much like an X-ray. Although the transducer is located above the surface, aimed downward, the picture can be viewed from "plan" or "elevation" ("profile") perspective. Another huge advantage of this technology is that a continuous profile of the dielectric values is available. In fact, layer thickness profiles or complete contours of the layer can be developed in a short time period. Currently, the technology requires operators with special technical skills to interpret the data that have physical meaning to the quality of construction. Software programs are avail- Figure 8. Manual DCP in operation (courtesy of able that provide color-coded charts and contours of the Minnesota Road Research Section, Office of Materials, material. This system has been used to determine layer thick- Minnesota DOT). ness at a reasonable accuracy--when layers with different dielectric values are tested. The accuracy of the analysis pro- grams requires cores to accurately measure the in-place thick- between the hammer and base for the hammer. Furthermore, ness and other volumetric properties. soils or materials with boulders or large aggregate particles Most of the data reduction-presentation programs, how- (refer to Figure 9) can cause refusal of the device. When this ever, still require some volumetric properties to be assumed occurs, the test point should be moved and the test redone. in estimating density, air voids, and other volumetric prop- An automated trailer mounted DCP is available, but is more erties. These assumptions result in error of the properties that expensive (see Figure 10). Only the manual DCP was used in are calculated from the dielectric values. The assumptions the field evaluation of NCHRP Project 10-65. are believed to be a reason why the GPR's analysis and inter- The manual DCP is considered to have potential for QC pretation from the Part A projects did not coincide with some use on a day-to-day basis, but an additional contractor and of the other NDT devices. There are programs available that agency staff person would probably need to be assigned to use do not require many assumptions, but all of the known pro- the DCP under normal practices; however, the training and grams are proprietary. These proprietary programs were maintenance of this device is considered minimal. not used in the Part A field evaluations, but were included in the Part B summary at a few facilities. Data from some of these proprietary programs is presented and discussed in 1.5 Ground Penetrating Radar Chapter 2. GPR is a pulse echo method for measuring pavement layer Calibration is another issue that is important to the suc- thicknesses and properties. GPR uses radio waves to penetrate cess of GPR antennas in estimating volumetric properties the pavement by transmitting the wave energy into the pave- of materials. Cores have to be recovered and the physical
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32 Large aggregate particles in the embankment soil caused refusal of the DCP in localized areas. These particles found near the surface also had an impact on the DSPA and GeoGauge readings. Figure 9. DCP test and large aggregate particles encountered at some of the projects, resulting in refusal of the test. Figure 10. Automated DCP attached to a trailer (courtesy of Minnesota Road Research Section, Office of Materials, Minnesota DOT).