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33 GPS Receiver GPS Data Acquisition & Storage GPR Antenna a. Air-Coupled GPR Antenna Attached to Survey Vehicle b. Ground-Coupled Antenna Arrays Attached to Survey Vehicle Figure 11. GPR antennas attached to a standard survey vehicle. properties of those cores determined and correlated to the 1.6.1 Electrical Density Gauge dielectric values measured by the GPR prior to and during construction. This requires that control strips be used at the An electrical density gauge was used in the Part A field beginning of a project and the correlations periodically con- evaluation projects, because of the equipment's perceived ease firmed during construction. Many agencies are eliminating of use and application to a diverse set of unbound materials or not requiring the contractor to use control strips, especially and soils. The specific gauge used was the one manufactured for small projects. Thus, this technology has limited use in QC applications, but has greater potential for use in accept- able programs--especially those for which thickness is included in the price adjustments or pay factors. 1.6 Electric Current/Electronic Methods This family of systems includes those that rely on technology such as electrical sensing fields, impedance, electric current, and radio waves to determine the quality of HMA pavement, base, or embankment (see Figures 12 and 13). The training and technical skills required to operate this technology are no different than those required for nuclear density gauges. In addition, the calibration requirements to improve on the accu- racy of testing specific materials with the non-nuclear gauges are similar in detail and extent for nuclear density gauges. Figure 12. Electrical density gauge.
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34 tests. This technology does not require more personnel than are now being used for QC/QA of unbound layers. The system and devices should be easier to maintain and the operators of the equipment can be easily trained in its use--similar to a nuclear density gauge. The most time-consuming but critical part of the system is developing a proper soil model for density and moisture content measurements. To date, other more traditional tests (such as sand cones) are performed in specific locations that cover the range in density and water contents. A regression model is then developed based on correlations between the EDG values and the density and water contents measured from other tests. It is expected that this test will be improved with time, but at present, its use as a practical device for con- trolling construction of unbound layers is limited. 1.6.2 Pavement Quality Indicator Figure 13. Purdue TDR method (courtesy of Durham The PQI (see Figure 15[a]) uses a constant voltage, radio fre- Geo website). quency, electrical impedance approach, in which a toroidal electrical sensing field is established in the material being tested. by EDG, which is confined to use on aggregate base layers, This allows the PQI to make quick, in-situ measurements of embankments and subgrades, or any unbound layer (see Fig- pavement density. The sensor consists of a set of flat plates that ure 14). The system uses 6-in. darts that are driven into the are interconnected to form the electrodes of a planar capacitor. soil within a 1.8 square foot area. This allows the system to Variations in density are determined through changes in the measure a 1.0 cubic foot volume of material. dielectric constant of the medium between the capacitor plates. The system uses a 3-MHz radio signal, producing a current Using this technology, the PQI can be used like the nuclear of a certain voltage and phase, which allows measurements of density gauge, with the exception that it has the capability to the capacitance, resistance, and impedance. The connected adjust for moisture variations and mix type. The device also data acquisition program uses algorithms and ratios of the has an onboard, real-time system that takes the readings and measured parameters to determine the density and water keeps a record of them, allowing it to be integrated seamlessly content of an unbound layer (refer to Figure 14). into the paving process. This test takes several minutes to perform, but it appears to have huge potential for use in replacing the nuclear density gauges and other traditional QA tests, such as the sand-cone 1.6.3 PaveTracker The PaveTracker (see Figure 15[b]) is a light weight non- nuclear device for measuring the uniformity of HMA mixtures. The measurements are practically instantaneous when the device is placed on an HMA surface. Areas of segregation, lower density levels along longitudinal joints or other non- uniformity areas can be detected by the PaveTracker Plus, which allows the operator to correct the problem before con- struction is complete. The advanced software, built-in reference plate, and enlarged display screen are some of the features offered by the Pave- Tracker. The large display screen is an advantage, because the device is compact and close to the ground. Like the PQI, the PaveTracker can be used exactly like the nuclear density gauge, without the use of any nuclear device. The PaveTracker also has an onboard, real-time system that takes the density readings and keeps a record of them for future use, allowing Figure 14. Electrical density gauge in the field. the device to be easily integrated into the paving process.