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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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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.
