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26 Figure 3. DSPA and PSPA being used to test different materials. being used for control or acceptance of flexible pavement con- GeoGauge were found to be a function of the material's struction. In fact, the same technician using the nuclear density moisture content and density. Stiffness readings were also gauges or taking cores from the HMA layer could also operate reported by the test equipment and were a function of the the PSPA and DSPA at the same time. Its main disadvantage is structure. training the operators to determine a "false" reading. The process followed by the GeoGauge operator is almost In summary, the ultrasonic technology can be used in day- identical to that followed by an operator of the current state- to-day QA operations to assist contractor and agency personnel of-the-art nuclear density gauge, except that the GeoGauge in judging construction and materials quality by itself or in operator spreads a thin layer of sand on the pavement surface to tandem with other geophysical and/or ground truth sampling set the instrument on before taking the reading (see Figure 4). programs. The operator clears the surface to be tested with a small broom or other device to remove loose surface particles (see Figure 4). A thin layer of moist sand is used on rough surfaces to fill in 1.2 Steady-State Vibratory-- surface voids to ensure that the ring under the gauge is in con- GeoGauge tact with at least 75 percent of the test surface. Moist sand This system is applicable to HMA and unbound materials should be used because the gauge vibrations will cause dry and soils, and is similar to the roller-mounted devices that sand particles to shift under the gauge and disturb the reading. are described in Section 1.7. The GeoGauge, however, is only The layer of moist sand should only be thick enough to fill the used for testing unbound materials and soils. The GeoGauge surface voids of the material being tested. A light pressure and provides elastic modulus values that are displayed on the rotation of the GeoGauge was also used to ensure good contact gauge or stored in the device and downloaded to a computer with the test surface. at a later date. The resulting values were found to be similar Each test takes 75 seconds, as compared to the nuclear to the resilient modulus values measured in the laboratory density gauge's 60 seconds. Thus, this test takes about twice or calculated from the resilient modulus regression equa- as long as the nuclear density gauge, including the time for tions developed through the FHWA-LTPP program (Yau spreading the sand. The test procedure is still quick enough and Von Quintus 2002). The elastic modulus values from the not to be a hindrance to the contractor's progress and does