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21 Similar PSPA modulus values were measured at higher temperatures and corrected for temperature using a master curve in comparison to those measured in the laboratory. An important condition that the NDT device needs to consider is the effect of time and varying moisture content on the properties of the HMA mixture near construction and how those properties will change in service. There have been various studies completed on using the PSPA to detect stripping in HMA mixtures. For example, the PSPA was used in combination with GPR to successfully locate areas with stripping along selected interstate highways in Georgia (Hammons et al. 2005). The test results from the NCHRP 10-65 study support a similar conclusion. However, the PSPA does have some limitations regarding full-scale use in QA programs. Use of the PSPA should be delayed after rolling to allow the mix to cool. Dr. Nazarian's rec- ommendation is to delay all testing for one day after HMA placement and compaction. If required, this time restriction is considered a disadvantage for use in QA programs. A measure of the mixture density or air voids is also required in judging the acceptability of the modulus value or durability of the HMA mixture. The two devices that deserve further evaluation include the GPR and non-nuclear density gauges. The GPR provides full coverage in a short period of time. The non-nuclear density gauges are also well suited for QA because they can be readily incor- porated into control programs. Some contractors are already using the non-nuclear density gauges in controlling the compaction operation. This technology was also used to identify anomalies at a reasonable rate and can be used to identify tender mixtures and the effects of rolling in the temperature sensitive zone. Variations in water have a definite effect on the HMA density measured with the PQI. The manufacturer's recommendation is to measure the density immediately after compaction, prior to allowing any traffic on the HMA surface. This type of time restriction is considered a disadvantage to the use of the PQI in a day-to-day practical QA program. This time effect, however, was not found within the Part A test program, but the moisture effect was observed in Part A of the field evaluation. Use of other non-nuclear density gauges (PaveTracker) did not exhibit this moisture sensitivity. However, the effect of water on these gauges was not included in the field evaluation as a primary variable. Measurements were taken after heavy rains in areas where the readings were previously taken prior to the thunderstorms. The same density values were measured after removing and drying all free water at the surface. This potential bias of free water on the surface is not considered a limitation but must be considered in taking measurements for control purposes. Use of the GPR technology using the single antenna method, even with mixture calibration, requires assumptions on specific volumetric properties that do vary along a project. Using the multi-antenna method is expected to improve on the measurement of the volumetric prop- erties and identification of areas with deficiencies or anomalies. Thus, the GPR is suggested for continued research studies, especially with the multiple antenna system, which is a propri- etary analysis system. The proprietary system needs additional validation prior to full-scale implementation into a QA program. The FWD is not suggested for use in QA programs, because this technology was unable to identify some of the anomalies. In addition, the FWD has high variation in elastic modulus values, and those values are influenced by the strength of the underlying materials and layers. Recommendations The research team's recommendations are based on the evaluation of NDT devices for imme- diate and practical use in QA programs. Thus the GeoGauge can be used for estimating the modulus of unbound layers, while the PSPA is the device suitable for use with HMA layers.

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22 The PaveTracker can be used in establishing and confirming the rolling pattern for HMA mix- tures. Other NDT devices may provide useful data for pavement and materials testing purposes. Each has its own benefits and advantages for evaluating and designing pavements. The IC or instrumented rollers can be valuable to a contractor in terms of controlling the com- paction operation. These rollers that operated without problems were used on too few projects to suggest their immediate inclusion in QA programs. Nonetheless, they can assist the contractor in optimizing the compaction of the material. Their disadvantage for HMA layers is the temper- ature of the mat issue. Decreases in temperature will cause the stiffness of the mat to increase. Thus, other devices still need to be used with the IC rollers for control. The use of IC rollers for acceptance is not suggested at this time. Research with the multi-antenna GPR device and proprietary data interpretation system should not be abandoned and should be validated in future studies. This system definitely shows promise in providing the volumetric properties for HMA mixtures. The data can be collected at highway speeds, and the proprietary data interpretation system can provide results on a real-time basis. The disadvantage of this system is that it also needs field cores for calibrating the method to project specific conditions. These cores should be taken periodically to confirm the calibra- tion factors being used in estimating the volumetric properties.