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18 · GPR technology for thickness determination of HMA and unbound layers; suggested for use in acceptance plans. The data analysis or interpretation is a limitation of this technology. The GPR data require some processing time to estimate the material property. The time for layer thickness esti- mates is much less than for other layer properties. This technology requires the use of cores for calibration purposes. Cores need to be taken periodically to confirm the calibration factors used to estimate the properties. Use of this technology, even to estimate layer thickness, should be used with caution when measuring the thickness of the first lift placed above permeable asphalt treated base (PATB) layers. GPR can be used to estimate the volumetric properties of HMA mats, but that technology has yet to be verified on a global basis. Measurements using this technology cannot be calibrated using laboratory data. · IC rollers; suggested for use in a control plan, but not within an acceptance plan. The instrumented rollers may not identify localized anomalies in the layer being evalu- ated. These rollers can bridge some defects (may have insufficient sensitivity to identify defects that are confined to local areas). Temperature is considered an issue with the use of IC rollers for compacting HMA layers. Although most IC rollers measure the surface temperature of the mat, the effect of temperature on the mat stiffness is an issue--as temperature decreases the mat stiffness will increase, not necessarily because of an increase in density of the mat. Delaying the com- paction would increase the stiffness of the mat measured under the rollers because of the decrease in temperature. The instrumented rollers also did not properly indicate when checking and tearing of the mat occurred during rolling. The non-nuclear density gauges (PaveTracker) successfully identified this detrimental condition. Measurements using this technology and associated devices cannot be calibrated using laboratory data. Conclusions Unbound Layers and Materials · The GeoGauge is a self-contained NDT device that can be readily incorporated into a QA program for both control and acceptance testing. This conclusion is based on the following reasons: It provides an immediate measure of the resilient modulus of the in-place unbound material. It identified those areas with anomalies at an acceptable success rate (second only to the DSPA). It adequately ranked the relative order of increasing strength or stiffness of the unbound materials. It provided resilient modulus values that were correlated to the dry density over a diverse range of material types. The normalized dispersion is less than for the other NDT devices that provide an estimate of stiffness. The training and technical requirements for this technology are no different than what is required when using a nuclear density gauge. Two disadvantages of using this device in a QA program are (1) the need for measuring the water content and density using other methods, which is also the case for the DSPA and other
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19 modulus estimating devices and (2) the need to calibrate the test results to the material and site conditions under evaluation. The latter is the more important issue and is discussed in more detail. The GeoGauge should be calibrated to the project materials and conditions to improve on its accuracy, especially when testing fine-grained soils. This calibration issue requires that laboratory repeated load resilient modulus tests be performed on each unbound layer for judging the quality of construction. Most agencies do not routinely perform resilient modu- lus tests for design. Eliminating the laboratory resilient modulus tests from the calibration procedure will reduce its accuracy for confirming the design values, but not for identifying construction defects. For those agencies that do not have access to or the capability to perform resilient modulus tests, use of the FHWA-LTPP regression equations is an option that can be used to calculate the target resilient modulus at the beginning of construction. The target resilient modulus should be the value used in structural design. For the MEPDG, this is the average value measured in the laboratory. · The DSPA is also a self-contained unit that was successful in many of the areas noted for the GeoGauge. It was the device that had the highest success rate in identifying areas with different physical conditions or anomalies. An additional advantage of the DSPA is that the results can be calibrated to the specific unbound material being tested prior to construction, when the M-D relationship is measured in the laboratory. This calibration procedure allows the DSPA to be used to detect volumetric, as well as physical, changes in the materials during construction. In other words, the DSPA modulus is measured on the M-D samples prepared at different water contents and dry densities. In short, the DSPA can be used in day-to-day operations to assist contractor and agency personnel in judging construction and materials quality by itself or in tandem with other geophysical and/or ground truth sampling programs. Two disadvantages of the DSPA are that it consistently resulted in a higher normalized dis- persion measured over a diverse range of conditions and materials, and that it requires more sophisticated training of technicians to correctly interpret the load pulse and responses to ensure that satisfactory data have been collected by the device. · The DCP was also successful in many of the areas noted for the GeoGauge. However, testing takes much more time, especially for stiff materials and layers with large aggregate. In addition, the test results were found to be more dependent on aggregate size than the other NDT devices. The normalized dispersion was also found to be much higher than for the DSPA and GeoGauge. Conversely, the DCP does have the capability to readily estimate the strength of thicker unbound layers and can measure the modulus gradient with depth. In fact, it can be used in conjunction with the GeoGauge and DSPA in adjusting the modulus values from those devices to laboratory conditions for fine-grained soils for agencies that do not have resilient modulus testing capability in the laboratory. Use of the DCP can be considered an option in adjusting the test results for the GeoGauge for those agencies that have no plans to incorporate a resilient modulus testing capability within their design or materials departments. · The GPR (single antenna method) was found to have a poor success rate in identifying anom- alies. It did not provide a measure of modulus or strength of material. In addition, using the single antenna method requires that either the density or water content be assumed and the other parameter calculated. Both vary along the project, resulting in higher variations of the property being calculated. Using an inaccurate value can lead to an incorrect finding. For example, the GPR found some of the areas tested to have the highest density, while most other NDT devices found that area to be the softest and least dense. It was successful, however, in measuring the layer thickness of the unbound materials. Two other disadvantages of this system are in the training requirements for using this technology and the need to calibrate the dielectric values to physical properties of the in-place
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20 material. Samples need to be recovered and tested to determine the water contents and den- sities of those areas prior to using the results for QC or acceptance. This requires that control strips be used prior to construction, and these calibration factors should be checked periodically during construction. Many agencies are not requiring control strips, or the first day of con- struction is the control strip. Training is another issue; this system requires more sophisticated training for the operator to interpret the measurements taken with the GPR. Thus, with its current limitations, it is not suggested for future use in testing unbound materials to determine the quality characteristics of the in-place material. However, it is suggested that research with the GPR continue because of its continuous coverage and speed of data collection. · Similar to the GPR, the EDG was found to have a poor success rate in identifying areas with anomalies. However, this device is believed to have potential to provide volumetric data on the unbound materials for use in a QA program with continued use. The density estimated from this device is definitely related to resilient modulus across a wide range of unbound materials. However, further improvements in the measurements will require a program to obtain addi- tional data. The variability of the water contents measured with this device was found to be very low. Other agencies are beginning to use this device in their research programs. For example, Texas and Nevada have ongoing programs that could provide improvements to the equipment and procedures in the near future. As a result, further detailed evaluation of this device and technology to improve its accuracy are warranted. · The deflection-based methods (LWD and FWD) were found to have limited potential for QC purposes. The LWD devices have greater mobility than the FWD, which is an advantage for their use over the FWD. These devices have more potential for use in acceptance programs of the final structure and certainly in forensic areas for evaluating the interaction between the pavement layers and foundation. The following summarizes the conclusions reached on these devices: Technology was unable to consistently identify those areas with anomalies. The modulus values can be influenced by the underlying layers, resulting in lower or higher and more variable modulus values. The normalized dispersion was found to be high, relative to the other NDT devices. The relationship between modulus from this technology and dry density was poor. Any error in thickness of the layer being tested can result in large errors and more variabil- ity that could lead to wrong decisions being made by the contractor and agency about the construction operation. HMA Mixtures · The PSPA is a self-contained NDT device that can be readily incorporated into a QA program for both control and acceptance testing of HMA mixtures. As noted for unbound materials, an advantage of this technology is that the device can be calibrated to the specific materials being tested during the mixture design stage for HMA mixtures. This calibration procedure allows the PSPA to be used to detect volumetric, as well as physical, changes in the materials during construction. In short, the PSPA can be used in day-to-day operations to assist con- tractor and agency personnel in judging construction and materials quality by itself or in tan- dem with other geophysical and/or ground truth sampling programs. This conclusion is based on the following reasons. The PSPA is the NDT device found best suited for QA applications because it adequately identified all but one area with anomalies. The PSPA provides a measure of the dynamic modulus that is needed for pavement structural designs, even before adjusting the PSPA modulus for laboratory conditions. The PSPA modulus was found to be correlated to the dynamic modulus at elevated temperatures using the master curve developed from labo- ratory dynamic modulus tests.