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NDT Technology for Quality Assurance of HMA Pavement Construction (2009)

Chapter: Chapter 1 - Applicability of NDT Technologies on Construction Projects

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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
×
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
×
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
×
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
×
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
×
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
×
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
×
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
×
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Suggested Citation:"Chapter 1 - Applicability of NDT Technologies on Construction Projects." National Academies of Sciences, Engineering, and Medicine. 2009. NDT Technology for Quality Assurance of HMA Pavement Construction. Washington, DC: The National Academies Press. doi: 10.17226/14272.
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24 Some NDT devices initially were operated by a representa- tive of the manufacturer and then used by field technicians or engineers. Those devices that were found to have a reasonable success rate in identifying anomalies were used by the con- tractor and agency staff in their daily QA operations, in accor- dance with manufacturers’ guidelines. Clustered tests were performed using each NDT device to determine the repeata- bility and accuracy of each system in evaluating its effective- ness in defining construction quality. The time and personnel requirements to perform each test were recorded. This infor- mation was considered in rating the level of impact that each device may have on construction. Since the technology was of primary interest (not a particular system or manufacturer), reports on each system are presented under the heading of the technology used by the system. 1.1 Ultrasonic—PSPA and DSPA This system is applicable to HMA, unbound aggregate base, and embankment soils. The PSPA is used to test HMA, while the DSPA is used for unbound materials and soils. Both devices consist of a stand linearly connected by a stiff arm to a source and two receivers and by wire to a computer, as shown in Figure 2. The source contains a hammer which is dropped several times at regular intervals. The receivers, containing quartz-crystal accelerometers, measure the acceleration of the Rayleigh waves induced by the dropping of the hammer and report the resulting electrical charge to the data acquisition system. An FFT transforms the electrical charge or data into the frequency domain. There is also a temperature sensor in the system. Sturdiness of the laptop is an important feature. The PSPA test can be and was performed on cold material one or multiple days after placement, as well as on surfaces at elevated temperatures immediately after compaction. The system’s temperature gauge is used to incorporate the tem- perature into the calculation of the material’s modulus. The rubber pads beneath the receivers deteriorate more rapidly when used on surfaces at elevated temperatures. In fact, they have been known to melt when used on HMA surfaces shortly after placement. The operator needs to check these periodi- cally to ensure adequate coupling between the receivers and the surface. These pads are easily replaced. Both devices work properly as long as all points are in firm contact (coupled) with the surface being tested. Adequate cou- pling is the system’s primary limitation. The speed of data collection makes this technology a good candidate for QC applications, assuming that the temperature of the material is properly considered by the modulus calculation process. None of the PSPA and DSPA devices (including the laptops) used exhibited any problems. The main operational issue was inspecting and replacing the rubber pads of the receivers to ensure good contact with the surface being tested. The data interpretation program that comes with the PSPA and DSPA devices uses this information to provide the output in the form of the mean Young’s modulus to a particular depth. The spacing of the receivers determines the depth of measurement. The operator needs to be trained to visually inspect the load pulse and response data on the output screen for judging the suitability of an individual test (see Figure 3). This training is considered more sophisticated than what is required for a nuclear density gauge. The operator also needs to ensure that the spring-loaded receivers are in contact with the surface between each test. If one of the receivers gets stuck, the result will be a data anomaly or “false” reading. With proper training, the operator can easily identify false readings by viewing the shape of the load pulse and receiver response. The shapes of the load pulse and receiver response are visually displayed on the laptop screen for each reading. The PSPA is used to test HMA mixtures, while the DSPA is used to test crushed aggregate base layers, embankments, and prepared subgrades. The DSPA was used on the shoulders of the US-280 reconstruction project instead of on the main roadway because the roadway base layer had been chip-sealed. This type of surface reduces the repeatability of the ultrasonic C H A P T E R 1 Applicability of NDT Technologies on Construction Projects

device, as well as other NDT devices, because the points of the receivers and source are not always in good contact with the surface tested. Ensuring good contact with the surface being evaluated is important for both the PSPA and DSPA. The system initially converts the readings of the load pulse and response to a seismic modulus of the material. The seismic modulus is internally adjusted to a modulus at a specific con- dition (temperature and load frequency for HMA). Each test location requires three to five tests for this system. Each test took 10 to 20 seconds to complete. Therefore, the entire process (3 to 5 readings at a point) takes only slightly longer than the system currently used for QC, the nuclear density gauge, which is generally set for one 60-second reading. This system can also be used to estimate the elastic proper- ties parallel and perpendicular to the direction of the rollers (refer to Chapters 2 and 3). Measuring the seismic properties in different directions actually increases the perceived vari- ability of the device. The variability can be reduced slightly by always taking the readings in one direction. All other NDT devices result in an average or equivalent value at a test point. The spacing of the receivers can also be changed easily for testing thin and thick layers. Layer thickness variation that occurs along a construction project can have less of an impact on the resulting seismic modulus values than on the resulting values from other NDT technologies. Another advantage of this technology is that the system can be calibrated easily to the specific materials being tested during the mixture design stage for HMA materials or in developing M-D relationships for unbound materials. This calibration procedure allows the PSPA and DSPA to be used to detect volumetric, as well as physical, changes in the materials during construction. The DSPA can be used to develop modulus growth with compaction relationships during the first day of construction for the unbound layers and periodically during the project. Use of the PSPA to develop HMA modulus growth relation- ships can be problematic because of the elevated temperature. It is more applicable to warm-mix projects. The equipment (including the laptop) was found to be durable, and it did not require more personnel than those now 25 Carriage case recently developed for facilitating the use of the PSPA & DSPA in data collection. Figure 2. PSPA in operation for testing HMA layers. The DSPA is used for testing unbound layers.

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

not require more personnel than those now being used for control and acceptance. As for the DSPA, the same technician using the nuclear density gauge or running sand-cone tests could also operate the GeoGauge at the same time. The train- ing and technical capability of the operator is no more than what would be required for operating a nuclear density gauge. Similar to the DSPA, the GeoGauge can easily be used to develop relationships between modulus growth and com- paction effort in unbound layers. Such relationships can be initially developed at the start of the project to optimize the compaction process and then be periodically verified throughout the project. This feature becomes advantageous when the water content significantly varies from the optimum value measured in the laboratory. The GeoGauge should be calibrated to the project materials and conditions to improve on its accuracy, because of the potential influence of the supporting materials. 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 per- form resilient modulus tests for design or for forensic evalu- ations, even though the 1993 AASHTO Design Guide suggests that they be performed (AASHTO 1993). Eliminating the laboratory resilient modulus tests from the calibration proce- dure will reduce its accuracy for confirming the design values, but not for identifying construction defects. As a replacement to the repeated load resilient modulus test, the regression equations developed from repeated load resilient modulus tests included in the LTPP program (Yau and Von Quintus 2002) or the use of the DCP is permissible. The disadvantage of the GeoGauge is that it will result in high variability when testing non-cohesive, well-graded sands or similar soils. In addition, the elastic modulus readings from the gauge represent an equivalent modulus for the upper 10 to 12 in. of the layer. Thus, the gauge in its current form should not be used to test thin (less than 4 in.) or thick (greater than 12 in.) layers without proper material calibration adjustments or changing the diameter of the ring under the gauge. In summary, the GeoGauge has potential use in day-to-day QA programs by both the contractor and the agency personnel. 1.3 Deflection-Based Methods 1.3.1 Falling Weight Deflectometer The FWD is a large, expensive apparatus that is mounted on a trailer and pulled behind a tow vehicle. The operator works a computer and locates the apparatus for testing (see Figure 5). This system is capable of applying dynamic loads to the pavement surface, similar in magnitude and duration to 27 Figure 4. Humboldt GeoGauge.

that of a single heavy moving wheel load. It is being used within the LTPP program, and most state agencies have access to at least one FWD. Thus, it is already being used in most agencies’ day-to-day practice. The response of the pavement system is measured in terms of vertical deformation, or deflection, over a given area using seismometers or geophones. An FWD enables the user to deter- mine a deflection basin caused by a controlled load. These results make it possible to determine the stiffness of existing pavement structures for use in M-E based rehabilitation design methods. The falling weight strikes a set of rubber buffers mounted to a 300-mm circular foot plate, which transmits the force to the pavement (see Figure 5). A thin-ribbed rubber pad is always mounted under the footplate. By varying the mass or the drop height or both, the impulse load can be varied. This load may be varied between 10 kN and 140 kN. Sensors measure the surface deflections caused by the impulse load. Most agencies use seven sensors at the spacing recommended by LTPP. However, fewer or more sensors can be used, and those can be spaced uniformly or at some other spacing selected by the user. Peak deflections are recorded, stored, and displayed. In some cases, one of the geophones or sensors can be incorrectly placed on the test surface by the sensor bar, especially on rough surfaces. The data acquisition software will identify this anomaly, notifying the operator that the test should be rejected and redone. The test takes about 2 minutes to complete, including the use of seating drops. Seating drops are important and should be used at each test point. This does not include time to con- figure the trailer and set up the data acquisition system, which should only have to be done once per day for each project. It takes about 30 minutes to configure the trailer and 2 to 3 min- utes to set up the data acquisition program. Similar to the PSPA, the operator needs more technical and sophisticated training in setting up the equipment and visually interpreting the deflection basin data. A separate data interpretation system or software is required for producing elastic modulus values from the measured deflection basins—Young’s modulus for each layer. The calculated elastic modulus values are structure dependent. Most data interpretation or analysis programs used back- calculation techniques for calculating layered elastic modulus values. Backcalculation programs do not determine unique 28 Figure 5. Trailer mounted FWD.

modulus values for each layer and are sensitive to layer thick- ness variations. Forward-calculation procedures have been developed that result in unique layer modulus values for a particular deflection basin, but these values are thickness dependent. Any errors in the layer thickness will increase the error and variability of the processed data. Its use for acceptance of individual layers by the agency should be limited to the use of the forward-calculation proce- dure. Because the backcalculation procedures do not result in unique layer modulus values, it would be difficult to defend in contractor disputes where material has been rejected or pay- ment penalties issued to the contractor. The device can be used to check or confirm the final flexible pavement for new con- struction or HMA overlays of existing pavements, but would probably create many disputes with the contractor when the entire pavement structure is rejected at the end of the project. In addition, the resulting values for the upper layer are dependent on the stiffness and variability of the supporting layers. Calculating the elastic modulus of layers is generally restricted to those that are thicker than 3 in. The FWD may also require one additional field technician and tow vehicle. The expense, size of the system, time needed to perform each test, and data interpretation software make this system less practical for QC and acceptance. Thus, the FWD is believed to be less practical and effective for the QA uses that are the focus of this study. 1.3.2 Light Weight Deflectometer The LWDs use the same theory as the FWD, but offer an advantage of being much more portable. In addition, the training and technical requirements for the LWD operators are no different than for nuclear density gauges, with one exception—the operator needs to understand and be aware of the factors and physical features that affect layer modulus calculated from the measured deflections. Results from the LWDs were significantly influenced by the supporting materials on some of the projects. All three LWD devices used on selected projects have similar features. Only the Dynatest and Carl Bro devices are discussed in the following paragraphs. 1.3.2.1 Dynatest Prima 100 LWD Device The Prima 100 is manufactured by Dynatest and consists of the weight (hammer) on a pole and the sensors (geophones) in a plate on the ground, all encompassed in one, connected, portable structure (see Figure 6). The sensors were connected to a handheld computer by wireless remote technology. The unit tested was somewhat flexible and the frame came apart on multiple occasions. Besides slowing down the process, this resulted in questionable data because the wireless remote would sense the jolt from the frame coming apart as a sepa- rate test, resulting in a deflection and modulus value for that anomaly. The wireless remote was troublesome and kept losing con- tact with the apparatus. This happened anytime the technician carrying the apparatus came within a few feet of the technician holding the computer. This slowed down the operation because the computer had to be re-started each time it occurred. When using the system on particularly stiff base material, the hammer can bounce high enough, such that it can strike the apparatus again—resulting in an appreciable rebound load. The rebound load can cause the remote to mistake that rebound as a second or separate test. The software, as written, causes the actual test results to be deleted and replaced by a reading of the rebound. The system, however, is fast. One test takes about 10 seconds, so the five tests conducted (and averaged) at each location take approximately the same amount of time that a nuclear density reading takes at one location. However, the apparatus is bulky to handle, so the time that most non-nuclear systems gain by not having to deal with the steps of transporting the nuclear device are lost. 29 Figure 6. Dynatest Prima 100 LWD.

1.3.2.2 Carl Bro LWD Device The Carl Bro system looks exactly like the Dynatest system, except that it has additional sensors that are not attached to the frame. These extended geophones do not change the theory and applications. Although, the algorithms are slightly differ- ent to include input from the additional sensors, the theory and application appear to be the same. The geophones are arranged linearly at set distances from the plate. Since the sensors are connected to each other by a bar, but separate from the loading plate, connecting and placing them at a specific distance from the plate for each test becomes problematical. It is expected, however, that these perceived disadvantages can be resolved in future modifications to the equipment. The process, from the beginning through the last of the five drops, takes an average of about 5.5 minutes. The procedure followed for using the system is listed. 1. Locate test point (surface must be even (flat) and must be cleared of anything that could cause part of the plate to lose contact with the surface). 2. Set the loading plate on the surface to be tested (plate must be flat on the surface). 3. Measure for geophone location. 4. Set the geophone arm and line up the sensors. 5. Set data acquisition key for collecting the deflection data. 6. Drop hammer (first drop “seats” the plate and is not read). 7. Repeat last two steps for five drops at each location (including the one to seat the plate). This system had a wired connection to a laptop computer and was more cumbersome to set up because of the additional geophones. In addition, the seating drop of the plate some- times moved the plate. This increased the variability in the data gathered from the geophones and increased the number of anomalies. The system is comparable in cost to the Prima 100. 1.3.2.3 Summary This technology was tested on crushed aggregate base material, embankments, and prepared subgrades. However, there should be no difference between the procedures and the device’s reaction to a hard base material and those of HMA mixtures. A key advantage of this technology is that it gives the operator a reading of the elastic modulus in about the same time required to obtain a nuclear density gauge reading. The disadvantages are that the devices have limited reliability because of the range and reliability of the wireless remote and its software logic. In addition, the resulting values for the upper layer are dependent on the stiffness and variability of the supporting layer. It is expected that these disadvantages of the equipment can be easily resolved with future modifications. These devices will likely make the technology and device more expensive. It does, however, provide the agency with elastic modulus val- ues that can be used to confirm design assumptions with proper calibration. In summary, the LWDs are believed to be less practical and effective for the uses that are the focus of this study. 1.4 Dynamic Cone Penetrometer The DCP is used to estimate the strength and modulus of unbound materials and soils. The DCP is much like the LWD in appearance (see Figure 7); however, it uses a 15-lb (6.8-kg) steel mass falling 20 in. (50.8 cm) that strikes the anvil to cause penetration of a 1.5-in. (3.8-cm) diameter cone (45° vertex angle) that has been seated at the surface or in the bottom of a hand augered hole (see Figure 8). The blows required to drive the embedded cone a depth of 13⁄4 in. have been correlated by others to N values derived from the Standard Penetration Test (SPT). Experience has shown that the DCP can be used effectively in augered holes to depths of 15 to 20 ft (4.6 to 6.1 m). The system has been used in the past for the testing of soils more than anything else. The technical skills and training requirements for the DCP operator are no different than for a nuclear density gauge. Advantages of the DCP include its simplicity, low maintenance (using disposable tips, making sure that the allen screws are kept tight, etc.), mobility, and low cost. It can also be used to test thick embankment layers, unlike some of the other NDT technologies and devices. Conversely, the manual apparatus is slow (tests took 5 to 10 minutes at each location), its use is physically demanding, and the test is actually destructive to bases and pavements, that is, the test creates a hole in the material. Use of the device can also be dangerous, if the operator’s hand gets caught 30 Figure 7. DCP before assembly for use in measuring the in-place strength of unbound materials and layers.

between the hammer and base for the hammer. Furthermore, soils or materials with boulders or large aggregate particles (refer to Figure 9) can cause refusal of the device. When this occurs, the test point should be moved and the test redone. An automated trailer mounted DCP is available, but is more expensive (see Figure 10). Only the manual DCP was used in the field evaluation of NCHRP Project 10-65. The manual DCP is considered to have potential for QC use on a day-to-day basis, but an additional contractor and agency staff person would probably need to be assigned to use the DCP under normal practices; however, the training and maintenance of this device is considered minimal. 1.5 Ground Penetrating Radar GPR is a pulse echo method for measuring pavement layer thicknesses and properties. GPR uses radio waves to penetrate the pavement by transmitting the wave energy into the pave- ment from a moving antenna. These waves travel through the pavement structure and echoes are created at boundaries of dissimilar materials. An air-coupled horn antenna attached to the back of a small SUV (see Figure 11) was used in the field evaluation of NCHRP Project 1065 to evaluate HMA, unbound aggregate base, and embankment soils. The speed of data collection is one of the biggest advan- tages of GPR technology. There should be no impact to the contractor’s operation, because this system collects the same information regardless of material temperature and is capa- ble of taking measurements at speeds of up to 40 miles per hour. Higher speeds have been used on more recent projects through enhancements made to the equipment and data acquisition systems. The disadvantages of the technology are the interpretation of the dielectric values that are measured and personnel requirements for calibrating and maintaining the equipment and data interpretation software. The system is simple to operate and provides results imme- diately, at least in terms of dielectric values. The results are in the form of a “picture” of the pavement system, much like an X-ray. Although the transducer is located above the surface, aimed downward, the picture can be viewed from “plan” or “elevation” (“profile”) perspective. Another huge advantage of this technology is that a continuous profile of the dielectric values is available. In fact, layer thickness profiles or complete contours of the layer can be developed in a short time period. Currently, the technology requires operators with special technical skills to interpret the data that have physical meaning to the quality of construction. Software programs are avail- able that provide color-coded charts and contours of the material. This system has been used to determine layer thick- ness at a reasonable accuracy—when layers with different dielectric values are tested. The accuracy of the analysis pro- grams requires cores to accurately measure the in-place thick- ness and other volumetric properties. Most of the data reduction-presentation programs, how- ever, still require some volumetric properties to be assumed in estimating density, air voids, and other volumetric prop- erties. These assumptions result in error of the properties that are calculated from the dielectric values. The assumptions are believed to be a reason why the GPR’s analysis and inter- pretation from the Part A projects did not coincide with some of the other NDT devices. There are programs available that do not require many assumptions, but all of the known pro- grams are proprietary. These proprietary programs were not used in the Part A field evaluations, but were included in the Part B summary at a few facilities. Data from some of these proprietary programs is presented and discussed in Chapter 2. Calibration is another issue that is important to the suc- cess of GPR antennas in estimating volumetric properties of materials. Cores have to be recovered and the physical 31 Figure 8. Manual DCP in operation (courtesy of Minnesota Road Research Section, Office of Materials, Minnesota DOT).

32 Large aggregate particles in the embankment soil caused refusal of the DCP in localized areas. These particles found near the surface also had an impact on the DSPA and GeoGauge readings. Figure 9. DCP test and large aggregate particles encountered at some of the projects, resulting in refusal of the test. Figure 10. Automated DCP attached to a trailer (courtesy of Minnesota Road Research Section, Office of Materials, Minnesota DOT).

properties of those cores determined and correlated to the dielectric values measured by the GPR prior to and during construction. This requires that control strips be used at the beginning of a project and the correlations periodically con- firmed during construction. Many agencies are eliminating or not requiring the contractor to use control strips, especially 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. 1.6.1 Electrical Density Gauge An electrical density gauge was used in the Part A field evaluation projects, because of the equipment’s perceived ease of use and application to a diverse set of unbound materials and soils. The specific gauge used was the one manufactured 33 Figure 12. Electrical density gauge. b. Ground-Coupled Antenna Arrays Attached to Survey Vehicle GPS Receiver GPR Antenna a. Air-Coupled GPR Antenna Attached to Survey Vehicle GPS Data Acquisition & Storage Figure 11. GPR antennas attached to a standard survey vehicle.

by EDG, which is confined to use on aggregate base layers, embankments and subgrades, or any unbound layer (see Fig- ure 14). The system uses 6-in. darts that are driven into the soil within a 1.8 square foot area. This allows the system to measure a 1.0 cubic foot volume of material. The system uses a 3-MHz radio signal, producing a current of a certain voltage and phase, which allows measurements of the capacitance, resistance, and impedance. The connected data acquisition program uses algorithms and ratios of the measured parameters to determine the density and water content of an unbound layer (refer to Figure 14). 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 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 The PQI (see Figure 15[a]) uses a constant voltage, radio fre- quency, electrical impedance approach, in which a toroidal electrical sensing field is established in the material being tested. This allows the PQI to make quick, in-situ measurements of pavement density. The sensor consists of a set of flat plates that are interconnected to form the electrodes of a planar capacitor. Variations in density are determined through changes in the dielectric constant of the medium between the capacitor plates. Using this technology, the PQI can be used like the nuclear density gauge, with the exception that it has the capability to adjust for moisture variations and mix type. The device also has an onboard, real-time system that takes the readings and keeps a record of them, allowing it to be integrated seamlessly into the paving process. 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 the device to be easily integrated into the paving process. 34 Figure 13. Purdue TDR method (courtesy of Durham Geo website). Figure 14. Electrical density gauge in the field.

1.7 Intelligent Compactors/Rollers with Mounted Response Measuring Devices These systems offer real-time pavement quality measure- ment with no negative impact to the contractor’s progress. They use accelerometers to measure parameters of the com- pactor’s vibratory signature. Other sensors are also used to gain information about the pavement. Information from the sensors is then used to make decisions about pavement quality. Although these roller-mounted systems have been shown to be beneficial to a contractor from a control standpoint, they have not been used for acceptance and confirmation of the design-modulus values. Two of these systems were used in the demonstrations sponsored by FHWA at the NCAT and MnROAD facilities and included in the NCHRP Project 10-65 field evaluations. They are described in the following paragraphs. 1.7.1 Asphalt Manager and Varicontrol System This system, developed by Bomag, contains an onboard pavement analysis system based on the electrical charge gen- erated by strategically mounted quartz-crystal accelerometers that measure the acceleration of the vibratory drums of the compactor. An onboard computer transforms the data from the sensors using an FFT into the frequency domain. This transformation allows the computer to calculate the material’s modulus. There is also a temperature sensor in the system, which feeds data into the computer for use in modulus calcu- lations. In addition, the system takes this reading and alters the compaction effort of the roller to avoid the damaging effects of over-compaction. Stiffness readings are taken continuously and presented as a modulus value developed by Bomag and called Evib, in the form of MN/m2. The Evib value should be related to the dynamic modulus of the material being compacted. However, this computed value is expected to be affected by the underlying support conditions. To date, the Evib value has not been evaluated or checked against dynamic modulus values measured in the laboratory or esti- mated through other NDT devices. The system is fully integrated into a vibratory roller that is part of an operational paving train (see Figure 16). The true test of this “intelligent compaction” system is whether it actually saves time (fewer passes), improves uniformity of the mat, and renders accurate, consistent readings. As for this part of the analysis (impact on the contractor’s progress), assuming that the system does what it claims, it can only help the contractor’s progress. 1.7.2 Ammann Compaction Expert Ammann-America, the U.S. branch of the Swiss manufac- turer Ammann Compaction, Ltd., has introduced the Ammann Compaction Expert (ACE) to the U.S. market. The goal of the ACE is the same as for the Asphalt Manager. The major differ- ence is that the ACE seems to take the paving environment 35 (a) PQI Non-Nuclear Density Gauge (b) PaveTracker Non-Nuclear Density Gauge Figure 15. Non-nuclear, non-roller-mounted devices used to measure the density of HMA layers.

into account more than the Asphalt Manager does in an auto- mated fashion. The computer in the ACE system is capable of receiving information such as lift thickness, number of passes, mix or soil type, which is used in the calculation of the mate- rial’s stiffness or modulus. Just as with the Asphalt Manager, the system is fully integrated into a vibratory roller that is part of an operational paving train. 1.7.3 Summary The true test of this “intelligent compaction” system is whether it actually saves time (fewer passes), improves uni- formity of the mat, and results in accurate, consistent readings. For impact on the contractor’s progress, assuming that the roller-mounted devices do what is claimed, they can help the contractor’s progress and provide information so that the contractor can make better decisions in real-time regarding compaction of pavement layers. 1.8 Summary of Process Impact Table 13 provides a summary of process impact on flexible pavement construction for different NDT technologies and devices regarding their use in QA programs. 36 a. BOMAG Asphalt Manager IC Roller b. AMMANN IC Roller c. Caterpillar IC Roller d. Vibratory Roller Instrumented by TTI for Use on Research Projects Figure 16. Fully equipped rollers measuring the stiffness of the material being compacted.

37 Table 13. Process impact of different NDT technologies and devices on QA programs. NDT Technologies Deflection-Based DCP Non-Nuclear Devices Impact Topics or Issues Ultrasonic Gauges Steady- State Vibratory Trailer Portable Manual Automated GPR Non-Roller- Mounted Roller- Mounted Easily used to develop density or modulus growth curves? HMA- No Unbound- Yes Yes No Yes No No No Yes Yes Resulting Value Seismic Modulus Elastic Modulus Deflection Deflection & Elastic Modulus Penetration Rate or Index Penetration Rate or Index Dielectric Values Density & Water Content Stiffness or Density Conversion required to adjust readings? Yes No No No No No Yes No No Requires calibration to specific materials or soils? Yes Yes Yes Yes Yes Yes Yes Yes No Can readily test thin layers (<3 in.) Yes No No No Yes Yes Yes Yes Yes Can readily test thick layers (>12 in.) Yes No Yes Yes Yes Yes Yes No No Readily applicable to control? Yes Yes No Yes Yes Yes No Yes Yes Readily applicable to acceptance? Yes Yes No, only final structure Yes Yes Yes Yes Yes No Additional auxiliary equipment needed? No No Yes, tow vehicle No No Yes Yes, vehicle No No Additional staff needed? No No Yes, operator No No Yes Yes No No Equipment readily available on commercial basis? Yes Yes Yes Yes Yes Yes Yes Yes Yes Software readily available on commercial basis? Yes NA Yes Yes NA NA No; for Proprietary NA NA

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TRB's National Cooperative Highway Research Program (NCHRP) Report 626: NDT Technology for Quality Assurance of HMA Pavement Construction explores the application of nondestructive testing (NDT) technologies in the quality assurance of hot-mix asphalt (HMA) pavement construction. Supplementary material to NCHRP Report 626 was published as NCHRP Web-Only Document 133: Supporting Materials for NCHRP Report 626

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