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24 CHAPTER 1 Applicability of NDT Technologies on Construction Projects Some NDT devices initially were operated by a representa- when used on surfaces at elevated temperatures. In fact, they tive of the manufacturer and then used by field technicians or have been known to melt when used on HMA surfaces shortly engineers. Those devices that were found to have a reasonable after placement. The operator needs to check these periodi- success rate in identifying anomalies were used by the con- cally to ensure adequate coupling between the receivers and tractor and agency staff in their daily QA operations, in accor- the surface. These pads are easily replaced. dance with manufacturers' guidelines. Clustered tests were Both devices work properly as long as all points are in firm performed using each NDT device to determine the repeata- contact (coupled) with the surface being tested. Adequate cou- bility and accuracy of each system in evaluating its effective- pling is the system's primary limitation. The speed of data ness in defining construction quality. The time and personnel collection makes this technology a good candidate for QC requirements to perform each test were recorded. This infor- applications, assuming that the temperature of the material mation was considered in rating the level of impact that each is properly considered by the modulus calculation process. device may have on construction. Since the technology was of None of the PSPA and DSPA devices (including the laptops) primary interest (not a particular system or manufacturer), used exhibited any problems. The main operational issue was reports on each system are presented under the heading of the inspecting and replacing the rubber pads of the receivers to technology used by the system. 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 1.1 Ultrasonic--PSPA and DSPA in the form of the mean Young's modulus to a particular This system is applicable to HMA, unbound aggregate depth. The spacing of the receivers determines the depth of base, and embankment soils. The PSPA is used to test HMA, measurement. The operator needs to be trained to visually while the DSPA is used for unbound materials and soils. Both inspect the load pulse and response data on the output screen devices consist of a stand linearly connected by a stiff arm to for judging the suitability of an individual test (see Figure 3). a source and two receivers and by wire to a computer, as shown This training is considered more sophisticated than what is in Figure 2. The source contains a hammer which is dropped required for a nuclear density gauge. The operator also needs several times at regular intervals. The receivers, containing to ensure that the spring-loaded receivers are in contact with quartz-crystal accelerometers, measure the acceleration of the the surface between each test. If one of the receivers gets Rayleigh waves induced by the dropping of the hammer and stuck, the result will be a data anomaly or "false" reading. report the resulting electrical charge to the data acquisition With proper training, the operator can easily identify false system. An FFT transforms the electrical charge or data into readings by viewing the shape of the load pulse and receiver the frequency domain. There is also a temperature sensor in response. The shapes of the load pulse and receiver response the system. Sturdiness of the laptop is an important feature. are visually displayed on the laptop screen for each reading. The PSPA test can be and was performed on cold material The PSPA is used to test HMA mixtures, while the DSPA is one or multiple days after placement, as well as on surfaces used to test crushed aggregate base layers, embankments, and at elevated temperatures immediately after compaction. The prepared subgrades. The DSPA was used on the shoulders of system's temperature gauge is used to incorporate the tem- the US-280 reconstruction project instead of on the main perature into the calculation of the material's modulus. The roadway because the roadway base layer had been chip-sealed. rubber pads beneath the receivers deteriorate more rapidly This type of surface reduces the repeatability of the ultrasonic
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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. device, as well as other NDT devices, because the points of the The spacing of the receivers can also be changed easily for receivers and source are not always in good contact with the testing thin and thick layers. Layer thickness variation that surface tested. Ensuring good contact with the surface being occurs along a construction project can have less of an impact evaluated is important for both the PSPA and DSPA. on the resulting seismic modulus values than on the resulting The system initially converts the readings of the load pulse values from other NDT technologies. and response to a seismic modulus of the material. The seismic Another advantage of this technology is that the system can modulus is internally adjusted to a modulus at a specific con- be calibrated easily to the specific materials being tested during dition (temperature and load frequency for HMA). Each test the mixture design stage for HMA materials or in developing location requires three to five tests for this system. Each test M-D relationships for unbound materials. This calibration took 10 to 20 seconds to complete. Therefore, the entire process procedure allows the PSPA and DSPA to be used to detect (3 to 5 readings at a point) takes only slightly longer than the volumetric, as well as physical, changes in the materials during system currently used for QC, the nuclear density gauge, construction. which is generally set for one 60-second reading. The DSPA can be used to develop modulus growth with This system can also be used to estimate the elastic proper- compaction relationships during the first day of construction ties parallel and perpendicular to the direction of the rollers for the unbound layers and periodically during the project. (refer to Chapters 2 and 3). Measuring the seismic properties Use of the PSPA to develop HMA modulus growth relation- in different directions actually increases the perceived vari- ships can be problematic because of the elevated temperature. ability of the device. The variability can be reduced slightly by It is more applicable to warm-mix projects. always taking the readings in one direction. All other NDT The equipment (including the laptop) was found to be devices result in an average or equivalent value at a test point. durable, and it did not require more personnel than those now