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27 Figure 4. Humboldt GeoGauge. not require more personnel than those now being used for but not for identifying construction defects. As a replacement control and acceptance. As for the DSPA, the same technician to the repeated load resilient modulus test, the regression using the nuclear density gauge or running sand-cone tests equations developed from repeated load resilient modulus could also operate the GeoGauge at the same time. The train- tests included in the LTPP program (Yau and Von Quintus ing and technical capability of the operator is no more than 2002) or the use of the DCP is permissible. what would be required for operating a nuclear density gauge. The disadvantage of the GeoGauge is that it will result in Similar to the DSPA, the GeoGauge can easily be used to high variability when testing non-cohesive, well-graded sands develop relationships between modulus growth and com- or similar soils. In addition, the elastic modulus readings from paction effort in unbound layers. Such relationships can the gauge represent an equivalent modulus for the upper 10 to be initially developed at the start of the project to optimize 12 in. of the layer. Thus, the gauge in its current form should the compaction process and then be periodically verified not be used to test thin (less than 4 in.) or thick (greater than throughout the project. This feature becomes advantageous 12 in.) layers without proper material calibration adjustments when the water content significantly varies from the optimum or changing the diameter of the ring under the gauge. value measured in the laboratory. In summary, the GeoGauge has potential use in day-to-day The GeoGauge should be calibrated to the project materials QA programs by both the contractor and the agency personnel. 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 1.3 Deflection-Based Methods tests be performed on each unbound layer for judging the 1.3.1 Falling Weight Deflectometer quality of construction. Most agencies do not routinely per- form resilient modulus tests for design or for forensic evalu- The FWD is a large, expensive apparatus that is mounted ations, even though the 1993 AASHTO Design Guide suggests on a trailer and pulled behind a tow vehicle. The operator that they be performed (AASHTO 1993). Eliminating the works a computer and locates the apparatus for testing (see laboratory resilient modulus tests from the calibration proce- Figure 5). This system is capable of applying dynamic loads dure will reduce its accuracy for confirming the design values, to the pavement surface, similar in magnitude and duration to
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28 Figure 5. Trailer mounted FWD. that of a single heavy moving wheel load. It is being used within displayed. In some cases, one of the geophones or sensors can the LTPP program, and most state agencies have access to at be incorrectly placed on the test surface by the sensor bar, least one FWD. Thus, it is already being used in most agencies' especially on rough surfaces. The data acquisition software day-to-day practice. will identify this anomaly, notifying the operator that the test The response of the pavement system is measured in terms should be rejected and redone. of vertical deformation, or deflection, over a given area using The test takes about 2 minutes to complete, including the seismometers or geophones. An FWD enables the user to deter- use of seating drops. Seating drops are important and should mine a deflection basin caused by a controlled load. These be used at each test point. This does not include time to con- results make it possible to determine the stiffness of existing figure the trailer and set up the data acquisition system, which pavement structures for use in M-E based rehabilitation design should only have to be done once per day for each project. It methods. takes about 30 minutes to configure the trailer and 2 to 3 min- The falling weight strikes a set of rubber buffers mounted utes to set up the data acquisition program. Similar to the to a 300-mm circular foot plate, which transmits the force to PSPA, the operator needs more technical and sophisticated the pavement (see Figure 5). A thin-ribbed rubber pad is always training in setting up the equipment and visually interpreting mounted under the footplate. By varying the mass or the drop the deflection basin data. height or both, the impulse load can be varied. This load may A separate data interpretation system or software is required be varied between 10 kN and 140 kN. Sensors measure the for producing elastic modulus values from the measured surface deflections caused by the impulse load. deflection basins--Young's modulus for each layer. The Most agencies use seven sensors at the spacing recommended calculated elastic modulus values are structure dependent. by LTPP. However, fewer or more sensors can be used, and Most data interpretation or analysis programs used back- those can be spaced uniformly or at some other spacing calculation techniques for calculating layered elastic modulus selected by the user. Peak deflections are recorded, stored, and values. Backcalculation programs do not determine unique
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29 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. Figure 6. Dynatest Prima 100 LWD. 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 would sense the jolt from the frame coming apart as a sepa- training and technical requirements for the LWD operators rate test, resulting in a deflection and modulus value for that are no different than for nuclear density gauges, with one anomaly. exception--the operator needs to understand and be aware The wireless remote was troublesome and kept losing con- of the factors and physical features that affect layer modulus tact with the apparatus. This happened anytime the technician calculated from the measured deflections. Results from the carrying the apparatus came within a few feet of the technician LWDs were significantly influenced by the supporting holding the computer. This slowed down the operation because materials on some of the projects. the computer had to be re-started each time it occurred. All three LWD devices used on selected projects have When using the system on particularly stiff base material, the similar features. Only the Dynatest and Carl Bro devices are hammer can bounce high enough, such that it can strike the discussed in the following paragraphs. 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 220.127.116.11 Dynatest Prima 100 LWD Device actual test results to be deleted and replaced by a reading of the The Prima 100 is manufactured by Dynatest and consists rebound. of the weight (hammer) on a pole and the sensors (geophones) The system, however, is fast. One test takes about 10 seconds, in a plate on the ground, all encompassed in one, connected, so the five tests conducted (and averaged) at each location portable structure (see Figure 6). The sensors were connected take approximately the same amount of time that a nuclear to a handheld computer by wireless remote technology. density reading takes at one location. However, the apparatus The unit tested was somewhat flexible and the frame came is bulky to handle, so the time that most non-nuclear systems apart on multiple occasions. Besides slowing down the process, gain by not having to deal with the steps of transporting the this resulted in questionable data because the wireless remote nuclear device are lost.