Minnesota DOT, and Nebraska DOT continued the research under the federally funded SHRP 2 project. ADOT&PF participated in two dem- onstration projects with technology company MOBACorporation, first with the PAVE-IR Bar and more recently using the PAVE-IR Scanner, a paver-mounted thermal profiler. New system in action ADOT&PF conducted demonstrations of the PAVE-IR technology in 2011 and began using IC in 2013. While visiting an infrared showcase, Giessel saw a demonstration of GSSI’s PaveScan RDM. “I got to walk behind it and watch what it was doing and I was pretty excited about the technology and what it could do for us in Alaska,” Giessel said. Alaska roads can be really affected by freeze-thaw cycles; the capital city of Juneau is affected most, with about 50 freeze-thaw cycles per year, giving them extremely difficult conditions to maintain. Giessel got the opportunity to use all three components together as part of the recently completed Glenn Highway: Hiland Road to Eklutna Pavement Preservation project — 15 miles of four-lane expressway requiring a bit more than 50,000 tons of asphalt. This was a 2-inch mill and fill project, where contractors milled out 1.75 inch or 2 inches and put back 2 inches of material. Beginning in 2016, ADOT&PF used the IC and PAVE-IR paver- mounted thermal scanner technologies as contractor pay factors as part of project acceptance. The PaveScan RDM asphalt density assessment tool was used as a technology demonstration only. They obtained full- coverage data but did not connect the information to any pay factors. Giessel is extremely pleased with the results of the project, noting some unexpected findings. One had to do with the PaveScan RDM’s data concerning compaction at the road shoulder. The project called for milling out and leaving a paved shoulder with a rumble strip. QA engineers found that in some cases, the asphalt was not stacked high enough above the paved shoulder to get really good compaction. In the past, they had rarely cored near the edge, so had not observed this area of low compaction; the random selection almost never got within a foot of the rumble strip. Now, the PaveScan RDM measures every couple feet across the road, and engineers were picking up on the shoulder compaction issue. Another interesting finding observed during calibration is that PaveScan readings are much closer to the cores than the nuclear gauge, which had always been their standard proxy for cores. The correlation coefficient (R2) for the nuclear gauge is only 0.82 (82 percent accurate), while the R2 with the PaveScan was 0.93-0.95 (93-95 percent accurate), which really boosted their confidence in the data they were getting. One other significant finding is the ability to use PaveScan to “see” de- fects they have never seen before, for example, density variation across a longitudinal joint. “We had never been able to see what was going on at those longitudinal joints before,” Giessel said. “Instead of just running parallel down the highway, I turned the PaveScan 90 degrees and went across those joints perpendicularly. We could see as much as a 10 percent compaction dip right at the joint between the milled
PaveScan assessing a longitudinal joint during night paving.
The last element in the system is the GPR asphalt density meter, which evaluates void content, rolling along and taking a measurement every square foot of the pavement after compaction. This is a huge advance compared with the prior random sampling method. Instead of 50 to 100 random tests, the GPR method provides 2.5 million tests — one on every square foot. This enables QA personnel to finally be able to find defects. Since the method provides geo-located data, they can go back and fix issues before they lead to premature failures such as road raveling, cracking, and deterioration along joints. The ability to measure voids is a major benefit, since inadequately compacted asphalt deteriorates much more quickly than properly com- pacted material. Asphalt with too many air voids ages at a more rapid rate due to oxidation of the binder. Over-compaction may cause crush- ing of aggregate or low air void content, leading to rutting or flushing. In either case, the asphalt is subject to early failure and road lifetime is less than asphalt that contains the optimal air void content. Con- struction engineers are always looking for optimal compaction, and by measuring density variations they can calculate the air void content. This approach is in line with research Giessel had been following as part of the Federal Highway Administration’s Strategic Highway Research Program 2 (SHRP 2) Renewal Project R06C (Using Both Infrared and High-Speed Ground Penetrating Radar for Uniformity Measurements on New Hot Mix Asphalt (HMA) Layers). As part of SHRP 2, the Texas Transportation Institute (TTI) developed a way of using IR and radio wave technologies to improve the assess- ment of asphalt mixture and compaction quality. TTI later worked with Geophysical Survey Systems, Inc. (GSSI) to package the radio wave components into the PaveScan RDM asphalt density assessment tool, a streamlined and operator-friendly device that provides real-time profil- ing of asphalt mixture uniformity. After this first phase, Maine Department of Transportation (DOT),
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march 2018
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