The Alaska Department of Transportation & Public Facilities is on the front line when it comes to promoting new technologies that give them continuous, full-coverage asphalt pavement density testing for road paving projects. Unlike traditional random sampling, continuous testing uses multiple inputs to “look for” failure zones and has a high probability of detecting defects before they lead to premature failures. The system they opted for combines intelligent compaction (IC), a paver-mounted infrared (IR) thermal profiler/scanner, and ground-penetrating radar (GPR) rolling density meter. Now, Alaska DOT is moving towards a system that will tie project acceptance to data from this system.
The ability to measure voids is a major benefit, since inadequately compacted asphalt deteriorates much more quickly than properly compacted material. The ground-penetrating radar (GPR) asphalt density meter used in the system evaluates void content, rolling along and taking a measurement every square foot of the pavement after compaction.
This is a huge advance over the prior random sampling method. Instead of 50-100 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 like road raveling, cracking, and deterioration along joints. Geophysical Survey Systems, Inc (GSSI) worked with Texas Transportation Institute (TTI) to package the radio wave components into the PaveScan® RDM asphalt density assessment tool, a streamlined and operator-friendly device that provides real time profiling of asphalt mixture uniformity.
Alaska DOT conducted demonstrations of the PAVE-IR technology in 2011 and began using intelligent compaction in 2013. Beginning in 2016, Alaska DOT used the IC and PAVE-IR paver-mounted thermal scanner technologies as contractor pay factors as part of project acceptance.
Implementation yielded some unexpected findings. One had to do with the PaveScan RDM’s data concerning compaction at the road shoulder. 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.
One other significant finding is the ability to use PaveScan to “see” defects they have never seen before, for example, density variation across a longitudinal joint. Engineers could see as much as a 10 percent compaction dip right at the joint between the milled edge and the new hot mix were the hot and cold edges had not melted together well. Another interesting finding observed during calibration is that PaveScan readings are much closer to the cores than the nuclear gauge.
As the first state proposing to use the new system for acceptance, Alaska has received encouraging support from the Federal Highway Administration, which supports the idea of not having to fund another project prematurely because something was not done well. This technology and implementing this type of specification has the potential to save the state millions of dollars in annual maintenance.
Alaska DOT recently received a STIC (State Transportation Innovation Council) incentive award from the FHA, and they want to use PaveScan RDM on a project in the near future for actual acceptance of the compaction of the highway. Instead of the drilled cores that they are currently using, they intend to use the drilled cores only to calibrate the machine at the beginning of the project and accept the density on the rest of the project based on the GPR readings. For the upcoming STIC project, Alaska DOT proposes paying the contractor based on the average compaction value.
“We hope their game will be dialed up a bit when they have to go back and fix areas they missed if they do not compact well. It will raise everybody’s awareness of quality work,” says Rich Giessel, Alaska Department of Transportation & Public Facilities Statewide Quality Assurance Engineer, “Basically, we will have no potholes left behind with this program.”