| APPLICATIONS
Will Infrared
Thermography Highways throughout the United States are typically engineered to last 15 years and more, but have been failing much earlier due to potholes, cracks, raveling, and other problems. by Leonard A. Phillips
To improve the longevity of these roads — more than 90% of which are paved with stone matrix, hot-mix asphalt — the Transportation Research Board of the National Academies coordinated a five-year-long $150-million Strategic Highway Research Program. This program created a set of optimized construction methods and standards called SuperPave. The TRB conservatively projected that if the new SuperPave procedures achieve only a 25% increase in highway service life, state and federal agencies could save $785 million annually in avoided repair costs, and motorists could save between $1.3 billion and $2.1 billion a year in maintenance-related delays and vehicle wear and tear, plus the value of improved safety conditions. However, even after SuperPave procedures were adopted, premature mat failure persisted. To find out why, a series of research studies of hot-mix asphalt during road construction were launched at the University of Washington, subsequently involving the Washington State Department of Transportation, and the Roadtec subsidiary of Astec, a major manufacturer of infrastructure equipment. The studies determined the cause of premature HMA road failure to be excessive thermal differentials in the hot mix caused by surface and contact cooling during truck transport from the batch plant to the construction site. New help The joint WSDOT/UW research effort began using a FLIR ThermaCAM PM-280 infrared camera in 1998. The camera was used to image and measure to an accuracy of 1 degree F HMA temperatures in the trucks and as extruded by the paver with and without the use of material transfer vehicle or device. The latest infrared cameras are much smaller, weighing only 1.5 pounds, and are even more accurate. The density of the imaged hot pavement areas was then evaluated by nuclear densitometry and the density results were correlated with the temperature measurements. It was found that in areas where thermal differentials in the hot mix after laydown were greater than 25 degrees F, air voids in the material increased by approximately 2% after curing, lowering the density and, therefore, the resistance of the affected areas to wear and tear. Previous extensive field data showed that each 1% increase in air voids over a base threshold of 7% (based on a Rice Test maximum of 155) causes a 10% reduction in pavement life from physical and environmental wear and tear. On this basis, WSDOT correlated the thermographic data, nuclear density readings, and projected pavement life. The result — for the first time, a state DOT had a practical field test method and an economical tool, the infrared camera, to conduct quality assessments of HMA pavement mat during laydown that would accurately predict pavement life. Reblending in the field
After an exhaustive series of tests of available road-building equipment, WSDOT concluded that a particular piece of equipment called the Shuttle Buggy, which is made by Roadtec, mitigated thermal segregation effectively, and far better than any other device tested. The Shuttle Buggy is called a material transfer vehicle, because it can accept HMA loads from trucks or pick up HMA from windrows, thoroughly remixes it with a powerful auger, and then transfers it into the paver machine. With the Shuttle Buggy in the paving train, hot mat temperatures consistently were well within the 25-degree window. With the problem identified and a pragmatic and economical test method and solution in hand, WSDOT implemented a systematic density specification on 10 projects in 2002, and is applying the specification to all HMA road construction this year and thereafter — a significant step for reducing premature road failure.
The standardization of density profiling and the use of a thermographic protocol to identify density problems in road-building specifications offer important benefits for the paving industry and federal and state specifying agencies. These benefits include: longer lasting roads; improved return on road construction investment; maximization of the value of Superpave procedures; an extended paving season for contractors; and stimulus for the development of new protocols to maintain thermal consistency in batches of HMA during transport and laydown. Leonard A. Phillips is a senior writer for infrared applications, FLIR Systems. Reprinted from Better Roads
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