| APPLICATIONS
Will Infrared Thermography
Revolutionize Asphalt Paving?
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
 Premature road failure unnecessarily wastes millions of taxpayer
dollars every year and threatens the strategically critical National
Highway System, which carries more than 40% of all highway traffic, 75% of
heavy truck traffic, 90% of tourist traffic, and virtually all of our
military traffic.
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
 Intuitively, the researchers speculated that reblending the hot mix in
the field prior to loading it into the paver machine might solve the
problem, but if so, how would the remixing be achieved?
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 specification has teeth and can be used to penalize contractors
with price disincentives and other penalties when work does not meet state
density standards. Kim Willoughby of WSDOT estimates conservatively that
the use of thermography and reblending can save the state over $9 million
annually in road repair costs by ensuring maximum highway lifetimes. Add
the difficult-to-calculate hidden costs that are also avoided from delays
to commuters and transporters, police details, damage to vehicles, and
noise from construction vehicles. Now consider such savings on a
nation-wide scale. The State of Washington has 3,384 national highway
system miles — about 2% of the national total of 163,734 miles. So,
projected savings could be $450 million a year.
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 Magazine
December 2003 |
