TTI Turns Up the Heat on Asphalt Cold Spots

One objective of SHRP 2 is to develop technologies that result in long-lasting transportation facilities. Researchers at the Texas A&M Transportation Institute (TTI) did just that.

Initial work on the new technology developed by TTI began nearly 10 years ago. A serious problem identified with black-top roads was cold spots forming in asphalt mats during the laydown process.

“We design these pavements for all these stresses and strains, but that’s not how they’re failing. They’re failing because of defects that are built in on day one during construction,” says TTI Senior Research Engineer Tom Scullion, who manages TTI’s Flexible Pavement Program. Scullion says it’s the area beneath the pavement where problems develop first. “It may take 2 years; it may take 10 years. But moisture gets into those areas, sits there, and then the stones start to ravel out and it’s a failure.”

Researchers call it thermal segregation, which is a large problem nationally. “The story we get from the contractors is ‘We know it’s a problem. Give us a tool we can use,’ ” Scullion explains.

The TTI research team’s first goal was to measure the temperature of the mat the moment it’s laid down. The result was the PAVE-IR bar. PAVE-IR includes an array of sensors mounted on the back of a laydown machine that allows contractors to detect temperature problems in real time and make adjustments.

Top: an installed PAVE-IR system trailing above a newly layed asphalt mat; Bottom: color graph produced from the temperature variations of an asphalt mat recorded by the PAVE-IR system

A PAVE-IR system installed on the back of an asphalt laydown machine. Over 40 of these systems are now in use in the United States. The colors in the graph illustrate the temperature variations in the asphalt mat.

“Normally, [the problem is] what’s called truck end segregation…cold spots in the mat, usually at the end of every truckload. Essentially, you see [them] every 150 feet as you go down the road. These things are about 5 to 6 feet across. Once you’re tuned in, you can see these everywhere,” Scullion says.

Compacting a mat with cold spots is also hit or miss. “What we find is that some of the mixes we place are a lot more forgiving,” observes Scullion. “They will stand that temperature variation and still compact reasonably. Other ones are completely non-forgiving. If it goes down cold, there’s no way you can compact it.”

PAVE-IR provides 100 percent coverage of mat temperature issues prior to rolling. The best way to get similar coverage and to detect low-density areas is with ground-penetrating radar (GPR). The TTI team is now focusing their efforts on developing a GPR-based density-measuring system, which can, in real time, provide close to 100 percent coverage of new mats. The challenge is to develop a GPR system that is small enough and tough enough to take the daily abuse of construction.

“What we’re busy putting together right now is a three-antenna system, which can go on the back of a pickup truck. These are very small antennas, about the size of a cigar box. You’ll drive over the section immediately after it’s been rolled, and they’ll tell you in real time if you have any low-density areas,” Scullion said. A private-sector company helped develop the PAVE-IR system. That successful model is being used to develop the GPR system.

working prototype of a three-horn 2.6-GHz GPR system

A working prototype of a three-horn 2.6-GHz GPR system designed to be mounted on pickups
and vans. This system was tested in November 2012.

graph: profile view of expected air-void distribution of  a road section

This plan profile view of expected air-void distribution shows a 1,000-foot road section where the GPR was calibrated to hot-mix asphalt core air-void content.


Monica A. Starnes, SHRP 2 senior program officer, is enthusiastic about the technologies. “The use of both technologies will really improve the quality of hot-mix asphalt pavements,” she explains. “Improving quality control during construction and fixing problems on the spot will save money because contractors won’t have to come back and replace trouble spots later.”

Numerous states, including Texas, Florida and Pennsylvania, have volunteered to work with researchers during the field testing, which is scheduled to begin in spring 2013. Implementation is sure to follow.

Is this just the beginning for exciting new technologies in road building? Tom Scullion thinks so. “Some technologies don’t work; others work very well. Over the years, it’s been really encouraging and gratifying to see some of these new technologies coming along. Hopefully we can keep on [developing] products like this,” he says.

Starnes adds, “I was in a recent meeting with our technical coordinating committee. At that meeting, Tom Baker, Washington State Department of Transportation materials engineer, mentioned that over 40 PAVE-IR units were already in use around the country. It’s one of those technologies that is so ready for full deployment and implementation.”

This Issue

From Texas to the Nation

Texas Transportation Researcher: Volume 48, Number 4

Volume 48, Number 4
December 2012
Issue Overview


SHRP 2 R06(C)

On this page:

“The use of both technologies will really improve the quality of hot-mix asphalt pavements. Improving quality control during construction and fixing problems on the spot will save money because contractors won’t have to come back and replace trouble spots later.”
Monica A. Starnes, SHRP 2 senior program officer

“We design these pavements for all these stresses and strains, but that’s not how they’re failing. They’re failing because of defects that are built in on day one during construction. It may take 2 years; it may take 10 years. But moisture gets into those areas, sits there, and then the stones start to ravel out and it’s a failure.”
Tom Scullion, TTI senior research engineer

For more information:

Tom Scullion
(979) 845-9913
t-scullion@tamu.edu