Developing Better Asphalt Mixes

Test Weeds Out Designs Prone to Cracking

The Texas Transportation Institute at Texas A&M developed tests to measure a pavement’s susceptibility to cracking but didn’t stop there. It also developed toughness tests to help refine the ideal mix for overlays.

By Mary Lou Jay

The reintroduction of an old pavement overlay test in Texas is helping ease the frustration of the state’s highway engineers and researchers over rapid cracking of newly laid pavement.

After the federal Strategic Highway Research Program brought out new asphalt design procedures in the early 1990s, the hot-mix industry expected it to follow up with new strength or performance tests.  When that didn’t  happen, the departments of transportation in various states began using wheel tests to measure a pavement’s rutting resistance. Pavement engineers reformulated mixes to pass the wheel tests, which measure a pavement’s susceptibility to rutting and moisture by rolling a steel wheel over a pavement sample that was immersed in hot water.     

“To pass the wheel test, contractors started making the mixes stiffer and stiffer, which eliminated the rutting,” says Tom Scullion, senior research engineer for the flexible pavement program at the Texas Transportation Institute at Texas A&M University. TTI studies a broad range of transportation-related issues including highway design, construction and maintenance. “But the mixes became brittle and didn’t have any flexibility to them. They were also difficult to work, to compact.”

TxDOT found that pavements made with these stiffer mixes often cracked within a few months of placement.

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“Many would fail in one cycle,” Scullion adds.

The team then ran tests on core samples of pavements that were performing poorly and others that were performing well. It found that the overlay test results matched field performance, Scullion says.

“If you did well in those tests, you had a good flexible pavement that could absorb movement, but if you did badly the pavement would crack straight through,” Scullion says. “The tests tell you very quickly if you have a bad material.”

Tony Yrigoyen, formerly with TxDOT and now working for Pate Transportation Partners, a division of Pate Engineering, a civil engineering firm with six offices in Texas, says the overlay tester is a simple piece of equipment.  “In Texas we have used the Hamburg wheel test for toughness, but there was nothing for flexibility,” he adds. “Now the overlay tester is allowing us to evaluate this flexibility, so we can properly balance those two important properties.”

TTI’s version of the overlay tester measures approximately 32 in. wide by 30 in. deep by 44 in. high and includes a laptop computer loaded with the necessary testing software.  With its automatic temperature control feature, the overlay tester can test samples in a range from 32 F to room temperature. It costs about $42,000 and is available commercially from Shedworks.com.

TxDOT has been so impressed with TTI’s modified overlay tester that it has ordered several for its central and district offices. The Center for Advanced Infrastructure and Transportation in New Jersey has also bought a tester and is finding that the lab results it yields closely correspond to actual field performance results, according to Tom Bennert, a senior researcher at CAIT.  Scullion says that Oklahoma is also evaluating the overlay tester.

Research yields improved pavements

In Texas, TTI’s job wasn’t finished after predicting cracking, however. After analyzing overlay test results, researchers continued investigations into the reasons for poor performance, and they found several, Scullion says. 

“They were not putting enough asphalt in to the mixes—they were too dry,” he adds. “The other thing was that they had gone to stiffer binders. Both of those things contributed to the cracking problems.”

The other problem was the quality of the aggregates. The limestone frequently used in Texas tends to absorb the light oils from the asphalt itself, making the mix drier overall. 

“To fix this problem, we went back and put much tighter requirements on the type of aggregates to use,” Scullion says.

The new mixes based on this research have shown good performance in both cracking and toughness. Three years ago in the Houston district, TxDOT tested a two-overlay system that incorporates I in. of a crack-attenuating mix, also known as a rich bottom layer, and 2 in. of porous friction course on top. 

RBL, as TxDOT calls it, is a fine, high-asphalt-content layer that passes both the rutting and cracking tests. Initial tests of this pavement were so successful that the Houston District now uses it routinely as a leveler over concrete or severely cracked asphalt pavements, and TxDOT has included it in its specification standards, according to Charles Gaskin, TxDOT’s director of construction in the Houston District.

TxDOT is also experimenting with the use of RBL as a surface course. A tough, flexible l-in. overlay would offer several advantages in urban areas, where thicker overlays can interfere with curbs and gutters, Scullion says.

An initial test of surface RBL on U.S. Highway 83 in the Childress District has held up well so far, says Ronald Hatcher, a laboratory supervisor for the district.

“We left a 500-ft. section on U.S. 83 just to see what it would do,” says Ronald Hatcher, a laboratory supervisor for the district. “We were afraid it would get slick because it has about 8% asphalt in it. But it will be down two years in September and it doesn’t show signs of getting slick. It seems to be working just fine.”