Asphalt innovations

Researchers find new ways to improve attractiveness of road material

By Candy McCampbell

The alternatives to asphalt are getting more attention as producers are pressured by higher costs for petroleum and aggregates as well as environmental issues involving plant emissions.

This reality has many asphalt proponents developing and promoting new asphalt applications that enhance its attractiveness.

Asphalt prices have doubled in the last two years, says Chris Abadie, materials research administrator at the Louisiana Transportation Research Center in Baton Rouge.

“If they keep doubling, I think the answer will be yes” to using substitutes or blending other materials with asphalt, he adds.

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Binders, resins or other additions are costly now, but the relative cost lowers as the price of asphalt goes higher, says Margaret Cervarich, vice president of marketing and public affairs for the National Asphalt Pavement Association in Lanham, Md.

“If you show them a better way, a cheaper way, (contractors) are going to use it,” she adds.

Right now, growing cost is a big issue for asphalt pavers in the South Central region because state budgets are virtually frozen.

“The state money has not increased and materials cost more, so that means less amount of roads paved,” says Mel Monk, executive director of the Alabama Asphalt Pavement Association. 

Developments on the horizon aim to lower costs, change materials in the mix and increase recycling of older pavement. Following are a few of the more popular innovations currently being implemented.

Perpetual pavement. “Perpetual pavement is a way of designing and building asphalt pavement so the pavement will be there (virtually) forever,” Cervarich says.

Maintenance is simple: Mill off the surface and replace it every 20 years. Re-mix the rocks, heat and tumble them with asphalt cement and put the mixture back down on the road. 

This type of pavement offers solutions to environmental concerns, emissions and fuel consumption, Cervarich adds.

Of course, perpetual pavements require a base that can handle the weight and forces generated by vehicles traveling the route, but there has been a lot of research, both in the lab and in the field, that show its effectiveness, she says.

In addition, state departments of transportation are accepting the idea.

Warm mix. Many South Central DOTs are undecided about the merit of substituting warm mix for hot-mix pavement.

The production of hot mix requires heating aggregates and asphalt cement to more than 300 degrees, but the warm-mix process calls for temperatures of 200 to 250 degrees.

The benefits include a savings for fuel, lower emissions from burning fuels and less fumes and odors produced at the warm-mix plant and the paving site.

The idea has been around for several years in Europe and at least five versions have cropped up worldwide, Cervarich says. All of the versions have differing characteristics.

“If you can do it at 300 (degrees), you can do it at 250, maybe even lower,” she says. She adds that even though warm mix generates fewer emissions and uses less fuel, there can be compacting problems.

Research on warm mix is under way at the National Center for Asphalt Technology at Auburn University in Auburn, Ala.

One small NCAT project with Astec Technology Inc. of Chattanooga, Tenn., tested a foaming head that shoots water and hot air into asphalt to create a foam “the consistency of shaving cream,” says Andrea Kvasnak, NCAT engineer.

The product “compacted nicely out in the field,” she adds.

A side benefit was a reduction in odor, and the lower temperature needed meant workers could wear regular shoes on the fresh pavement.

Another test showed a clear difference in emissions, Kvasnak says. Smoke coming from trucks of traditional mix was evident, but there was no smoke or steam from the warm-mix side, she says.

She acknowledges that contractors are “not comfortable” with the lower temperatures of warm mix and are worried about compaction, but “we’re getting compaction and they don’t have to do anything different.”

The Louisiana Transportation Research Center is starting a two-year study of warm mix, LTRC’s Abadie says.

“Warm mix is really an ideal, and I would love to see it work,” he says. “But you would have to prove that water, which is inherent in aggregate,” can be removed at the lower temperature.

Even after a couple of presentations on the process to the Arkansas Asphalt Paving Association, executive director John Suskie says he doesn’t know of “any substantial use” of the process in the state, a sentiment expressed by other South Central states.

Recycled Asphalt Pavement. Thousands of roads are being repaired every day, generating tons of asphalt paving material that can be re-used.

“Asphalt is the most recycled material in the country,” Cervarich says.

More than 70 million tons – or 80% of the 90 million tons milled up – go back down on roadways or shoulders.

“It drives costs down,” she adds. “You don’t have to dispose of it, you don’t have to quarry rock, you don’t have to blast, you don’t have to crush” or do any of the other processes needed to ready new rock for a roadbed.

In addition, any of the asphalt captured on the old pavement “is still as good as it was the day it was used.” It can be heated again, mixed with new material and be used for paving.

States are still slow to buy in, especially for use of recycled asphalt pavement, or RAP, in a surface mix, so they limit the percentage from as low as 10% to as much as 50%.

“Some states are more comfortable using it for shoulders and definitely for base mix rather than surface mix,” Cervarich says.

Mississippi uses up to 30% RAP in the paving mix, says Tone Garrett, executive director of the Mississippi Asphalt Paving Association. 

Superior Asphalt Inc. and APAC of Mississippi, both of Jackson, Miss., working as a joint venture, recently milled about 47 tons of asphalt for a $31 million, 9 mi project on Interstate 55 in Jackson.

Mike Carroll, project manager with Superior, says crews milled at night, beginning at 7 p.m. and ending by 6 a.m. The project also required multiple lane closures. 

The job started in June 2006 and was scheduled for completion in July, but it finished early in mid-May.

A project in Louisiana also calls for RAP, but only for the shoulders. Austin Bridge & Road of Irving, Texas, has a $36.1 million contract to widen and rebuild 12 mi of U.S. Highway 171 from the Sabine Parish line to South Mansfield in DeSoto Parish, providing a better hurricane evacuation route between Lake Charles and Shreveport. >>

About 50% of the job is abandoning the existing highway and rebuilding near it, says Lane Fouts, project manager.

But about 7 mi of pavement will be milled up for re-use, then get a new overlay.

The job includes three bridges, two over the railroad track that runs parallel to the highway and a third over the existing highway, forming a figure 8 as the road swings out to the right and back to the left of the existing road.

Alternative binders. In another initiative, the Louisiana Transportation Research Center is adding rubber to the asphalt, Abadie says.

It’s in the form of styrene butadiene styrene at a rate of about 3%-5% to the liquid asphalt, adding $4 to $5 per ton to the cost. That overage, though, will level off as the cost of asphalt climbs, Abadie adds.

Mississippi and Florida both allow the use of waste tires in the mix and Louisiana is about to approve it, he says.

A polymer modified asphalt is going down on a 12-mi stretch of Interstate 65 in Conecuh and Escambia counties in Alabama, where APAC Southeast Inc. of Birmingham is doing a $17.7 million resurfacing job.

They are using Super Pave binders and surfaces, and stone matrix asphalt, with a top layer that is an open-grade friction course to reduce hydroplaning, says Garrett Pass, APAC vice president.

“It’s an all-polymer liquid and one-size rock that looks like it has a lot of voids in it that gives a place for water to run into,” he says.

This type of pavement is nothing new for APAC Southeast, he says.

“It’s resistant to rutting, a lot tougher than asphalt and takes traffic loads over time,” Pass adds.