Warm-Mix Asphalt Heats Up

Technology Shows Promise After Local Demonstration Efforts

By Scott Judy

The science and technology of asphalt paving is always evolving. The implementation of the Superpave process and the popularization of the "Perpetual Pavement" concept were the most recent major developments to hit the industry.

And now, another evolution might be taking place.

Warm-mix asphalt paving - as opposed to the standard hot-mix asphalt - is the next potential trend to impact asphalt producers, contractors and project owners around the country. The concept is fairly simple: producing and paving at significantly lower temperatures to produce numerous potential environmental, economic and health benefits.

Various mix additives currently being developed and marketed would enable this paving work to be done at the lower temperatures.

While various companies are testing the potential of warm-mix asphalt, The Hubbard Group of Orlando and its subsidiary, Blythe Construction of Charlotte, have conducted what is believed to be the first demonstration of this emerging technology on a city street.

The company has taken the step as part of its role as the national U.S. marketer and distributor for Aspha-Min, the additive product that Hubbard's parent company, France-based Eurovia has developed in Germany.

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Conducted last fall as part of Blythe's reconstruction of Old Statesville Road in Charlotte, the demonstration was a follow-up to the contractor's other, earlier initiatives to encourage the implementation and technical reviews of warm-mix technology.

"We've been working on it in the U.S. for about two years," said R. Wayne Evans, interim president and CEO for The Hubbard Group. "We hired the National Center for Asphalt Technology at Auburn University to do some research on warm mix using Aspha-Min. We have a draft of the final report, and it is very promising for us."

NCAT's report will be available on-line at www.NCAT.us this summer.

The Technology

Aspha-Min is referred to as a synthetic zeolite and is a chemical mixture of natrium, aluminum and silicate that has been thermally crystallized. Distributed as a very fine powder, it is 21 percent crystalline water by mass.

Bags of the Aspha-Min product are added to the asphalt mix at a rate of approximately 0.3 percent by mass. As the sand and aggregate mix is heated up, the product's crystalline water is released, creating a microscopic foaming action within the asphalt binder and thereby increasing the mixture's workability at lower temperatures.

In addition to allowing higher workability of the mix, results have indicated an improved compactibility compared to standard hot-mix techniques. The addition of Aspha-Min improved compaction down to 190°F, NCAT found.

Reduced energy costs at the plant are also achieved due to the decrease in mix temperature, which can be as much as 50° F lower than normal. On the Charlotte project of Blythe's, for example, the contractor ultimately produced the warm mix at a temperature of about 265-270° F, compared to the 315° F it used for the conventional sections of the same project.

According to Hubbard and Blythe, the lowering of an asphalt plant's energy consumption is linked to a lower emission of nitrous oxides as well as volatile organic carbons, the combination of which is linked to the creation of ground-level ozone, a major contributor to smog.

The warm-mix technology also is reported to reduce the emissions of carbon dioxides as well as sulfur oxides, a suspect in the creation of acid rain.

All of this could prove especially important to parts of the country designated as "nonattainment" areas by the Environmental Protection Agency. Such designation could impact asphalt contractors by making permitting an asphalt plant extremely difficult or impossible.

"If you're in an urban area that's in nonattainment with EPA, then you might be able to permit a plant using this technology, where you couldn't permit a plant taking the mix to conventional temperatures," Evans said.

These nonattainment areas can also experience reductions in federal highway funding.

NCAT Results

Hubbard commissioned NCAT to perform a lab study examining the Aspha-Min technology's effect on compaction, turn-over to traffic, rutting susceptibility and moisture sensitivity.

In addition to the compaction results reported above, the NCAT study found that the ability to turn over recently placed pavements to traffic remained unchanged compared to conventional hot-mix methods.

"Testing indicated that traffic can be returned to Aspha-min warm mix as rapidly, or at the same temperatures, as hot-mix," said Brian Prowell, assistant director for NCAT. "There was no difference in the rutting susceptibility … between Aspha-min warm mix and control mixtures without zeolite produced at the same temperature."

NCAT's findings added, however, that rutting increased at lower temperatures due to reduced aging of the binder.

"If plant temperatures are above approximately 275°F, there shouldn't be any problem," Prowell commented. As a result, he added, for lower production temperatures, where rutting is a concern, a stiffer binder may be required.

The report also stated that lower temperatures may reduce tensile strength ratios determined for moisture sensitivity. NCAT added that this can be counteracted by the addition of anti-stripping additives, such as hydrated lime.

The Blythe Project

Blythe's test project in Charlotte was another first step in testing the true capability of warm-mix technology. In addition to being the first demonstration on a city street - earlier tests have involved parking lots - it also marked the first time that plant emissions were monitored. The choice of Charlotte was also notable because the city is not in compliance for ground-level ozone.

"There are threats down the road that this whole region could have federal funds cut off for anything that adds capacity to the roadway system," said Allen Hendricks, vice president with Blythe Construction. "Local officials are trying to demonstrate the reduction in emissions."

Blythe had already been awarded the Old Statesville Road project, which was a total reconstruction of a 2-mi. section of this city-maintained road. The contractor approached the city of Charlotte, the owner, with the idea of paving one section with warm mix.

The city agreed to it, with the result being that a roughly 1,500-ft.-long, 15.5-ft.-wide section that would include a 1.5-in. top layer of warm mix, for a quantity of about 220 tons, said Berry Hall, quality control manager for Blythe.

The overall paving project, designated as Superpave, was easy to convert to warm mix, Hall added.

"We took our Superpave mix that we normally produce and then added 0.3 percent of the synthetic zeolite," he said. "That's all we did. We did not have to change the grade of our asphalt cement at all. We just added this zeolite to make it workable at lower temperatures."

Blythe placed the warm mix on the same day as it was laying down the hot mix, thereby allowing a direct comparison of plant emissions. The results were positive, and consistent with findings from European projects.

Hall reported that there was a 3.2 percent increase in carbon monoxide levels but significant decreases in others: nitrous oxides fell 6.1 percent; sulfur oxides, 17.6 percent; volatile organic carbons, 35.3 percent; and carbon dioxides, 2.3 percent.

"There are no visible fumes either at the plant or around the paver," Hendricks said. "At the plant it was pretty dramatic. It was perfectly clear. And when we loaded our trucks there was zero blue smoke." Usually with standard applications, a "blue smoke" is visible at the plant and other areas.

The compaction results were also positive and required no alteration from the standard Superpave method.

"We did not alter our rolling pattern at all," Hall said. "We were averaging a high 92 percent maximum density (with the conventional mix) and when we ran the warm mix we were in the mid-93s. It was about half a percent increase without altering anything."

Hendricks added that using Aspha-Min produces a pavement that is virtually identical to the conventional sections.

Next Steps

NCAT also examined a field trial placed at one of Hubbard's parking lots in the Orlando area, alongside some conventional Florida DOT mix. This took place in February 2004, and could serve to predict how the Charlotte pavement will perform.

Here, NCAT's Prowell said, "Field results were impressive. Using the same roller train and pattern, the same density was obtained for the control and Aspha-min sections, even though the compaction temperature for the Aspha-min warm mix was 35°F lower.

"After a year in place, the hot mix and warm mix were both performing well and appeared identical," Prowell continued.

Moving forward, Hubbard is working with the Federal Highway Administration and the American Association of State Highway and Transportation Officials, both of which are interested in the technology's potential.

Next, "we'll probably work with some local agencies and then maybe with some other state DOTs," Evans added. He said the company was discussing performing a demonstration project with the Florida DOT, and it intends to reach out across the country.

"We've taken calls from New York and Maryland and had a lot of interest from universities doing research," Evans said. "We're going to promote it where the states show the most interest."