Better Roads Staff | August 2, 2011
Warm Mix Asphalt Rolls into Place
Technologies taking form under the watchful eyes of the National Asphalt Pavement Association and the Federal Highway Administration
By Mike Anderson
There is little doubt that even the most staid and traditional of road agencies and contractors are at least warming to the idea of a cooled-down approach to asphalt production and application only unveiled to the United States nine years ago. The ability to cut fuel consumption, greenhouse emissions and employee risk has that effect.
Some industry stakeholders are even convinced warm-mix asphalt (WMA), regardless of how the ever-evolving category here is ultimately defined, will turn out to be the equivalent to what hot-mix asphalt (HMA) has long been. In other words, there won’t be any more HMA applications per se; they’ll all be what we are now loosely terming as WMA.
The Federal Highway Administration (FHWA) and National Asphalt Pavement Association (NAPA) may not be ready for such a proclamation quite yet; the two organizations, however, continue to team up to try to get the asphalt industry’s collective head around WMA. Their Second Annual International Warm-Mix Conference is scheduled for Oct. 11-13 in St. Louis, Mo. A dedicated website maintained for FHWA by NAPA, www.warmmixasphalt.com acts as an ongoing source of education, news, research and products on the market.
More than 30 warm-mix asphalt products or technologies are being marketed and sold in the U.S., “which has been a challenge for us,” says Mike Corrigan, FHWA program manager, “although a good challenge, because it’s made us focus really on warm mix as a category. Instead of focusing on individual technologies, we’ve had to take a step back and say, ‘Let’s focus on warm mix as a material and what the performance of that warm mix is.’”
Speaking at NAPA’s Asphalt in Depth Conference held in June in Nashville, Corrigan did issue a disclaimer about the products listed on the website, ranging from emulsions to mixing systems, sourced from long-established suppliers and market newcomers alike. “The one thing I do like to point out is that just because it’s on warmmixasphalt.com does not mean it’s been through any kind of large scrutiny or evaluation whether it’s appropriate technology. A lot of them have had a lot of successes, but the only criterion for us to really list them on the site is the fact that they have their own website that we can link to, to provide people information. We’re not trying to say that we’re endorsing that technology or that it’s a proven technology, just that there is information available.”
Ongoing research, including at the National Center for Asphalt Technology (NCAT) test track at Auburn University in Alabama, is addressing a wide range of questions, reports NAPA and FHWA:
Can warm-mix pavements be opened to traffic quickly after construction?
What are the performance characteristics of these pavements?
In the case of technologies developed in other countries, can they be adapted to the U.S. where climate conditions are often more extreme?
If the production temperature is lower, does that mean that the binder does not age as much?
Will the potential for thermal cracking be reduced?
Will the potential for rutting be different?
Will the contractor have to use a different grade of asphalt binder?
What changes for the mix design procedure will be required?
And will the performance-graded binder in a warm mix perform differently from pavements produced at a higher temperature?
“A lot of questions get asked about higher-volume traffic – ‘Are we building warm mix on Interstates?’ – and the answer is, ‘Yes,’” says Corrigan, pointing to established usage in such traffic-heavy states as Texas, Florida and now New York.
Speaking at the Nashville conference, contractor Mike Law outlined conclusions from a Kentucky evaluation of WMA application, including:
Average densities were equal to if not superior to HMA counterparts.
Temperatures were on average 60 degrees F cooler than HMA control sections.
Workability still seems to be a concern for some contractors.
Actual savings will depend on plant efficiency, fuel prices, additive costs and aggregate moisture.
Average permeability readings were similar in HMA and WMA sections.
And, on average, WMA holds its temperature between the paver and roller better than HMA.
“Quality of warm mix must be as good as or better than HMA, or it is not acceptable,” says Law, of Scotty’s Contracting & Stone, Bowling Green, Ky.
Asphalt production expert Bob Frank of New Jersey-based Compliance Monitoring Service puts ongoing evaluations another way: What are the concerns with HMA? “Quite generally,” he says, “it’s things being where you don’t want them, whether it’s residual moisture in the aggregate where the dryer didn’t adequately dry the mix at the lower production temperatures, or moisture showing up in your baghouse fines due to the lower exhaust temperatures.”
Of the 358 million tons of asphalt produced nationwide last year, about 10 percent was warm mix, says Corrigan. “That was a tremendous increase in percentage from the year before,” he says, “and I think we’re going to continue to see that steady climb.” When he introduced Corrigan to the Nashville gathering, NAPA President Mike Acott called WMA “the future of flexible pavement in the United States.”
A heads-up, says Kentucky contractor Law: “If you’re not doing warm mix today, your competitor is.”
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