What if asphalt mixes could be produced at temperatures 50 to 75 degrees lower than they are produced now (or even at a temperature as low as 140 degrees)? That was the intriguing premise of a recent article written by Dave Newcomb in the National Asphalt Pavement Association's Hot Mix Asphalt Technology magazine (HMAT, July/August 2005).

The benefits are obvious. First, the plant would require less energy to produce the mix and would realize less plant wear. There may also be reduced emissions, fumes and odor. And, because of the lower temperature, the materials would "age" less in production and may therefore be more resistant to cracking.

Newcomb, a professional engineer, Ph.D., and NAPA's vice president of Research and Technology, detailed four "warm mix" technologies in the article: the Warm Asphalt Mix (WAM) foam; the Aspha-min® process that uses a mineral additive; a process that involves the use of an organic additive; and Evotherm, which uses a chemically modified binder.

WAM, he said, has been produced successfully in batch and drum plants with reductions in temperatures from 310 to 230 degrees reported. It has performed as well or better than Hot Mix Asphalt (HMA) in several projects with a 30-percent reduction in energy consumption and CO2 emissions.

Aspha-min, marketed by Eurovia with a mineral additive called zeolite, is added to the mix in the temperature range of 250 to 295 degrees. It can also be done in a batch or drum plant and handles the same as a conventional mix with similar field densities. Eurovia reports a reduction in energy consumption of about 30 percent.

Two organic additives being used — Fischer-Tropsch paraffins and low molecular weight ester compounds — have shown good field performance for five years. Experience suggests that the material be delivered at 300 degrees or less and compaction can start at about 250 degrees.

Evotherm uses a chemically modified binder produced by MeadWestvaco. In South African field trials, the manufacturer noted no problems mixing and placing the materials with production and paving temperatures as low as 140 degrees.

After reading the article, I wanted to know more, so I called Newcomb with questions about the cost effectiveness of warm mix and how the mixes were performing where they've been tested. I also wanted to know if asphalt paving could really be done with materials at 140 degrees.

"Without even attempting to predict the ever-changing cost of energy, the technology is new enough now that any energy savings could be negated by the higher cost of the materials or additives," Newcomb said. "In the future, if the technology becomes more accepted and more comes onto the market, then the cost will decrease. It's like anything when you have new technology. It always costs more until it is incorporated into the market."

As far as performance, Newcomb called demonstrations of the warm mixes "promising." Europe has much more experience with the technology than we do but the United States' knowledge base is growing with trials and demonstration projects.

Producing and paving at temperatures as low as 140 degrees, as was reported in South Africa, is a manufacturer's claim, Newcomb pointed out. "I think 140 degrees would make a lot of people uncomfortable," he said, "mainly because you have to have the material workable for construction purposes yet when it gets into service you want it to be able to resist rutting at high temperatures. And 140 degrees is a number that represents a maximum temperature in many parts of the country. Their claim is that they have gone down to 140 degrees but I don't think they always do that necessarily."

Finally, I asked Newcomb how low he thought the temperatures could realistically go. And I couldn't resist suggesting that instead of Hot Mix Asphalt, we could maybe call it "Lukewarm Asphalt Mix" or "Tepid Asphalt Mix" (and, to his credit, he was nice enough to laugh).

"I don't know how low it will go," he said. "It is promising but we need to study these processes to see if there is a benefit to using warm mix in the United States.

"And I don't know what type of new terminology will evolve. It could be that we'll have to redefine 'Hot Mix Asphalt.'"

Speaking of advances in asphalt paving, this issue also features an introductory article on "Intelligent Compaction," which may hold benefits for contractors, roadway owners and the traveling public.

As with warm mix asphalt, intelligent compaction has had more use outside the United States and is another industry development worth watching.