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Credit: National Center for Asphalt Technology

MeadWestvaco tested its Evotherm warm mix at the 1.7-mile test track operated by the National Center for Asphalt Technology at Auburn, Ala. After 500,000 equivalent single-axle loadings with trucks, test sections showed 1 mm or less of rutting.

“Environmentalists say they want warm mix asphalt technology now,” said Eric Harm, assistant chief engineer with the Illinois DOT, speaking to several hundred attendees at the World of Asphalt, held in Orlando, Fla., in March.

“We don't have five years to wait,” said Harm. “The credibility will go down the tubes if we wait five years to get this technology accepted as a mainstream option.”

Warm mix asphalt technologies typically use an additive to the mix or to the binder to create the asphalt mixture at a temperature range of 250º to 275º F. In contrast, hot mix asphalt runs from 275º to 325º F. Because warm mix is made at lower temperatures, the heating and mixing process reduces gaseous emissions and odors, compared to hot mix.

Warm mix processes have been tested in short trial runs in the United States, but not on full-scale projects. While these preliminary results are promising, Harm cautioned that further tests are warranted before agencies begin to write generic specifications. “We have to be sure that we're getting equal or better performance than we get with hot mix asphalt,” Harm said to World of Asphalt attendees. “We don't want to try this before we're ready and have early failures.”

Three warm mix technologies have been successfully tried in the United States. As presented at World of Asphalt, they are:

  • Aspha-min zeolite, developed by Eurovia. This is a granular material that contains 2% water by weight. The water is released under the heat of mixing.
  • Sasobit, developed by Sasol International. Sasobit wax is a long-molecular-chain hydrocarbon produced from coal gasification. Sasobit reduces viscosity at mixing and compaction temperatures, yet has the same or increased viscosity as hot mix at in-service temperatures.
  • Evotherm, made by MeadWestvaco and the only one of these three to originate in the United States (Stamford, Conn.), employs a chemical package in an emulsion that improves coating and workability.
  • At a 660-ton project in Indiana, Evotherm achieved substantial reductions, compared to the hot mix control, in stack gas emissions: nitrous oxides, total particulate matter, total organic matter, and benzene soluble matter, said Everett Crews, a technical manager for MeadWestvaco/Asphalt Innovations, speaking at World of Asphalt. The plant produced warm mix at 300º F in the drum and 210º F at discharge to the trucks.

    Sasobit comes in the form of small pellets, or flakes, and can be blended with the binder at a terminal or injected into the mix at the hot plant, said Larry Michael, an asphalt consultant who represents Sasol Wax Americas Inc. and is a retired regional materials engineer from Maryland. More than 6 million tons of Sasobit mix have been placed in Europe and China with zero failures, Michael said.

    For Aspha-min zeolite, the cost is about $3.60 per ton of mix. “That's probably a little high, because it's experimental,” said Jeffrey L. Pochily, quality control manager for Pike Industries Inc., Belmont, N.H. Late last year, Pike placed about 500 tons of warm mix at its asphalt plant in Hookset, N.H.