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Credit: Photo: National Asphalt Pavement Association

After three years, the 250 tons of warm-mix asphalt that Charlotte, N.C., placed on this section of Old Statesville Road showed no appreciable rutting or other deterioration. Core comparisons with a hot-mix section show that hot and warm mixes densify at about the same rate.
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Credit: Photo: National Asphalt Pavement Association

Produced and placed at lower temperatures than hot-mix asphalt, warm mixes eliminate fumes and increase worker comfort during paving operations.

Last year, representatives from NAPA, the American Association of State Highway and Transportation Officials (AASHTO), FHWA, the Asphalt Institute, contractors, and consultants visited warm-mix projects, plants, and laboratories on another European tour. After examining pavements constructed as early as 1999, they agreed that WMA is viable, and that U.S. agencies and the asphalt industry should continue working cooperatively to implement it. One of the team's goals “is that WMA should be an acceptable alternative to HMA at the contractor's discretion.”

THE ROAD TO ACCEPTANCE

Based on European experiences as evidenced by last year's tour, as well as the U.S. field trials and laboratory research performed to date, there doesn't appear to be any serious challenges associated with warm-mix asphalt. Still, there are good reasons for continuing study and discussion.

“The industry doesn't change very fast, and the conservatism of the pavement engineers is understandable,” says Margaret Blain Cervarich, NAPA's vice president for marketing and public affairs. “You don't want to claim that a particular pavement will last 10 or 20 years, if none has been around for that long.”

Besides the issue of long-term durability, there are others yet to be resolved. NAPA Vice President for Research and Technology David Newcomb enumerates several in his paper titled “An Introduction to Warm-Mix Asphalt,” including:

  • What affect does the treatment of asphalt binder in the warm-mix process have on the pavement's susceptibility to moisture damage?
  • What changes might be needed in mix designs for production at lower temperatures?
  • What new guidelines might be needed for quality control and quality assurance of the mixing process?
  • Is there a difference in cure time that would prevent warm-mix pavements from being opened to traffic at least as quickly as conventional hot-mix pavements?
  • Can the reductions in fuel consumption and emissions be quantified to ascertain the cost benefits of warm-mix asphalt?
  • Is warm-mix asphalt suitable for the high production rates in U.S. plants?

Ongoing research at Ohio University and continuing field trials around the country should answer some of those questions, while two projects being funded by the National Cooperative Highway Research Program (NCHRP) will provide valuable information. NCHRP is administered by the Transportation Research Board and sponsored by AASHTO member departments in cooperation with FHWA.