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Warming up to new asphalt mixes

Warming up to new asphalt mixes

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    With lower temperatures than hot-mix asphalt, warm-mix paving eliminates smoke and reduces fumes and odors on the jobsite. This results in a safer working environment for paving crews.

The National Asphalt Pavement Association (NAPA) introduced warm-mix asphalt (WMA) technology to the United States from Europe in 2002, spurring immediate interest among hot-mix asphalt (HMA) users with its potential environmental and engineering benefits. At least 45 states now actively use warm-mix technology or have constructed trial projects.

While laboratory tests have indicated an increased potential for rutting and moisture susceptibility for some warm mixes, early field performance of warm-mix asphalt projects has been equal to or better than that of conventional hot-mix.














Temperature readings of freshly placed hot- and warm-mix asphalt in a recent NCAT test project show the warm-mix is 50° F cooler. Photos: National Center for Asphalt Technology

Lower temperatures could result in incomplete drying of absorptive coarse aggregate, and compromise the bond between asphalt and aggregate. With reduced mixing temperatures, there is also less binder aging, which can produce lower tensile strength and/or increase rutting in lab tests.

Besides the laboratory testing, field evaluations of warm-mix are underway across the country (see sidebar). To date, they have not shown any practical difference in rutting between warm-mix and control hot-mix sections. Two recent case studies in Missouri and Ohio tested several warm-mix technologies ? Aspha-min (a zeolite), Sasobit (a wax), and Evotherm ET (a chemical additive). The wam-mix sections performed as well or better than hot-mix in terms of tensile strength and moisture susceptibility.

Warm-mix asphalt research
Ongoing research projects across the country include:

  • The National Center for Asphalt Technology (NCAT) has begun a comprehensive laboratory testing program and a certification program to evaluate well-controlled test sections on the center's pavement test track.
  • National Cooperative Highway Research Program (NCHRP) research projects - in three major studies, a team of pavement engineers is comparing engineering properties of warm- and hot-mix, and evaluating energy savings and emissions reductions for warm-mix production. Researchers will monitor short-term warm-mix performance over a two-year period and will use the NCHRP Mechanistic-Empirical Pavement Design Guide to predict long-term performance. The studies are:
    • Project 9-43 (completed) - results will be published as Report 691, Mix Design Practices for Warm Mix Asphalt, in June.
    • Project 9-47 & 9-47A - Phase I focuses on engineering properties, emissions, and field performance; Phase II studies short-term performance.
    • Project 9-49 & 9-49A - the Texas Transportation Institute is investigating moisture susceptibility concerns, and will publish guidelines for identifying and minimizing moisture susceptibility. Long-term field performance will be further studied under NCHRP 9-49A.
    • Two more projects are scheduled to begin in 2012:

    • Project 9-52 - Short-term Laboratory Conditioning of Warm-Mix Asphalt Mixtures for Design and Performance Testing
    • Project 9-53 - Asphalt Foaming Characteristics for Warm-Mix Asphalt Applications

    THE BASICS & BENEFITS

    The term "warm-mix asphalt" encompasses a variety of technologies that allow paving mixtures to be produced and compacted at lower temperatures without loss of workability. Temperature reductions of 50° to 100° F between hot and warm mixes reduce fuel consumption and greenhouse gas emissions. (Hot-mix asphalt generally ranges between 285° and 340° F, while warm mixes reach 220° to 275° F.)

    Other potential benefits that make warm-mix appealing to pavement engineers and road managers include the ability to pave in cooler temperatures, to use longer haul distances while obtaining density targets, and to incorporate higher percentages of reclaimed asphalt paving at reduced temperatures. Some highway agencies use warm-mix asphalt in overlay applications, to eliminate or reduce bumps caused by the expansion of underlying crack sealant. As reported in the WMA European Practice Report published by the National Asphalt Pavement Association, tests have also shown that warm mixes significantly reduce asphalt aerosols/fumes and polycyclic aromatic hydrocarbons as compared to hot mixes.

    Warm-mix can be used for all asphalt types, including dense-graded, stone-matrix, porous, and mastic asphalt. It can also be used in various layer thicknesses and applications, from parking lots and low-traffic roadways to interstate highways.

    TECHNOLOGIES & COST

    Warm-mix technologies can be classified in several ways. The first is by degree of temperature reduction, as there is a wide range of production temperatures in warm-mix asphalt that separate it from half-warm-mix asphalt.

    They can also be classified by distinguishing those that foam the asphalt binder from those that use some type of chemical or organic additive to improve mix workability at lower temperatures. Asphalt foaming techniques include adding small amounts of water to the hot asphalt binder just prior to mixing with aggregate, and using damp aggregate (or zeolite) that contains adsorbed water. When that water turns to steam at atmospheric pressure, it's dispersed into the hot asphalt, expanding the binder and reducing mix viscosity.

    Organic additives or specially formulated waxes reduce the viscosity of the asphalt binder above the melting point of the wax. The wax must be carefully selected to ensure that its melting point is higher than temperatures expected to occur when the asphalt is in service, and to minimize asphalt brittleness at low temperatures.

    The third type of warm mix technology uses chemical additives that include specially formulated surfactants to improve aggregate coating, adhesion, and binder lubricity.

    There are additional material costs to consider when using additives in warm-mix asphalt. Alternatively, the water injection/foaming process requires an initial equipment investment for asphalt producers, but essentially no additional material costs.

    DESIGN, PRODUCTION, & PLACEMENT

    Recently completed research conducted by Advanced Asphalt Technologies LLC, through the National Cooperative Highway Research Program (NCHRP) project 9-43, recommends specific changes to asphalt mix design procedures to accommodate warm-mix technologies. These modifications would update the American Association of State and Highway Transportation Officials' Superpave asphalt mix design and analysis system developed by the Strategic Highway Research Program in the 1980s.

    However, hundreds of warm-mix projects have been successfully built using standard hot-mix design procedures. Quality control and quality assurance testing is performed at the warm-mix temperature and must meet standard hot-mix asphalt requirements.

    Some warm-mix technologies require producers to make minor plant modifications and adjustment of fuel burners to efficiently operate at lower temperatures. Placement practices usually don't differ between warm- and hot-mix asphalt, with the important exception of lower compaction temperatures. Improved compactability is often cited as a potential benefit of warm-mix asphalt.

    Project 9-43 recommends conducting performance testing to assess warm-mix designs before full-scale production. For more information, visit www.warmmixasphalt.com. PW

    Contributed by the National Center for Asphalt Technology