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What is Warm Mix Asphalt?

Warm Mix Asphalt - Fewer Emissions

The hottest topic in asphalt pavements these days is warm—Warm Mix Asphalt (WMA), that is.  Introduced in Europe in 1997 and in the US in 2002, WMA is rapidly gaining acceptance.  WMA is a green product that lowers manufacture and lay down temperatures by 30° to 100°F.  By reducing the temperatures, WMA doesn’t need as much energy to produce.  It’s been documented to lower energy costs by 30-60%.  WMA reduces greenhouse gas and hydrocarbon emissions from the manufacture and construction of asphalt pavements.  And the lower temperatures make it safer for workers in the plant and at your asphalt paving project job site.  WMA has been especially successful in areas where air quality has been a problem (nonattainment areas).

WMA is manufactured, constructed and used with only minor adjustments.  European and American companies have developed several technologies that all increase workability at the lower temperatures.  There are chemical additives to improve workability and mechanical techniques to foam the asphalt binder.  Special equipment is attached to the asphalt plant to create the foaming process, which coats the aggregates at lower temps. Some of the additives can be easily pre-blended with the liquid asphalt or added at the asphalt mix plant. These additives will be specified in the job mix formula (JMF).

Conventional Asphalt Mix

WMA Advantages

  • The lower warm mix asphalt production temperatures reduce emissions of volatile organic compounds (VOC) at mix plants.
  • Warm mix asphalt reduces fume exposure for workers at the job site.
  • The lower construction temperatures reduce burn hazards.
  • WMA is more easily compacted, which is beneficial during extreme weather conditions, for stiff mixes and mixes with reclaimed asphalt pavement (RAP), and for reducing the amount of roller passes.
  • Warm  mix asphalt production and construction conserves fuel by requiring significantly less energy.
  • The lower temperatures and longer workability allow longer transport, enabling construction at more remote locations as well as more flexibility in transportation schedules.
  • Almost all of the WMA products and processes have been designed to use existing mix design technology, existing asphalt plants and existing transportation and construction equipment.
  • Warm mix asphalt may be (and has been) used for base, binder and wearing courses.
  • Because the pavement is constructed at lower temperatures, it may be opened to traffic sooner, which reduces user-delay costs.
  • The lower temperatures mean that the asphalt binder is age hardened less during the mixing process, providing longer pavement service life.
  • Some WMA additives can be pre-blended with the liquid asphalt at the refinery.
  • The additives improve workability and constructability and may the improve moisture resistance of the mix.

WMA Disadvantages

  • WMA may be more expensive than traditional asphalt mixes.
  • Lower temperatures used for WMA can result in incomplete drying of the aggregates.  The resulting trapped water in the coated aggregates may cause moisture damage.  Care must be taken to monitor this.
  • Performing handwork and luting with some WMA technologies can be difficult at the lower temperatures.
  • WMA mixtures are still being reviewed for long term performance.  To date no known pavement failures or concerns have been observed.

For more information please visit National Clearing House for Warm Mix Asphalt.