What Is It?
What is Warm Mix Asphalt (WMA)? How is it different from Hot Mix Asphalt (HMA)? In the simplest of terms, warm mix asphalt is the combination of aggregates and liquid asphalt, just like hot mix asphalt. However, WMA requires less heat to produce than HMA due to the incorporation of an additive to the mixing process. These additives, of which there are currently four on the market, appear to reduce the viscosity of the liquid asphalt at a given temperature. As a result, the liquid asphalt coats the aggregates at reduced temperature compared to HMA.

History
Although efforts to reduce mixing temperatures have been underway since the 1970s in the United States, the warm mix asphalts movement of today started in Europe. The European Union has made a commitment to significantly reduce greenhouse gases produced as a result of manufacturing. One area of focus in meeting these reduction goals was the hot mix asphalt industry. That industry has been building asphalt pavements with WMA technology for several years. WMA activity in the United States to date has occurred through test projects throughout the country.

Benefits
Warm mix asphalt has the potential to allow the producers of hot mix asphalt to lower the temperatures at which the material is mixed and placed on the project. To date, documented reductions in temperature during production have ranged from 30 to 100 degrees Fahrenheit. These reductions in heat required to produce the mix mean less fuel is used and consumed, fewer greenhouse gases are produced and emissions are reduced. This could be especially beneficial in ozone non-attainment areas where the reduction of emissions is essential in the effort to get into compliance with the Clean Air Act.
In addition, WMA may provide the contractor greater latitude in terms of the turnaround time for placing the mixed asphalt after it has left the plant. Given that WMA has a lower temperature than HMA to begin with, it takes longer to cool off than HMA.

Additives
There are currently four technologies on the market which are used to produce warm mix asphalt:

Aspha-Min (www.aspha-min.com)
WAM-Foam (www.wamfoam.com)
Sasobit (http://www.sasolwax.com/Sasobit_Technology.html)
Evotherm (www.evotherm.com)
To learn more about these products and their role in the production of WMA, you can click on each product name and connect to the company website for that product.

Future
In exploring whether warm mix asphalt is the wave of the future, the following questions must be addressed:

Can WMA pavements be opened to traffic as quickly as HMA pavements after construction?
What are the performance characteristics of these pavements?
In the case of technologies developed in other, can they be adapted to the U.S. where climate conditions are often more extreme?
Since the production temperature is lower does the binder not age as much?
Will there be a reduction in the potential for thermal cracking?
Will there be a difference in the potential for rutting?
Will the contractor have to use a different grade of binder?
Will there need to be changes for the mix design procedure?
Will the performance-graded binder in a warm mix perform differently from pavements produced at a higher temperature?