WARM MIX ASPHALT
Reducing the energy needed to heat
asphalt is a potential area of big savings
How is warm mix asphalt different from hot mix asphalt? In the simplest of terms, WMA is the combination of aggregates and liquid asphalt, just like HMA. However, WMA requires less heat to produce than HMA because an additive is incorporated in the mixing process. These additives appear to reduce the viscosity of the liquid asphalt at a given temperature. As a result, the liquid asphalt coats the aggregates at a reduced temperature compared to HMA.
Although efforts to reduce mixing temperatures have been under way in the United States since the 1970s, the WMA 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 HMA 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.
WMA has the potential to allow HMA producers 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 nonattainment 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.
In exploring whether WMA is the wave of the future, the following questions are being evaluated:
- 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 countries, can they be adapted to the United States 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?