U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
What is Hot Mix Asphalt (HMA)?
HMA is the traditional process for constructing asphalt pavements. HMA is manufactured in a central mixing facility (typically called a hot mix plant) and consists of high quality aggregate and asphalt cement. The two are heated and mixed while hot to completely coat the aggregate with asphalt cement. The aggregates and asphalt cement are heated above 300°F during mixing and kept hot during transport by truck, placement (where it is spread on the roadway by an asphalt paving machine), and compaction (where it is compacted by a series of asphalt roller machines) of the asphalt mixture. The mixture cools after compaction to form the asphalt pavement.
What is Warm Mix Asphalt (WMA)?
The traditional process for mixing, hauling, placing, and compacting asphalt mixtures uses HMA. WMA is a general term for technologies that reduce the temperature needed to produce and compact asphalt mixtures for the construction of pavements. With WMA, temperatures generally start 30 ° 70°F lower during mixing and remain lower during trucking, placement, and compaction. The mixture cools after compaction to form the asphalt pavement.
What materials make up the asphalt pavement mixture?Back to Top
Asphalt pavement mixtures are primarily composed of aggregates and asphalt cement (typically called asphalt binder). Aggregates are the granular material used in asphalt concrete mixtures and provide most of the load bearing characteristics of the mix. These are hard, inert mineral materials, such as crushed stone, gravel, and sand. Aggregates make up 90-95 percent of the pavement mixture, by weight. Asphalt binder is a dark brown to black, sticky substance produced by refineries during petroleum processing. Asphalt binder makes up approximately 4-8 percent of the pavement mixture, by weight.
What holds the asphalt pavement mixture together?
Asphalt Binder is a dark brown to black, sticky substance produced by refineries during petroleum processing. Asphalt binders are thermoplastic materials, meaning they are liquid when heated and solidify when cooled. At room temperature, they are a semi-solid, sticky material. The asphalt binder is the "glue" that holds the aggregates together to form the pavement. Generally, asphalt binder makes up less than 8 percent of the total pavement mixture, by weight.
How are the aggregates and binder combined into the asphalt mix?Back to Top
Two types of mixing facilities can be used to combine the correct amount of asphalt binder and aggregate at the correct temperature. Batch plants heat and dry the aggregate first. Then a separate mixer is used to blend the aggregate and asphalt binder one batch of asphalt mixture at a time. Drum plants heat and dry the aggregates and then mixes the aggregates with asphalt binder in a continuous process, using the same piece of equipment.
What makes WMA different from traditional HMA?Back to Top
High production temperatures are traditionally needed to make the asphalt binder fluid and less sticky during mixing to completely coat the aggregate and also have good workability during hauling, placement, and compaction. WMA technologies use water, water-bearing minerals, chemicals, waxes, and organic additives or a combination of technologies. The technologies are added to either the mixture or the asphalt binder to produce mixtures at lower temperatures. These technologies allow the asphalt binder to remain fluid at lower temperatures during mixing in order to completely coat the aggregates. It is the use of these technologies that allow the construction of asphalt pavements at lower temperatures.
Does WMA have any impact on workers at the paving site?Back to Top
Visible emissions and odors are reduced both at the plant and the paving site. Less emissions and odors are released at lower production temperatures. This creates a worksite that is cooler and more pleasant for workers during placement and compaction. Workers generally favor the cooler work environment and reduced odors, especially on hot summer days.
Does WMA impact plants that produce the asphalt mixtures?Back to Top
Because lower temperatures are also used at the plant, emissions and odors are reduced. Depending on the production temperature, a range of 15-70 percent reduction of carbon dioxide and other emissions during production have been reported.
Does using WMA affect costs?Back to Top
A benefit of producing WMA is the reduction in energy consumption required to heat traditional HMA to temperatures in excess of 300° F at the production plant. Because plant temperatures are lower, less fuel is needed. The reported reduction in fuel consumption typically ranges from 20-35 percent, with up to 50 percent reported for some technologies.
Is WMA also useful for patching roads?Back to Top
Because the cooling time of WMA is extended, paving and patching at cooler temperatures is possible.
Can recycled asphalt be used with the WMA process?Back to Top
Recycled asphalt pavement (RAP) can be used with WMA. The same RAP usage considerations for HMA are applicable to WMA.
Are temperatures significantly lower with WMA?Back to Top
WMA is produced at temperatures that are 30°F to 120°F lower than the typical HMA temperatures of 300°F to 350°F. Less fuel is needed to heat the mixture to the higher HMA temperatures.
Can WMA technologies be used at traditional HMA temperatures?Back to Top
Some WMA technologies were originally developed as compaction aids to help asphalt roller equipment compact the asphalt mixture. These technologies have been used as compaction aids at traditional mixing temperatures. Improvement in compaction alone will improve the performance and increase the life of asphalt pavements.
Where has WMA been used in the United States?Back to Top
As of 2009, WMA projects have been constructed in more than 40 States, and at least 14 State Highway Agencies have adopted specifications to accommodate WMA. In addition, 22 different named WMA technologies are actively marketed in the United States.
Does using WMA require plant and mix design modifications?Back to Top
The need for plant and mix design modifications depends upon the type of additive used. Most require relatively simple plant and mix design modifications to introduce the temperature-reducing technologies into either the mixture or asphalt binder stream. Some technologies are added to the asphalt binder by the supplier and do not require additional equipment at the plant. Other technologies require more substantial modifications. Technologies involving water-based foaming techniques or mixture additives require additional equipment installed at 1the plant to measure and deliver the additive.
What types of additives are used in WMA?Back to Top
WMA technologies may require water, water-based additives, water-bearing mineral additives, chemical additives, waxes, and organic additives or a combination of technologies. While many technologies were developed specifically for WMA use, others simply use a combination of existing technologies, such as liquid anti-strips, waxes, surfactants (soaps), and emulsifiers (asphalt-water-soaps) that have been used in the asphalt pavement industry for decades.