The future of hot mix asphalt might be cooling down a little bit...to warm mix asphalt. Warm mix asphalt (WMA) is an asphalt mix with an additive that allows the asphalt to be mixed and placed at cooler temperatures, says Annette Smith, project leader, Warm Mix Asphalt Technology Group, PQ Corp. "Not cold temperatures but usually between 50°F to 75°F cooler than you would typically make a hot mix asphalt," she says.
Warm mix asphalt initially started in Europe but made its way over to the United States around 2003-2004, Smith says.
The benefits of WMA are numerous. First, and one of the main reasons warm mix asphalt was created, is lower emissions. "Depending on the type of emissions you can reduce it up to 65%," Smith says. "There are reductions in emissions not only at the plant where they manufacture the asphalt but also behind the paver."
Contractors who use WMA to pave also benefit from the ability for longer hauls -WMA asphalt does not have to be placed as quickly as hot mix asphalt (HMA) - as well as the opportunity to place WMA in cooler temperatures. Being able to work in cooler temperatures can allow contractors to pave later in their normal paving season.
The additives, or "technologies," added to WMA reduce the amount of oxidation of the asphalt binder which creates a longer life for the pavement, Smith says. Another benefit is that warm mix asphalt acts like a compaction aid providing improved compaction or density with different mixes, she adds.
Warm mix asphalt also benefits the asphalt plants with reduced fuel costs, Smith says. "Because material is being placed at lower temperatures, fuel usage is decreased at the asphalt plant," she says. "Fuel savings changes depending on the mix design, but people have quoted anywhere from 10% to 30% fuel savings, which nowadays is becoming even more of an issue."
There is also the opportunity to benefit from combining reclaimed asphalt pavement (RAP) with warm mix asphalt, Smith says. The lower temperatures of WMA allow plants to include higher amounts of RAP in their mixes. Higher RAP amounts can help save costs and allow asphalt plants to use less virgin asphalt binder.
Perhaps the biggest benefit paving contractors can reap from the use of WMA is a better "quality of life" while on the job. When paving with WMA no black smoke is produced, Smith says. This means contractors won't get as dirty and will breathe less fumes. And since WMA is not as hot as HMA, the workers walking behind the paver or doing hand work have a less likely chance to get burns from the asphalt. Along with that, WMA gives off less heat, which is good for working in hot environments.
When WMA first took off in the U.S., there were three technologies available to add to HMA to lower its temperature, Smith says. Those three technologies were Advera, Evotherm, and Sasobit. It is important to note that the technologies used to create WMA are either added to the mix at the asphalt plant or at the terminal. None of the technologies can be added after the mix leaves the plant, Smith says.
Advera, a synthetic zeolite, is a mineral filler with a small amount of moisture. That moisture is emitted when the filler comes in contact with heat and lowers the mix's temperature. Evotherm has several technologies, some with water and some without water, which changes the binder to flow at lower temperatures. Sasobit is a wax that mixes with the binder and chemically changes the mix to create greater workability, Smith says.
These three technologies have been around in the United States since 2004 and have been widely tested, she says. But they aren't the only technologies available in today's market to create warm mix asphalt. To learn more about the current technologies visit www.warmmixasphalt.com or the Federal Highway Administration's website (www.fhwa.dot.gov).