What began in Europe is now the hottest thing in asphalt in Texas and the U.S. There are many ways to get there, but the goal is the same: to lower the temperature of asphalt by 35 to 100 degrees Fahrenheit prior to application.
“When we started the warm mix research project, I thought we’d have trouble getting test sections because we couldn’t get anybody interested,” says Cindy Estakhri, who conducted the first Texas study on warm mix asphalt. Estakhri manages the Texas Transportation Institute’s (TTI‘s) Recyclable Materials Program. “It’s grown so much now that we have difficulty keeping track of all the projects now.”
Why is warm mix so popular? Using it reduces emissions at the plant and on site, lowers energy costs for the contractor and expedites construction time. These and other advantages have convinced Texas road builders, and the Lone Star State now leads the nation in warm mix asphalt tonnage.
“Contractors see warm mix as a way to extend their paving season,” explains Estakhri. “If you can achieve compaction at lower air and pavement temperatures, then you may be able to extend your paving time by a few months every year.”
There are some 17 different methods for creating warm mix asphalt worldwide, and the Texas Department of Transportation (TxDOT) will soon fund a study to evaluate the chemistry involved in the different methods. “There’s a lot that’s new and proprietary going into the warm mix. You want to be sure that it won’t have some detrimental effect down the road,” Estakhri says.
One concern about using warm mix involves moisture content. In a regular hot mix, the aggregate is almost completely dehydrated. But laboratory testing has revealed that, if you use a moisture-susceptible aggregate, warm mix might be vulnerable to moisture damage. The significance of that potential problem is still a concern.
Jesse Fleming, TxDOT‘s area engineer in Graham, Texas, is a warm mix supporter. His district recently used warm mix on a section of U.S. 183 in Throckmorton County.
“Warm mix gives us a little more workability,” says Fleming. “We hope that it gives us more durability of the mix in the long run because we’re not cooking the lighter oils out of the warm mix.”
Using warm mix could also have a significant positive impact on transportation construction projects in and around air quality non-attainment areas. With warm mix technology, asphalt production plants can manufacture more tonnage without increasing plant emissions, a serious concern in an age of climate change. As the country gears up to repair its highways, the ability to create cleaner road construction materials is a strong selling point.
“The [performance] difference that we’ve seen between working with warm mix and working with hot mix is not significant,” says Fleming. “We believe that warm mix is going to have a big future in TxDOT.”