Warm mix asphalt (WMA) continues to gain momentum in the industry. As a result, tests are continuously being completed throughout the country to monitor the results of WMA compared to hot mix asphalt (HMA).
Matthew Corrigan, P.E., an asphalt pavement engineer with the Federal Highway Administration, travels throughout the country testing WMA. The testing completed has helped put together a framework for both HMA and WMA as well as emissions and environmental issues.
Operating out of a mobile trailer, the Federal Highway Administration completes several types of testing on WMA.
“Our mobile asphalt testing laboratory trailer supports industry and stakeholder initiatives to provide solutions and support of new materials, new test equipment, new devices, test procedures, provide training in those areas, and support quality assurance and quality control initiatives,” Corrigan says. “A combination of those programs came together to provide resources on the warm mix front.”
The testing completed ranges greatly. “We can do anything from typical or traditional asphalt mixture and binder tests which almost any laboratory in the country can run for acceptance or quality control,” Corrigan says. “In addition we have many advanced pieces of equipment and more advanced testing we’ve been working with for several years. Many of those technologies are ready for implementation across the country. There are more state DOTs and contractors purchasing and using that equipment.”
One benefit to using such equipment is being able to test innovative materials.
“The advanced testing devices give you more flexibility to use innovative materials like warm mix,” Corrigan says. “You can evaluate its performance properties within the mixture itself or binder itself rather than relying on some sort of recipe or spec that dictates the quantities.
“You can actually measure performance properties with that specific mixture whether it is WMA, reclaimed asphalt pavement (RAP), recycled asphalt shingles, (RAS), etc.,” he continues. “These kinds of equipment bring a lot of value to evaluate how these different materials and innovations act if it was a pavement under traffic conditions.”
Results of WMA testing
With 30,000 tons of WMA, Yellowstone National Park was one of the first big warm mix testing projects.
“It had two warm mix technologies and a hot mix control that was being evaluated for long-term performance,” Corrigan says. “We continue to do testing and evaluation on projects like that around the country in order to support the industry.”
Corrigan says that the findings of these tests are used by the Federal Highway Division offices as well as the state Department of Transportation where the project is being built.
“They are using our testing data to support their look at technology to see if warm mix has equal or better performance than hot mix,” he says.
Currently, there are few drastic differences between the two types of mix.
“When it comes to many of the traditional volumetric type of tests used for hot mix, in most instances those are comparable with warm mix,” Corrigan says. “There aren’t too many differences unless you get into a highly absorptive aggregate source or a low production temperature — then you can see differences between hot mix and warm mix volumetrics.”
When looking at the mechanical properties or engineering properties of the mixtures some differences do appear in laboratory performance testing.
“The differences that show up are driven by how low a production temperature the warm mix uses in comparison to the hot mix,” Corrigan says. “If you use a warm mix technology as a compaction aid at hot mix temperatures, than everything else is equal.
“If you just drop the temperature and don’t make any other changes to the mixture to accommodate that drop in temperature, you tend to have an initially less stiff mixture out of the production plant because you didn’t heat the mixture to hot mix temperatures.”