The biggest concern is the amount of water left on the aggregate due to lower processing temperatures. Is the asphalt trapping excess water as it coats the aggregate? If so, what are the long-term effects?
“A number of surveys from our project as well as others indicate this is not a problem with warm mix,” says Texas A&M Transportation Institute (TTI) Associate Research Engineer Amy Epps Martin, principal investigator on NCHRP 09-49: Performance of WMA Technologies. The project is assessing whether WMA technologies adversely affect the moisture susceptibility of flexible pavements, and is developing guidelines for identifying and limiting moisture susceptibility in WMA pavements.
To effectively reproduce field conditions in the lab for accurate testing, TTI closely, examined specimens that were mixed and compacted in the lab, and compared those to specimens that were mixed at the plant and compacted in the lab, and then to field cores, which are plant mixed and field compacted.
“The compaction differs between the field and the lab,” Epps Martin notes. “Conditioning and curing in the lab are key issues in proper testing of lab specimens. Properly simulating what’s happening at the asphalt plant and on the road is critical.”
Epps Martin says that this was one of the most complex asphalt research projects she has ever been involved with, requiring the help of departments of transportation in Iowa, Texas, Montana and New Mexico.
Researchers are also looking at what’s called a time horizon. “Many people think there’s a period in the early life of the pavement where warm mix hasn’t caught up to hot mix. We have seen that, too. After about a year’s time, warm mix seems to perform just as well as hot mix in terms of stiffness and moisture susceptibility,” Epps Martin explains.
Testing is under way to determine whether current moisture susceptibility tests capture any differences between warm mix and hot mix. An example question is: does the addition of anti-stripping agents — which are additives like lime or liquid chemical packages — help alleviate any moisture sensitivity problems if there are any?
“So far, we’ve determined proper curing times and temperatures for WMA. For example, at the design stage, two hours at 240 degrees helps simulate what’s going on in early life in the field,” Epps Martin says. The final report will include guidelines for testing with additives or foaming technologies.
“The work that Dr. Epps Martin is performing through this project is critical to the implementation of WMA by state and local agencies,” says Kim Willoughby, research manager for the Washington State Department of Transportation’s Construction, Materials, Bridges and Maintenance Research Office. “Being able to answer the question of whether WMA is more susceptible to moisture than hot-mix asphalt will facilitate the acceptance of these technologies throughout the nation.”