National efforts are underway to refine specifications for reclaimed materials in asphalt mixes.
A two-year review of asphalt-mix designs at the Indiana Dept. of Transportation has brought new attention to the long-standing use of reclaimed asphalt pavement (RAP) in paving projects, even though the practice itself is likely not the cause of premature aging in dozens of recently completed road projects statewide.
INDOT is testing samples from 173 projects done by 44 contractors, comparing the extracted binder's aggregate absorption value against the contractor-provided mix recipe. Consistent deficiencies found in the first project to be fully tested—a four-mile portion of the state Route 25 expansion in Cass County, completed in 2012—resulted in a $5.15-million fine for Fort Wayne, Ind.-based Brooks Construction Co. Brooks has denied any wrongdoing, claiming it complied with all INDOT specifications.
Robert Tally, INDOT deputy commissioner for materials, refuted suggestions that the blame falls on the state's increased amount of RAP as an alternative binder replacement in its asphalt-mix specifications over the past five years.
"There's no correlation between the use of RAP and percentage of problems we're experiencing," Tally says, adding that RAP is not a mandatory ingredient for pavement mixes on state road projects. The contractors may have been using aggregate with specific gravity values different than stated in their mix designs, resulting in low binder contents, he notes.
Other states that have increased allowable amounts of RAP in their pavement specifications as a cost-saving measure have likewise experienced problems, though not to the extent of Indiana's recent experience.
Yet the problem is hardly unusual, according to Ananth Prasad, national transportation practice leader for HNTB and former Florida secretary of transportation. "Every state DOT has had an issue with RAP in the mix design, even when the amounts were small," Prasad says, noting that factors such as the material's origin and consistency can affect the mix's performance. "Regardless of how much you use, it still has to meet performance tests. DOTs are continuously trying to optimize the quantity of RAP."
Others stand strongly behind the material. "RAP has been a success," agrees Audrey Copeland, vice president of engineering, research and technology for the National Asphalt Pavement Association. "The U.S. has been using pavement mixes averaging 20% RAP, and nothing indicates we should pull back."
Recycled pavement isn't the only non-traditional ingredient being used in asphalt mixes. Many states are incorporating more alternative materials in lieu of virgin binder, taking advantage of their high asphalt content. Among the most popular are reclaimed asphalt shingles (RAS), including manufacturing process waste and scrap from rooftop tear-offs. According to the Federal Highway Administration, 5% RAS by weight of the total mix—the maximum amount allowed by states when the material is permitted—can replace as much as an additional 25% of the total binder.
Like RAP, state RAS specifications are typically based on AASHTO-issued guidance. Mark Buncher, director of engineering for the Lexington, Ky.-based Asphalt Institute, cautions that states' specifications and mix-design procedures may not fully account for the extremely stiff nature of the RAS binder.
"Plant mixing temperatures are not hot enough for the RAS binder to get fully released and blended into the mixture," Buncher explains. "Unless more binder is added, the mix will be drier and more susceptible to early cracking and raveling."
Efforts are underway to refine AASHTO specifications for using RAP and RAS in asphalt mixes. A task group at FHWA is due to offer recommendations on RAS by April 2016.
Even with the new guidance, Rebecca McDaniel, technical director for the North Central Superpave Center at Purdue University, says INDOT's experience is a cautionary tale for other DOTs looking to increase the use of recycled materials in the name of cost-cutting.
"Contractors are responsible for a good mix," McDaneil says, "but DOTs are responsible for overseeing it."