Over the past year the price of asphalt binder, the “glue” in hot mix asphalt, has shot up more than the price of any other road construction material. Liquid asphalt binder costs more than doubled, from about $160 per ton in 2005 to $360 or even $400 in some areas.
To stretch their paving budgets, road agencies are increasing the use of a proven, yet under-used, technology: adding recycled asphalt pavement (RAP) to hot mix. The savings can be significant (see chart on page 40).
The Illinois DOT (IDOT), for example, is raising the percentage of recycled asphalt allowed in new pavements and overlays.
Starting this year, road contractors for IDOT can add up to 15% recycled asphalt in surface mixes for moderate- to high-volume roads—up from 10% in the past—and up to 25% on below-surface layers, up from 15%. And, for the first time, Illinois will allow up to 10% recycled asphalt in all layers of the highest-volume Interstate pavements.
Asphalt technologists agree that increasing recycled content is one of the most effective ways to lower hot-mix costs. Since the 1970s oil embargo, the hot-mix asphalt industry has recycled milled asphalt on a large scale, says David Newcomb, vice president for research and technology at the National Asphalt Pavement Association, Lanham, Md.
“It's time to consider ways of increasing recycled content even further,” says Newcomb. “Recycled asphalt pavement is a resource rich in asphalt and aggregate. In the same way we test and process virgin materials, so too should we judge the quality of RAP and process it.”
The technology exists to engineer pavements containing up to 40% or more of recycled asphalt by weight; those mixes are usually used below the surface layer. Newcomb says the sizing of the material will help to define its use in hot mix. Maintaining two or even three recycled-asphalt stockpiles, each with a different size material, gives a contractor greater flexibility in designing mixes for specific applications.
But talk to a few state asphalt pavement association executives, and they'll tell you that many local agencies don't permit the use of recycled asphalt. Why not? They may not have enough experience with recycled pavement. Or, they may believe that recycled materials are inferior to virgin materials.
“I'd say as high as 90% of the local agencies in Illinois don't use recycled asphalt at all, and I've heard that to be true in other states,” says David Lippert, an IDOT materials and research engineer based in Springfield. “It's been a major issue for us in Illinois. We approach it as an education problem, and so we've started a joint industry-government training program in the use of recycled asphalt pavement.” (For more info, visit www.dot.il.gov/blr/t2center.html or call Kevin Burke III, PE, at 217-785-5048.)