In 2005, contractors resurfaced this section of US 61/67 in St. Louis County, Mo., with hot mix containing 20% reclaimed asphalt. Photo: Missouri DOT
Reclaimed asphalt enters the outer drum near the burner end. Aggregate flows through the flame and mixes with the binder and used material as it moves in the outer drum from left to right. Photo: Astec Inc.
In Texas last year, hot-mix asphalt prices doubled. The primary culprit was the binder, which soared to $718/ton from $340/ton for a common grade. The story repeated itself across the nation as crude oil prices rose and supplies became pinched.
Reclaimed asphalt is simply the combination of used aggregate coated with used liquid binder; using more of it reduces the need for virgin aggregate and pricey new liquid binder. As a result, state road agencies are increasing their allowable amounts of reclaimed asphalt pavement (RAP).
Reclaimed asphalt has been milled from an existing roadway that's not yet deteriorated to the point at which full-depth reclamation becomes an option. Typically, it's trucked from the milling machine to a stockpile at an asphalt plant and stored there.
The transportation departments of Texas, New Jersey, and Illinois have recently increased their specified limits or are considering it. In and around Tucson, Ariz., agencies also are allowing it for the first time.
Properly designed and built, pavements with reclaimed asphalt will last as long or longer than roadways built with virgin materials, says Kent Hansen, director of engineering for the National Asphalt Pavement Association. He cites a 1980 Transportation Research Board study that analyzed the performance of various pavements up to seven years old and concluded that the mixtures performed at least as well as mixtures made with all-new materials.
Demonstration projects containing 25% to 50% of used asphalt have been implemented in nearly a dozen states, including Delaware, Illinois, Florida, South Carolina, Minnesota, Oregon, and Missouri, and reclaimed-material content of 50% and higher is becoming increasingly common.
“As raw material prices rose dramatically over the past year, the industry knew that the best way to keep hot-mix asphalt costs in check was to increase RAP contents,” says Randy West, director of the National Center for Asphalt Technology (NCAT) at Auburn University in Alabama. “The other big motivation has been the sustainability movement. Recycling means that we use less of the earth's nonrenewable natural resources.”SIZING UP USED AGGREGATE
The national average for surface mixtures is about 15% reclaimed asphalt, with the average for base and intermediate courses at about 20%.
When recycling began in the late 1970s, mixes with more than 50% reclaimed material were common. But there were notable failures, and agencies have been reluctant to try again. A few, such as the Arizona DOT, banned used material altogether.
“Some agencies want more performance data on higher-percentage mixes; others want to wait for more guidance on how to select the right grade of virgin binder,” West says. “Failures typically are not well-publicized or documented and so we often don't learn the real causes.
Today, however, changes in how used material is processed and advances in mixing technology make higher percentages more successful as well as more feasible.
One new development is fractionation: separating reclaimed material into particles of more than 1/2-inch or 3/8 -inch and particles of less than half an inch, enabling contractors to more easily develop fine and coarse mixtures.