Better Roads Staff
What percentage of RAP is best? It depends.
T here are powerful inducements today to reuse reclaimed asphalt pavement (RAP) in a variety of applications. And the big question about RAP has shifted from whether it belongs in mixes at all, to how much can safely be accommodated in a mix.
It is this question that is driving an enormous amount of attention and research today. Many road agencies are closely observing the research as they permit higher percentages of RAP in a mix. Environmental legislation at the state level also is compelling higher percentages of RAP.
In the meantime, research continues on the questions of to what degree does the residual asphalt on RAP replace performance-graded (PG) binders; how will larger percentages of RAP impact the type of PG binder that should be specified for a Superpave mix in a particular location; and how important is the processing and analysis of RAP stockpiles in allowing higher percentages of RAP in asphalt mixes.
New guidance was released this year and last from the Federal Highway Administration (FHWA) and the National Cooperative Highway Research Program (NCHRP) on higher amounts of RAP in asphalt mixes.
Today, critical-performance mixes – such as those for surface or friction courses – typically get a lower percentage of RAP allowed than the intermediate or leveling courses just below them. It’s being demonstrated that warm-mix asphalt (WMA) modifiers can permit higher percentages of RAP in a mix, and that warm mixes are very friendly to RAP. Use of a rejuvenator can allow vastly higher percentages of RAP in noncritical intermediate courses. And foamed asphalt- or asphalt emulsion-stabilized bases can use 100-percent RAP, as was done in a major recycling project on Interstate 81 in Virginia this year.
In 2009, researchers at the National Center for Asphalt Technology (NCAT) estimated that across the United States, RAP usage varied considerably, but the average RAP content was estimated to be around 15 percent. Boosting that average could reduce greenhouse gas emissions from roadbuilding substantially, National Asphalt Pavement Association (NAPA) president Mike Acott says. “Use of 25-percent RAP reduces total lifecycle greenhouse gas emissions by 10 percent, which equates to 2 million tons [of carbon dioxide] offset annually,” he says.
And more emissions reduction is possible, Acott implies: “A singular quality of asphalt cement in that it is rejuvenated when RAP is incorporated into new pavement, becoming an integral part of the binder. In view of the high reuse/recycling rate in lead states, including a preponderance of evidence that the quality of asphalt pavements incorporating RAP is equal to or better than pavements using all virgin materials, there is ample opportunity to double the quantity of RAP used within five years.”
Boosting the amount of RAP in mixes is a line item in the National Asphalt Road Map: Commitment to the Future, produced in 2007 by NAPA; FHWA; American Association of State Highway & Transportation Officials; Asphalt Institute; and National Stone, Sand & Gravel Association.
The asphalt road map lists Item No. 4.09: Develop High RAP Content Mix Design Procedure as one of its needed high-priority research projects. The road map also urges study on use of recycled materials other than RAP in asphalt mixes.
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