Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/115354
Rap Up Making high-RAP mixes – an art and a science M any state transportation departments have saved tens of millions of dollars by using reclaimed asphalt pavement. RAP is worth the same as the virgin materials it replaces. And to mill, haul and process RAP costs only a fraction of what virgin mixtures cost. With more RAP, contractors can produce a lower-cost mix and pass the savings along to owner agencies. Moreover, warm mixes made with a high RAP content have a fatigue life that is far superior to a 100-percent virgin mix, according to recent studies from the National Center for Asphalt Technology(NCAT). "The conventional wisdom has been that high-RAP mixes would crack," says Malcolm Swanson, vice president-engineering, Astec. "They'll crack in the laboratory, because of the way the tests are run. But the NCAT field tests show they didn't crack. Those results to me were just astounding." Today, large metropolitan areas are accumulating large amounts of RAP and they don't have space to store it. Even with 50-percent RAP mixes – which is higher than most agency specifications permit – only half the RAP that is milled up gets recycled. So the heat is on to run more RAP. But there are inherent problems with making high-RAP mixes. The RAP has to be heated in the drum mixer to the point that the recycled binder melts from its aggregate and even partially coats some of the virgin aggregate. That raises the problem of how to Better Roads March 2013 9