Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/258673
Better Roads February 2014 21 C orrectly running recycled asphalt shingles (RAS) and reclaimed asphalt pavement (RAP) at the same time can be a tricky business. The main economic advantage to doing so, of course, is that you replace expensive virgin binder with less expen- sive recycled materials. But if you try to run too much RAS and RAP, you may not get good mixing, and problems can develop. More experienced contractors do not usually run the maximum allowable percentages of RAS and RAP. They fi nd the mix designs that they can run consistently well and ef- fi ciently. If a state allows 5 to 7 percent RAS, they run 3 to 5 percent. One challenge is to transfer enough heat to the RAP and RAS to melt the liquid asphalt in both and achieve good mix- ing. Typically, this is accomplished by conducting heat from superheated virgin aggregate to the RAP and RAS. But that process has its limits. It takes a lot of heat to melt the liquid binder on a shingle chip or a piece of RAP. The ideal way to introduce RAP and RAS to the mixture is to introduce the shingles earlier than the RAP, says Malcolm Swanson, president of Astec Inc. That way, the stiffer asphalt in the shingles gets the benefi t of the higher temperatures in the superheated aggregate before it encounters the RAP. "Not a lot of plants are equipped that way, however," Swan- son says. "So in many cases, you're going to end up introduc- ing the RAP and shingles together." In a counterfl ow plant, that will occur at the RAP collar. Or, as Swanson points out, the Astec Double Barrel plant has a mixing chamber that permits the RAP and RAS to mix with the superheated aggregate in an inert, non-oxidizing environ- ment. "Pretty much everybody in the industry, even our com- petitors, acknowledges that the Double Barrel is a recycling beast," he says. A typical state specifi cation calls for RAS to be ground to 3/8-inch minus. But Ideker Construction Co., in St. Joseph, Running Shingles the Right Way Astec built a special bin for Ideker to run 1/10-inch shingles. HighwayContractor By Daniel C. Brown