Crunching the numbers of recycled asphalt
RAP segregated in piles
As CEO of ASTEC Industries, Inc. (the largest manufacturer of asphalt plants in the United States) and as the holder of approximately 90 patents, Don Brock is a bona fide expert on asphalt recycling. At the 2007 Minnesota Pavement Conference, he shared some nuggets of knowledge on the subject.
The times they were a-changing
Until the 1970s, Brock noted, the price of asphalt had remained about the same for about half a century. Then several factors dramatically changed the picture. First, when OPEC manipulated oil prices in the 1970s, the cost of asphalt increased by an order of magnitude: by 1979, it was about $200 per ton. This price spike coupled with the advent of milling brought the cost of recycling below that of virgin materials.
Why not separate recycled material?
But Brock pointed out a discrepancy in the way materials are handled today. A common practice, he said, is to separate virgin material by size but to throw all RAP (reclaimed asphalt pavement) together in one bin. As shown in Table 1, he explained that this leads to mix design problems because different sizes of RAP retain different amounts of asphalt.
“So as the rock is segregated,” Brock said, “it’s also segregating the liquid asphalt content.” This tends to limit the use of RAP in pavements to about 20 percent. But Brock explained that excellent products can be made if recycled material is separated just like virgin material.
RAP is worth it
Brock’s next point was that paving materials obtained from RAP are a bargain compared with the virgin materials they replace. For example, in 30,000 tons of recycled pavement, there are typically about 28,200 tons of aggregate and about 70 6,000-gallon transport loads of liquid—worth roughly $1 million. The cost to reuse that RAP is only the cost of the trucking and processing, since the same material is taken out of the road and then put back into the road. He assumed this trucking/processing cost to be $6.40 per ton. He then compared that with typical prices of virgin material: rock at $9.40 per ton + asphalt at $18.00 per ton = $27.40 per ton. Thus, the savings from using recycled material would be $21.00 per ton.
Overlay vs. inlay
Brock then compared overlays with mill-and-inlay operations: Putting an overlay on a rutted road is very likely to yield a rutted road again, but milling a pavement gets rid of the rutting. Also, the rough milled surface will interlock well enough with the inlay that a tack coat is probably unnecessary. Furthermore, Brock said, “One of the best benefits is we eliminate joint density problems. When you put down a 2-inch overlay, you put down 2½ inches, and as you try to compact it, you can’t contain it on the edges, so it spreads out. As it spreads out, you create low-density areas.” But with mill and inlay, the unmilled shoulder creates a dam on each side, so the material has no place to spread out.
Furthermore, with an inlay there is no need to raise either guardrails or shoulders—and no need to decrease the posted clearance under bridges or the weight limits on bridges. Finally, Brock said, if the tack coat isn’t necessary and a transfer machine is used, a paver can follow 200 feet behind the milling machine, and by double hauling (mix out, milled material back), save tremendously on hauling costs.
Brock did say recyclers need to work out some problems. For example, they need to allow more mixing time to allow water to evaporate, the superheated virgin material to cool, the RAP to heat up and melt the old liquid from recycled material, then mix new liquid. By allowing that evaporation time, they will produce a product that mixes the RAP and virgin material more homogeneously. Nonetheless, Brock ended by stating confidently that, as the cost of virgin materials continues to increase—and as we become more committed to decreasing our dependency on foreign oil—he expects all agencies to turn more and more to recycling.
Table 1: Liquid Asphalt Content in Recycled Pavement
Aggregate Size in RAP |
Typical Asphalt Content |
|---|---|
3/16 x 0 |
7% |
3/8 x 1/4 |
4% |
1/2 x 3/8 |
3% |
—Richard Kronick, LTAP freelancer