"All of our asphalt plants have two RAP bins," says DeVitto. "We screen it to 1/2-inch by 3/16 inch, and 3/16 inch minus. The finer material has more asphalt content, so that allows us to increase the RAP and decrease the virgin liquid that we run."
DeVitto explains. Suppose a hot mix design calls for 5% liquid binder. If the binder costs $400 per ton, that's $20 per ton of mix for binder.
Now, if your RAP has 4.8% binder in it, and you add 25% RAP, you can get a fourth of your liquid from the RAP - or 1.2% of the mixture.
So if you subtract 1.2% from 5% liquid, you only have to add 3.8% virgin binder. "So instead of adding in $20 worth of virgin binder, you only have to add $15," says DeVitto. "You save $4.80, or almost $5 per ton, just for binder, not counting your RAP cost.
But DeVitto figures the RAP costs $7 per ton to mill and process. So if you use 25% RAP, that's 25% of $7, or $1.75 per ton of mix in costs. You subtract $1.75 from $4.80 and get $3.05 net savings per ton of hot mix.
In middle Tennessee, LoJac Enterprises Inc. runs 12 asphalt plants, and six of the 12 have been converted with Astec's Double Barrel Green warm mix asphalt (WMA) technology (see Asphalt Contractor, December 2008, p. 54). Don Chambers, LoJac president, figures RAP costs him $2 to $3 a ton for milling, $2 to $3 to haul it, and $1 or $2 per ton to crush the RAP and screen it.
LoJac splits RAP into two sizes: 5/8-inch chips and 3/8 inch-minus material. "You can blend those two sizes back into your mix and use more RAP because of it," says Chambers. He figures that using RAP saves him between $6 and $11 per ton of mix, depending on the percent of RAP added.
NCAT's West says warm mix asphalt technology does not necessarily allow one to use higher percentages of RAP. However, mixes with high RAP contents can be difficult to place and compact - and a WMA technology can help with compactability.
"The advantages of the RAP and WMA combination include enhanced workability of high RAP mixes, big economic savings, plus the triple environmental benefits of reduced (plant) emissions,, reduced energy demand, and conservation of natural resources," says West. (Warm mix asphalt results in a mixture that is about 240 to 270 degrees F, not the 300 degrees used for conventional hot mix asphalt.)
When a conventional hot mix plant runs RAP, the virgin aggregate reaches 500 or 600 degrees F. because it must heat the RAP, says LoJac's Chambers. But with WMA technology, you don't heat the mixture as hot. That means you don't drive off the light oils from the virgin binder or the RAP.
Using warm mix technologies provides a better film thickness of binder on the aggregate, and the RAP is not scorched in the process.
In one demonstration project, LoJac ran 50% RAP in 9,000 tons of a binder course and 30% RAP in 6,000 tons of surface course mixture.
The hot mix was laid on a five-mile stretch of road that serves a regional landfill. The Tennessee DOT wants to see how the mix holds up under heavy truck traffic.
"If that mix holds up, the DOT may raise the percentage of allowable RAP," says Chambers.
Test track results
On the NCAT Test Track in Alabama, where accelerated loading tests are run on various asphalt mixtures, high RAP pavements have turned in some excellent rutting results. Basically the results show that the more RAP a mixture contains, the less rutting you'll get. For example, under 9 million equivalent single axle loadings (ESALs) a 20%-RAP mixture using a PG 67-22 binder showed a rut depth of 8.6 millimeters. By contrast, a 45%-RAP mixture with PG 76-22 binder showed only 0.5 mm of rut depth after 9 million ESALs.
NCAT's West explains. As RAP percentages increase, the binder stiffness also increases. And stiffer binders are more resistant to rutting. West does, however, express some concern with cracking of high-RAP mixtures.
Field performance results on cracking tend to vary, he says. In some cases the virgin mixes have performed better than RAP mixes and in others, the mixes with RAP have performed better than virgin mixes in terms of cracking.
"A properly selected virgin binder can mitigate issues with cracking," West says.