So when liquid binder prices spiked in 2008, West says the industry recognized that the best way to keep hot mix asphalt costs in check was to increase RAP contents. The other big motivation for higher RAP contents has been the sustainability movement. Recycling more today means using less of the earth’s non-renewable natural resources.

High RAP contents

Although many agencies were restrictive on RAP usage in the past, particularly for surface layers, there is a growing trend to allow higher RAP contents. The national average RAP content for surface mixtures is about 15 percent, and it’s about 20 percent for base and intermediate courses, says West. When asphalt recycling began in the late 1970s, RAP contents of more than 50 percent were used on some projects.

Florida provides a good example of increasing RAP usage. When binder prices jumped in 2008, Florida boosted the percentage of RAP allowed in surface courses to 15 percent, up from zero, says Jim Musselman, a Florida state bituminous materials engineer.

In the next year, the state raised the percentage again, to 20 percent.

But the increase came with qualifications. Florida has a problem with local aggregates that polish on the surface and don’t provide good friction characteristics. So when the state raised the RAP percentages allowed in surface courses, it also required the virgin aggregates to consist of 60 percent granite. The granite is imported from Georgia or from Nova Scotia.

“We use a very large volume of RAP in Florida,” Musselman says. “Historically, it’s accepted practice in every layer except surface courses. We took a big step up to allow it in surface courses. Our average RAP use today is 23 percent.”

A few pavement failures with high RAP contents decades ago likely caused some agencies to take a very cautious stance on RAP usage. “Failures typically are not well investigated or documented and so we often don’t learn the real causes,” West says. “Some agencies want more performance data on higher-RAP mixes; others want to wait on more guidance on how to select the right grade of virgin binder.”

Today’s RAP technology is more advanced than when the failures occurred. “A couple of new technologies are coming into practice to make these higher percentages of RAP more successful,” West says. One is the fractionation of RAP, which separates the material into two or more stockpiles based on the size of the RAP particles. It’s becoming more common to separate RAP into a coarse pile, with particles of more than 1/2-inch or 3/8-inch in size, and a fine pile with particles of less than one-half of an inch. That lets contractors use fine RAP in finer asphalt mixtures and coarse RAP in coarse mixtures.

Mixing RAP and WMA for compactability

“Second is the combination of warm-mix asphalt technologies with high RAP contents,” 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 – but 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 Fahrenheit, not the 300 degrees Fahrenheit used for conventional hot-mix asphalt.)

When a conventional hot mix plant runs RAP, you have to push up the virgin aggregate temperature to 500 or 600 degrees Fahrenheit to heat the RAP that is being added. 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.

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, less than 9 million equivalent single axle loadings (ESALs) a 20-percent-RAP mixture using a PG 67-22 binder showed a rut depth of 8.6 millimeters. By contrast, a 45-percent-RAP mixture with PG 76-22 binder showed only 0.5 mm of rut depth after 9 million ESALs.

LoJac Enterprises Inc., of Tennessee, ran 50 percent RAP in 9,000 tons of a binder course and 30 percent RAP in 6,000 tons of surface course mixture for a demonstration project.