Better Roads Staff
“The use of RAP in recycled asphalt pavement is well accepted practice by many federal, state and local agencies,” the road map says. “In many areas, almost all hot-mix asphalt (HMA) contains at least some RAP. However, with a few exceptions, the amount of RAP that can be added in hot plant mix asphalt mixtures is limited to relatively low percentages and in some areas the use of RAP is prohibited in certain types of mixtures, such as surface courses. Typically, the maximum percentage of RAP allowed is anywhere from 15 to 30 percent by weight of HMA mixture.”
The road map anticipates considerably higher percentages being implemented. “Laboratory and field studies have been performed on HMA with much higher percentages of RAP,” the road map says. “These investigations have concluded that HMA materials with percentages in excess of 50 percent can be produced to perform the same as ‘virgin’ mixes. It has been well established that agencies that are not currently allowing RAP into their HMA mixtures and those that are only allowing small percentages of RAP can safely increase the amount of RAP used without fear of shortening pavement life, provided that best practices are followed. . . . [T]he state-of-the-practice relative to the mix design procedures using high RAP content mixes needs to be established.”
To this end, last year, the leaders of trade associations that represent 150 million tons a year of asphalt recycling signed a cooperative agreement aimed at doubling the rate of reuse/recycling of asphalt pavements within five years.
Principal signatories to the agreement were NAPA and the Asphalt Recycling & Reclaiming Association (ARRA). Letters of support were provided by the FHWA and EPA. Under the agreement, NAPA and ARRA pledge to support each other’s efforts to deal with common challenges and build on each other’s strengths regarding asphalt recycling issues.
“Asphalt pavement is America’s most recycled material,” says NAPA’s Acott. “There are more than 18 billion tons of asphalt pavement already in place on the roads, streets and highways of this country. These same roads that Americans use every day are also a resource that future generations can use. Our goal is to increase the rate of recycling even further.”
“Reclaiming and recycling asphalt roads brings America the best possible pavements while conserving precious natural resources,” says Mike Krissoff, executive director of ARRA. “The members of both ARRA and NAPA are proud of the industry’s long track record of delivering quality and value.”
Efforts to boost RAP usage are restrained by the fact that larger-than-conventional doses of RAP – without binder adaptation – can complicate long-term mix performance.
One “rap” against RAP is that its composition varies because it’s sourced from a wide variety of locations. Therefore, advance knowledge of the composition of the residual binder in RAP – along with the separate stockpiling of different varieties, or blending of varieties to create a consistent product – is necessary for creation of reliable mixes.
One way to boost use and consistency of RAP in asphalt mixes is for producers to maintain sheltered, blended RAP stockpiles and, if needed, to reprocess, or fractionate, the RAP into individual gradations. Sheltered stockpiles are favored because RAP doesn’t shed water as easily as virgin aggregate.
The processing, or “fractionation,” of RAP replicates conventional best practices for virgin aggregate processing. With fractionation, RAP is screened, with oversize broken into smaller fractions and each stockpiled separately. Fractionated RAP may result in more uniform mixes, in which RAP fractions can be isolated, in contrast to general stockpiles in which large and smaller fractions may become segregated.
Long ago it was established that RAP was not just a “black rock,” but that its residual asphalt – oxidized and brittle as it is, and in varying amounts – still provides a bituminous portion to the overall mix design, permitting addition of a lesser amount of expensive liquid asphalt binder. The Asphalt Institute (AI)stated components of RAP have value, particularly true of asphalt binder, and the residual asphalt can reduce the amount of new asphalt binder in a mixture. A mix with 20-percent RAP with 5-percent asphalt content can result in a 1-percent savings in new asphalt binder, AI says.
RAP = Stiffer Mixture
Excessive amounts of RAP in the mixture can have substantial effects on pavement performance. Use of 15-percent or more RAP can result in a significant increase in stiffness of the mixture, which can enhance durability. But the use of RAP in hot-mix asphalt also can negatively affect low temperature cracking characteristics of the pavement.
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