Better Roads Staff
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.
“The aggregate in RAP should be considered as if it were just another stockpile of virgin aggregate,” states the Washington State DOT in an online tutorial. “RAP aggregate properties, as with virgin aggregate properties, may limit the amount of RAP that can be used in a particular mixture.”
The residual binder in RAP must be taken into account when designing a mix, WashDOT says. “The effect of RAP asphalt binder must be considered when using RAP in Superpave mix design (or any mix design),” the tutorial states. “RAP asphalt binder will blend with virgin asphalt binder in most any mix design, and the resulting properties of this blended asphalt binder must be understood.”
The reason is that RAP asphalt binder already is significantly aged because of its previous field life, WashDOT says. “This aged binder is generally stiffer than virgin asphalt binder and thus will cause the resultant binder blend to become more viscous (stiffer),” the tutorial says. “This, in turn, will cause the HMA to be more viscous.”
Therefore, successful RAP mix designs incorporating RAP above 15 percent by weight should analyze the stiffness of the existing residual binder in the RAP, and compare it to the stiffness of the virgin liquid binder, along with the proportions of each in the final product.
In their 2010 Transportation Research Board paper, A Backcalculation Method to Determine “Effective” Asphalt Binder Properties of RAP Mixtures, Thomas Bennert, Ph.D., senior research engineer, Rutgers University’s Center for Advanced Infrastructure and Transportation (CAIT), and Raj Dongré, Ph.D., Dongré Laboratory Services, Fairfax, Va., observe that it’s important to understand the effect that RAP has on the final asphalt mixture performance.
“Current recommendations for the use of RAP in asphalt mixtures follow those developed under NCHRP Project 9-12, Incorporation of Reclaimed Asphalt Pavement in the Superpave System,” they write. These include: no change in binder selection necessary for RAP percentages less than 15 percent; select a virgin binder grade one grade softer than normal for RAP percentages between 15 and 25 percent; and follow recommendations from blending charts when RAP percentages are greater than 25 percent. The results were based on laboratory testing of asphalt mixtures containing approximately 5-percent asphalt binder and using non-fractioned RAP.
‘Tiered’ Approach to Higher RAP
Similarly, New Jersey has taken an incremental approach in raising allowable percentages of RAP in its mixes, with evaluation of the residual RAP binder required for doses higher than 15 percent.
In his 2010 presentation, Higher RAP Mixes in New Jersey: Changes in Mix Design and Production, CAIT’s Bennert describes the efforts that New Jersey DOT is undergoing to incorporate higher levels of RAP in mixes while maintaining pavement durability.
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