Better Roads Staff
The Second Time Around
By Tom Kuennen, Contributing Editor
There is no stopping the growth of recycled and reclaimed materials in pavements.
The use of reclaimed asphalt pavement (RAP), reclaimed asphalt shingles (RAS), recycled concrete aggregate (RCA), and recycled granulated tire rubber (GTR) in pavement mixes and structures is growing dramatically as states accept them more and more in their specs.
But because RAP, RCA, RAS and GTR come from a variety of sources, they must be physio-chemically characterized prior to use in mixes.
• For RAP, which virgin aggregates does it contain? Do deleterious materials exist? How much residual asphalt remains after years of exposure to the elements and oxidation? How much liquid binder will the residual asphalt replace when reused in fresh asphalt mixes?
• For RCA, what is the extent and composition of the mortar or residual cement/sand blend? Were its virgin aggregates prone to alkali-silica reactivity (ASR) or is ASR present and in what degree? Are other deleterious materials present? Is the resulting RCA “good” enough to be used as aggregate in fresh asphalt or portland cement mixes, or is it going to be destined for road base, a much more common use?
• For RAS, the processed post-consumer (“tear-off”) shingle feed will come from a supplier that certifies the material meets state specs. The supplier will have sorted, ground and tested the RAS to make sure it does not contain asbestos, wood scraps or metal and is kept separate from pre-consumer (manufacturer waste) shingles (more on this below). Likewise, GTR will come from a supplier that maintains consistency.
Thus, physio-chemical analysis of beneficiated RAP, RCA, RAS and GTR by in-plant or supplier labs is essential for their continued usage. Because their source composition varies tremendously, these reclaimed materials must be chemically characterized and cataloged; then, blended stockpiles may be managed over time with more or less material added to maintain consistency.
Use of RAP and RCA as road base or fill is a less-critical application so a detailed analysis is not essential; here the research emphasis is on the possibility of leached pollutants finding their way into ground water, and long-term performance.
Processing Adds Value to RAP
Ideally the raw, stockpiled RAP or RCA will have been crushed and screened, or “beneficiated,” or screened or “fractionated” into homogenous stockpiles. While this costs the mix producer or contractor additional money, it adds value to the raw materials as they now are consistently sized.
Fractionation is the act of processing and separating raw RAP into at least two sizes, typically a coarse fraction (plus-1/2 or plus-3/8 inch) and a fine fraction (minus-1/2 or minus-3/8 inch), reports the Federal Highway Administration (FHWA) in its April 2011 publication, Reclaimed Asphalt Pavement in Asphalt Mixtures: State-of-the-Practice, by Audrey Copeland, formerly materials research engineer at FHWA, now vice president, engineering, research and technology at the National Asphalt Pavement Association.
“States allow higher amounts of RAP if it has been fractionated,” Copeland writes. “For example, in the Texas specification, unfractionated RAP is limited to 10, 20, and 30 percent by surface, intermediate and base layers, respectively. However, by special provision, fractionated RAP is allowed at up to 20, 30 and 40 percent in those same layers.”
Separately, RAP has to be chemically analyzed or characterized to determine its properties (below). That beneficiation or fractionation of RAP that’s been chemically characterized can permit significantly higher levels of RAP in Superpave mixes is borne out in a paper from the 2012 Transportation Research Board meeting, Fractionation of High Recycled Asphalt Pavement Content in Asphalt Mixtures for Superpave Mix Design Compliance, by Cory Shannon, E.I.T.; Yongjoo Kim, Ph. D.; Thomas Glueckert and Hosin “David” Lee, Ph.D., P.E., Department of Civil and Environmental Engineering University of Iowa-Iowa City.
“Due to the increased amount of fines created during the milling process and the corresponding increased surface area, high RAP content mixes have great difficulty in meeting the volumetric requirements of the Iowa DOT,” they write. “The fractionation method for this study focused on physical removal of RAP material below a certain sieve size to limit fine aggregate contribution.”
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