Better Roads Staff
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.”
Current Iowa DOT specifications limit contractors to a maximum of 30-percent virgin asphalt binder replacement by RAP materials in the surface course for any state-regulated project, the authors write. “The main objective of this study is to develop quality standards for the inclusion of RAP contents higher than 30 percent in asphalt mixtures,” they write. “First a sieve analysis was performed on the recovered aggregate materials from ignition oven burn-off testing to determine the aggregate and asphalt binder composition of the RAP materials. To remove excessive fine materials a fractionated RAP stockpile was produced by removing RAP materials passing the No. 30 (0.60 mm) sieve.”
The Superpave mix design was then performed with RAP inclusion levels of 30, 40 and 50 percent, based on virgin asphalt binder replacement for RAP materials randomly selected from the bulk stockpile (traditional RAP) and RAP materials from the fractionated stockpile consisting of materials retained at No. 30 sieve and larger (fractionated RAP).
The fractionated RAP materials produced a lower surface area requiring lower virgin asphalt content, they say, resulting in the increased asphalt film thickness on aggregates. The fractionated RAP mixtures also exhibited higher indirect tensile strength than traditional RAP mixtures for all levels of inclusion.
“It was determined that the use of the fractionation method to remove fine recovered aggregates contributed by the RAP materials is an effective method for improving mix design criteria compliance while also reducing the requirement of virgin asphalt for asphalt surface mixtures with a high RAP inclusion level up to 50 percent,” they conclude.
Determining AC Content
Lab testing requirements and testing frequency for binder (AC) content vary according to the category of RAP and the amount of RAP used in a mixture, Copeland writes in Reclaimed Asphalt Pavement in Asphalt Mixtures (download a copy by Googling “FHWA-HRT-11-021”).
RAP from multiple sources may be subject to more rigorous testing than RAP from a single source, she writes. For all RAP stockpiles, the asphalt binder content and aggregate gradation must be determined. The asphalt binder content may be determined according to AASHTO T308 or AASHTO T164.
The most common method for determining the AC in a sample of RAP is to use the ignition oven method specified in AASHTO T308. A Colorado DOT survey compiled in January 2008 includes responses from 29 state DOTs, and shows that almost half of them used the ignition oven to determine the AC of the RAP fraction for mix design purposes. About 30 percent of the respondents used solvent or chemical extraction, while three out of the 29 states used both solvent extraction and the ignition oven, reported FHWA in Reclaimed Asphalt Pavement in Asphalt Mixtures.
The oven can predict future performance of RAP mixes as well. Use of a lab oven for long-term aging or oxidation of various-content RAP mixes found that as RAP content increased, HMA mixes would stiffen at a slower rate than virgin mixes say Sean Tarbox, and Jo Sias Daniel, Department of Civil Engineering, University of New Hampshire, in their 2012 TRB paper, Effects of Long-Term Oven Aging on RAP Mixtures.
“Asphalt concrete mixtures undergo aging while in place during their service lives,” the authors state. “The aging process stiffens the asphalt, changing its mechanical properties and resulting performance under traffic loading. The major factor contributing to the increase in stiffness of asphalt concrete mixtures over time is the oxidation of the asphalt binder at the molecular level.”
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