Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/206514
Photo courtesy of Tom Kuennen and Randy West RoadScience Raw reclaimed asphalt pavement (RAP) awaits fractionation to clean, screened sizes Fractionating RAP for mix design One way to boost use and consistency of RAP for mixes is for hot-mix asphalt (HMA) producers to maintain sheltered, blended RAP stockpiles and, if needed, reprocess or fractionate RAP into individual gradations. Sheltered stockpiles are favored because RAP absorbs water more easily than virgin aggregates. "Fractionation of RAP is a huge advantage for mix design," said National Center for Asphalt Technology (NCAT) director Randy West, P.E., in a 2009 presentation to the Minnesota Department of Transportation (DOT). "It allows RAP to be used in practically any mix type." Fractionation of RAP describes a concept in asphalt recycling that falls back on conventional best practices for virgin aggregate processing. With fractionation, RAP is screened, with oversize broken into smaller fractions and stockpiled separately. Fractionated RAP results in more uniform gradations, in which RAP fractions can be isolated, compared to general stockpiles in which large and smaller fractions are jumbled. Fractionation is the processing and separation of raw RAP into at least two sizes, typically a coarse fraction (+1/2 or +3/8 inch) and a fine fraction (-1/2 or -3/8 inch). Many states allow higher amounts of RAP if it has been fractionated, reports Audrey Copeland, National Asphalt Pavement Association (NAPA) director of engineering. For example, in the Texas specification, she says, 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. One problem with RAP is its composition varies because it's 18 November 2013 Better Roads 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. Mobile impact crushers are ideal for producing large volumes of screened or fractionated RAP or RCA on a relatively small footprint in the plant. Impactors are best suited for RAP and RCA because they break up chunks of asphalt [or concrete] pavement or agglomerations of RAP, rather than downsize the aggregate gradation. Compression-type crushers such as jaws and cones tend to clog due to packing (caking) of RAP when the RAP is warm or wet, according to NCAT. Both impactors and jaws are suitable for RCA processing. Recycled concrete aggregate also must be crushed, screened, tested and stored in blended stockpiles to ensure consistency. Contaminants may be encountered during processing demolition concrete, such as soil and organic detritus, and reinforcing steel, dowel and tie bars. Here, crushers used with prescreens or grizzlies benefit because dirt and fines are removed before they ever enter the crushing circuit, preventing equipment wear, saving fuel and possibly creating a salable fill byproduct. High-performance rebar magnets should have a maximum discharge capacity, with the ability to be hydraulically lifted or lowered to accommodate larger sizes of rebar if present in the latest load of demolition concrete. Although most RAP or RCA processing equipment is