Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85930
cleaned and crushed into fine aggregate. Currently a national pooled-fund study involving multiple state DOTs – Performance of Recycled Asphalt Shingles (RAS) in Hot Mix Asphalt [TPF-5(213)] – is under way. Research in dif- ferent states includes mix performance comparisons, beam fatigue testing, dynamic modulus, flow number and binder property test results. "The use of reclaimed asphalt shingles (RAS) in asphalt paving mixtures is not a new concept," say NAPA's Kent Han- sen and Dave Newcomb, in NAPA Information Series No. 138: Asphalt Pavement Mix Production Survey. "The combination of a high asphalt binder content, high-quality fine aggregate, mineral filler, and fibers makes roofing shingles very compatible with asphalt pavement mixtures," they say. The fact that the asphalt cement in shingles is generally harder than that employed in paving mixtures, and that the other ingredients impact the volu- metric properties of the final mix, gen- erally limits its incorporation in asphalt mixtures to 5 percent or less, they add. "However, even at a relatively lower RAS content, there is somewhere on the order of 15- to 20-percent binder replace- ment in the final paving mixture," Hansen and Newcomb say. "Currently, 12 states allow the use of manufacturers' waste in asphalt mix and 10 states allow either manufacturers' waste or roofing tear-offs in their mixtures. It is estimated that there are 10 million tons of tear-off waste and 1 million tons of manufacturer waste available on an annual basis. If all these could be incorporated into asphalt paving mixtures, it would amount to approximately 1.8 million tons of asphalt binder replacement. Thus, there is great interest in utilizing waste asphalt roofing shingles in asphalt paving mixtures." Rubber as Performance Modifier Rubber from recycled tires is a common additive to asphalt on a regional basis, either as an asphalt modifier (wet pro- cess) – where it reacts with the liquid asphalt – or as a fine aggregate substitute (dry process). As a modifier, crumb rubber increases asphalt binder viscosity as it's blended in ranges of 18 to 25 percent rub- ber, reacting to produce an asphalt-rubber binder. Asphalt mixes in which ground rubber particles are added as fine aggregate are referred to as rubberized asphalt for open-graded mixes. The road to universal acceptance of granulated tire rubber (GTR) as an alternative to conventional polymer modifiers is long, and has been paved with doubts created by premature failures from technologies of the 1990s that were not well understood, or required too many chal- lenges to implement, says Doug Carlson, vice president of asphalt products for Liberty Tire Recycling. "Two more decades of research and development have dramatically changed the landscape by generating materials and process advancements that definitively position rub- berized asphalt as a viable alternative to polymer-modified asphalt in terms of performance and cost," Carlson says. Asphalt-rubber (A-R) is defined by the American Society for Testing and Materials (ASTM) Standard D6114 as "a blend of paving grade asphalt cement, ground recycled tire (that is, vulcanized) rubber and other additives, as needed, for use as binder in pavement construction. The rubber should be Crumb rubber increases asphalt binder viscosity as it's blended in ranges of 18 to 25 percent rubber, reacting to produce an asphalt-rubber binder blended and interacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles prior to use." Asphalt-rubber binder is field-blended (at a hot mix plant) – requiring mobile mixing equipment to produce – or as a terminal blend. The typical rubber content for asphalt rubber ranges from 18 to 22 percent. Granulated tire rubber used in asphalt rubber is in the 10-to-16 mesh range for maximum particle size. This binder is best suited for very thin overlays and heavy duty surface treatments to prevent cracking. "New technologies have emerged that allow GTR to be used as the primary modifier in performance-graded (PG) asphalt," Carlson says. "These binders are manufactured with 8 to 12 percent rubber content and may include a small amount of virgin polymer or other additives. The rubber particles have a 30-minus maximum size, but are small enough to fit into PG tests. They can be made onsite or delivered by an asphalt supplier. Mechanical or chemical suspension is needed for the binders that retain GTR par- ticles. These binders can directly replace polymer modified materials in dense graded mixes and chip seals." Rubber enables the use of more recycled asphalt pavement (RAP). Emerging technologies researched by Louay N. Mo- hammad and Samuel B. Cooper Jr. at the Department of Civil Engineering and Louisiana Transportation Research Center at Louisiana State University have shown that rubber mixes with up to 40 percent of RAP can perform as well as regular mixes with only 25 percent RAP, Carlson says. Better Roads September 2012 23