Better Roads

September 2014

Better Roads Digital Magazine

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Better Roads September 2014 7 Charts courtesy of Paola Bandini, New Mexico State University Among these innovations are use of recycled tire rubber as a modifier in PG asphalt binders, sustainable polymer modi- fication, quiet pavement designs, warm-mix technologies in combination with rubberized asphalt, binder stabilizers (fiber replacement) to prevent drain down in permeable, porous and open graded mixes, use of recycled tire rubber with reclaimed asphalt pavement and shingles, and reduced thickness designs for highly modified asphalt rubber mixes. Rubberized Asphalt Foundation chairman George Way, P.E., and Rubber Pavements Association executive director Mark Belshe, P.E., are providing input to FHWA towards the new document. What is asphalt rubber? Asphalt pavements utilizing rubber use it in different configu- rations or forms. Asphalt rubber (AR) binder consists of a blend of asphalt cement, ground recycled tire rubber (crumb rubber), and oth- er additives such as extender oil, natural rubber and polymers, as needed, says Paola Bandini, Ph.D., P.E., New Mexico State University, in his 2011 paper, Rubberized Asphalt Concrete Pavements in New Mexico. "The rubber content should be at least 15 percent (by weight) of the total blend to provide acceptable properties of the material, according to the Standard Specification for Asphalt- Rubber Binder (ASTM D6114/D 6114M–09). Higher rubber contents, between 18 and 22 percent by weight, are often used or specified." Bandini says the ground rubber should be blended suffi- ciently in the hot asphalt cement (347 degrees Fahrenheit) to cause swelling of the rubber particles and a considerable in- crease of the viscosity, which is strongly affected by the crumb rubber content and particle sizes. Because the AR binder is mixed and blended at the job site, it is also called field-blend asphalt-rubber binder based on its manufacturing process. Rubberized asphalt (RA) binder is also called terminal- blend or field-blend rubberized asphalt modified binder, depending on the manufacturing process, or just rubberized asphalt, Bandini says. "In practice, rubberized asphalt is mostly terminal blended [blended by the liquid asphalt supplier] and consists of asphalt cement with crumb rubber modifier or CRM binder (less than 15 percent by weight)," he says. "Rub- berized asphalt binder is often referred to as PG 76-22TR (tire rubber) or PG 76-22PM (polymer-modified) binder because these are the only RA binder types currently approved for the specifications of Caltrans." Historically, rubberized asphalt has contained up to 10 per- cent of CRM, thus it does not meet the requirements of ASTM for asphalt rubber. "However, in recent years, greater rubber contents have been used in RA binders in some projects," he says. "The RA binder is generally made with CRM smaller than Reasons for not using crumb rubber in pavement applications by state DOTs, 42 of 51 agencies responding in 2011 Reasons for using crumb rubber in pavement applications by state DOTs, 42 of 51 agencies responding in 2011

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