Better Roads

September 2014

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Page 9 of 35

RoadScience 8 September 2014 Better Roads 30 mesh and may contain 1 to 4 percent styrene-butadiene- styrene (SBS) polymer." Finally, there is the dry process as compared to the wet process. Crumb rubber recycled from waste tires in asphalt mixtures and pavement rehabilitation treatments can be achieved in two different ways, Bandini says. "Crumb or ground rubber can be used either as fine aggre- gate in the mixture [dry process], or as processed rubber added to the asphalt binder [wet process]," he says. "The dry process is any method that adds granulated or crumb rubber modifier from scrap tires as a substitute for a percentage of the aggre- gate in the asphalt concrete mixture, not as part of the asphalt binder. The crumb rubber is mixed with the aggregate fraction before adding the asphalt cement. The resulting product is of- ten called rubber-modified asphalt concrete mixture. Different gradations or sizes of granulated or CRM can be used." In this process, the asphalt cement is not modified signifi- cantly by the addition of the crumb rubber; however, the properties of the resulting HMA pavement are modified. The dry process can be used in dense-graded, open-graded and gap-graded mixtures to accommodate the rubber particles in the aggregate gradation, but cannot be used for cold mix, chip seals and surface treatments, Bandini says. The wet process is the method of modifying the asphalt binder with CRM from scrap tires before the binder is added to aggregate. "The resulting product is called asphalt rub- ber or rubberized asphalt," Bandini says. "The wet process requires thorough mixing of the CRM with the asphalt con- crete and other components of the modified asphalt binder at temperatures between 375 to 435 degrees Fahrenheit, and requires maintaining the blend at temperatures between 375 to 425 degrees Fahrenheit for a certain specified minimum time, generally 45 minutes." In addition to the performance benefits of RA or AR, the processes yield environmental benefits as they reduce the population of scrap tires. "Application of crumb rubber modified asphalt has been identified as one of the possible solutions to address the scrap tire issue while benefiting pave- ment industry," say Shahrzad Hosseinnezhad, Darius Holmes, and Ellie H. Fini, Ph.D., P.E., Department of Civil and Environ- mental Engineering, North Carolina A&T State University, in their 2014 Transportation Research Board paper, Decoupling the Physical Filler Effect and the Time Dependent Dissolution Effect of Crumb Rubber on Asphalt Matrix Rheology. "It is predicted that if just 10 percent of asphalt which is used during one year in the U.S. contained 3 percent rubber, approximately all scrap tire would be consumed for that year." Yet crumb rubber modified asphalt is not used in large volumes consistently from coast-to-coast, they observe. "Due to lack of in-depth understanding of the various interaction mechanisms between CR and asphalt binders, CRM asphalt is not widely used," they say. "Largely, such interactions depend on the physical and chemical properties of the asphalt binder and the CRM as well as the interaction environment including the rubber percentage, particle size and texture of the CRM as well as source of rubber and asphalt." Who uses asphalt rubber? Arizona has been the leader in using rubberized asphalt, but California, Florida, Texas, South Carolina, Nevada, Georgia, Pennsylvania and New Mexico are very active as well. The pen- etration of asphalt rubber into the 50 states was explored in a 2012 survey sponsored by the Rubberized Asphalt Foundation. That survey found that 70 percent of transportation agen- cies have previously used or currently use recycled tire rubber in asphalt. About half of the respondents indicate that they have a specification for using recycled tire rubber. The survey was conducted for RAF by the Highway Sustainability Re- search Center at the University of Massachusetts-Dartmouth. Agencies have utilized recycled tire rubber routinely for crack sealing (30 percent), chip seals (26 percent), dense- graded hot mix asphalts (15 percent), joint sealants (15 percent), stress absorbing membrane interlayers (SAMIs, 11 percent), and open-graded friction courses (OGFCs, 11 per- cent), the survey found. Other respondents reported the same uses for rubberized asphalt on an experimental basis. Another aspect of the survey notes the technologies being used by transportation agencies to incorporate recycled tire rubber into their pavements. These results show that termi- nal blending is the predominant method in use, chosen 59 percent of the time. Bandini also surveyed state DOTs to gauge use of the ma- terials in the nation. He sent out 51 surveys (including D.C.) and received 42, an 82 percent response. Of the 42 agencies who responded, about 55 percent have used, or currently use crumb rubber in one or more pavement application, and the remaining agencies (45 percent) have not used crumb rubber in flexible pavements (charts, page 7).

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