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Utilization of Recycled Materials in Illinois Highway Construction

Crumb Rubber

Shredding waste tires and removing steel debris found in steel-belted tires generates crumb rubber (CR). There are three mechanical methods used to shred apart these tires to CR: the crackermill, granulator, and micromill methods. CR can also be manufactured through the cryogenation method; this method involves fracturing the rubber after reducing the temperature with liquid nitrogen (1).
Physical Properties:
CR is fine rubber particles ranging in size from 0.075-mm to no more than 4.75-mm (1).
Engineering Value:
CR can be blended into bituminous concrete by either a wet or dry process. In the wet process, the CR acts as an asphalt modifier prior to the addition of aggregates; however, this process requires costly special equipment. In the dry process, CR constitutes a portion of the fine aggregate prior to the addition of the asphalt cement. In this process, limited equipment modification is necessary (1, 13, 14).
Potential Application:
During the early Nineties, the Department began efforts to use CR following a mandate which has since been lifted imposed by the 1991 Intermodal Surface Transportation Efficiency Act (ISTEA). To address the mandate, eleven experimental crumb rubber bituminous concrete projects were constructed and evaluated, which included ten dry processes and one wet process. For the dry process projects, the material was added into bituminous concrete at two rates: variable and fixed. The variable rate consisted of adding no more than five pounds of CR per ton of bituminous concrete, whereas the fixed rate consisted of adding at least twenty pounds CR per ton of bituminous concrete. Overall findings concluded that the fixed rate wet process method had shown fewer distresses than the control sections of conventional bituminous concrete. On the other hand, the dry processes compared poorly to conventional bituminous concrete. In addition to this, the fixed rate CR areas are currently displaying, on average, slightly higher smooth tire friction values than both the controlled or remaining test sections (13, 14).
Department Concern:
Even though the wet process is the only method that has provided an indication of higher performance, its final bid price was considerably higher (over 100 percent) than the average bid price of projects constructed with conventional bituminous concrete in that same year. Also, bituminous concrete suppliers in Illinois do not yet have the equipment required for the wet process. As a result of the higher cost and equipment requirements, implementation is not recommended (13).
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Updated: 04/07/2011

United States Department of Transportation - Federal Highway Administration