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Recycled Tire Rubber and Asphalt Pavement
Recycled tire rubber (RTR) from waste tires has been used in asphalt by the paving industry since the 1960’s. The rubber has been used as asphalt binder modifier and asphalt mixture additive in gap-graded and open-graded asphalt mixtures and surface treatments. The routine use of RTR in pavements has been limited to a few states. While performance is generally good, RTR cost has been high when compared to conventional practices. Local, State, and Federal regulations have also created an increase in the availability of RTR. This has driven a renewed interest in RTR as an asphalt binder modifier and mixture additive – with the goal of providing a long-life, cost competitive, environmentally-responsible pavement system.
In 1991, Section §1038(d) of the ISTEA required states to use a minimum amount of crumb rubber from recycled tires in asphalt surfacing placed each year beginning with the 1994 paving season. Although the mandate was lifted in 1995, a significant number of RTR asphalt sections were placed and national research was fostered. Many States discontinued use of RTR after the mandate was lifted. However Agencies such as Florida, Texas, and Rhode Island continued their use of RTR. In 2005, the State of California Public Resource Code Section §42700-42703 legislated the use of RTR.
The application of RTR modified asphalt binder has evolved with the development of terminal blended AR binders. This development was driven to reduce the need for asphalt mixture production plant modification (needed to incorporate RTR) and to address some performance concerns. A few RTR pavement failures had been linked to poor quality control with field blending practices.
In the Unites States, the predominate use of RTR asphalt pavements has been in warm climates. This has led some to believe that RTR modified materials will not perform well in cold climates. There have been issues with compaction and raveling of mixes in cold climates, but this has typically been a construction issue with unfamiliarity when working with high viscosity binders and trying to pave in cooler climates. In recent years RTR has been in cold climates. One significant property for pavement performance is achieving sufficient compaction on the roadway. Slightly higher binder contents in the RTR modified mixtures may help to achieve sufficient compaction. WMA technologies combined with RTR modified AR mixtures may help reduce production temperatures and also improve workability and compaction. This also could potentially reduce the exposure of workers to fumes that would otherwise be produced in greater concentration with higher mixture temperatures.
