The Rubber Pavements Association, a non-profit trade association dedicated to the greater use of recycled tire rubber in asphalt pavements, recognized three state transportation agencies for their use of Recycled Tire Rubber (RTR) in asphalt at their annual meeting in March in conjunction with the World of Asphalt in San Antonio, Texas.
Pennsylvania DOT was recognized as the 2012 Outstanding State DOT Program for its pilot project using an asphalt rubber gap graded friction course on I-78. The project was placed in October 2012 in Berks County from mile marker 11 to 16.
The purpose of the pilot is to study the benefits of asphalt rubber gap graded mixtures and also included the use of a warm mix additive to provide for lower mixing temperatures, workability and compaction.
The binder used in the mix is asphalt-rubber as defined by ASTM D6114 using a performance grade (PG) 64-22 base asphalt with over 15% RTR content that was manufactured on site using blending equipment provided by All States Materials Group. The contractor earned two bonuses on the job – a Ride Quality Incentive and Joint Density. The finished International Roughness Index was 52.64, much smoother than the average IRI on interstates in PA at 66 inches per mile as reported in 2006.
The project used over 730,000 pounds of RTR, which represents the beneficial reuse of over 60,800 end-of-life tires, or about half of the annual discards from Allentown, population 119,000. The project was featured in the Pennsylvania Asphalt Pavement Association’s 53rd Annual Conference held in Hershey, PA on December 11-13, 2012.
The South Carolina DOT was recognized for innovative use of RTR as a replacement modifier to SBS to provide a PG 76-22 binder in asphalt overlays on I-77 and I-20. A shortage of SBS in 2008 provided an incentive for DOT to explore the use of alternative asphalt modifiers.
The binder in the I-77 Chester and Fairfield counties project and I-20 Kershaw County projects was supplied from an asphalt terminal and used an additive in combination with the RTR to help provide for workability, heat retention and compaction.
A significant contribution of the RTR, besides replacing the conventional virgin polymer, was that the rubber particles provided a greater film thickness on the aggregate to negate the need for using fibers to prevent drain down on the open graded mix. The SCDOT also placed a “trackless tack” coat during the course of the project to evaluate if improvements in raveling resistance could be obtained in conventional open graded friction course mixes. The projects were featured in the SEAUPG Annual Meeting in November 2012 and the SCAPA Winter Conference held in Columbia, SC on January 31, 2013.
The Louisiana Department of Transportation and Development was recognized for its leadership role with the use of RTR as a modifier to meet modified performance grades in their pavements since 2008.
In conjunction with the many years of positive research by the LTRC, the DOTD has provided the first PG rubber modified standard specification. Louisiana has been researching rubber since the 1990s and has verified some improved performance characteristics, but widespread use did not occur due to the higher initial cost. However, during 2008, when SBS went into shortage and the price of asphalt remained elevated well above the cost of rubber, the LTRC research on the use of waste tires paid off and the use of crumb rubber as an alternative modifier to SBS in PG binder became a standard practice. It remains in use today because of the lowered cost.
Mix producers are able to manufacture the PG 82-22 RM binders on-site at the plant, and a QC program allows the PG binders to be verified. According to Jay Winford of Prairie Construction at the “Sustainable Materials for Pavement Infrastructure: Use of Waste Tires in Asphalt Mixtures” conference held in Baton Rouge in September 2012 the advantages he has seen with the rubber modified mixes were:
- Uses 1 tire per ton of hot-mix produced
- Good for environment
- Open graded mixes: no fibers
- Can be done at plant or pre-mixed
- Keeps tires out of landfills/bayous
- Makes for quiet and blacker pavements
- Cheaper than polymer
Congratulations to these leading state transportation agencies for capturing the value of recycled tire rubber in their roadways.