Florida Updates Rubber Binder Specification
A single update in its binder specifications is allowing the Florida Department of Transportation (FDOT) to use the same amount of ground tire rubber – if not more – as it has in the past. The July 2013 specification is geared more toward performance-based specifications, FDOT Bituminous Engineering Specialist Tanya Nash said in an interview with the Maryland-based Rubberized Asphalt Foundation (RAF).
The previous specification for FDOT rubber binder (ARB-5 and ARB-12) was a basic “dump and stir” methodology, Nash said. There was no real performance indication of either binder. “The only property that was monitored was viscosity and that was not really telling us much,” she said.
Over time, Nash said, polymer modified (SBS) binders have become the “gold standard” of performance binders for the department, typically a PG 76-22. Therefore, the idea of being able to create a binder that could compete in performance – but using ground tire rubber – became the new goal for the FDOT. “The idea is to use the ground tire rubber and get a performance binder out of it at the same time,” Nash said. The option to polymer modify the PG 76-22 (ARB) has also been added. This allows the polymer and the ground tire rubber to work together to create a performance binder. The number one complaint the industry has about using rubber binder is the issue of settlement in the storage tanks at the plant. With this new spec, the department now has a separation requirement for PG 76-22 (ARB) to help alleviate this problem.
The FDOT recently placed four test sections using the new PG 76-22 (ARB):
- US 19 – Hernando County (450 tons)
- US 19/US27 – Jefferson County (500 tons)
- SR 20 – Leon County (800 tons)
- SR 704 – Palm Beach County (450 tons)
FDOT set up monitoring and binder sample testing procedures for the projects with some specific goals in mind, Nash said. On the testing side, the binder was sampled at the time the transport was loaded for each test section.
This sample was split between the supplier and the FDOT for testing based on the requirements in the new July 2013 specification. The binder was sampled again sometime during the project. For the first three projects, the plant opted to pump the binder directly from the transport. For the fourth project, the plant had pumped the binder into their storage tank. The binder sample taken during the project is to check for possible cross-contamination and the effects on the binder properties, Nash said.
“During construction, we are looking for constructability issues – pumpability through the plant, workability of the mix, compaction (effort and issues), and temperatures. All the typical construction practices were monitored for possible differences in handling and indicators of performance,” Nash said.
Going forward, Nash said, it is difficult to predict how many projects will use the new binder. For existing projects, the contractor will have the option to change from ARB-5 or ARB-12 to the new PG 76-22 (ARB) at no cost to the department. It’s hard to say if it would be cost efficient to the contractor to do so, but the intent is for projects from July 1, 2013 and on for PG 76-22 (ARB) to replace the use of ARB-5 and ARB-12. According to FDOT, the PG 76-22 (ARB) and PG 76-22 (PMA) will not be freely interchangeable at this time. Ultimately, the department is heading in a market driven direction and will just specify a PG 76-22 modified binder, with the process of modification up to the contractor.
The FDOT produces approximately 5 million tons of hot mix asphalt a year. As a rough approximation, about 60 percent of the 5 million tons uses modified binder, SBS or GTR. Of that 60 percent, about 15 percent of the tonnage is GTR.
© Scrap Tire News, September 2013