Terminal blended asphalt rubber technology has been used since the mid 1980s in many states. In the terminal blending of rubberized asphalt, tire rubber is blended into the asphalt binder at the asphalt terminal or refinery and shipped to the hot mix production plant as a finished product with no additional handling or processing. The tire rubber is completely digested into the asphalt to provide styrene, butadiene, carbon black and aromatic oils yielding a homogeneous material that exhibits excellent storage stability and compatibility with the finished binder formulation.
Terminal blended rubberized asphalt (TR) was first used in Texas in the mid-1980s. It was initially used in Texas and Louisiana in dense-graded hot mix. A modified, hot-applied chip seal was adapted in Texas about the same time. Not long after, TR was used in open- and gap-graded surface courses in Texas, Florida, Louisiana, New York, Arizona, California and Nevada.
Rubberized asphalt material manufactured at the refinery or terminal differs from the “wet process,” which is field-blended asphalt rubber produced at the job site.
Terminal blends are manufactured at the refinery or terminal like any other polymer modified asphalt (PMA). The asphalt is heated under a controlled environment in a tank to an elevated temperature. The crumb tire rubber is then introduced into the tank and digested into the asphalt. During this process, an operator takes samples and runs a solubility test to ensure that the rubber is completely digested. Typical solubility of the finished material is above 97.5 percent, which makes it a homogeneous material.
“The idea with terminal blends is to produce a material in which the tire rubber is fully digested into the asphalt without leaving visibly discrete particles,” says Don Goss, Manager of Product Technical Services for Valero Energy Corporation. “There are some proprietary and other non-proprietary processes used,” he says, “but most of the processes generally employ a high-shear blending process in order to fully digest the tire rubber into the asphalt binder, resulting in a smooth, homogeneous product.”
Storage of Terminal Blends
The storage of terminal blended rubberized asphalts is similar to other blended asphalts. They are storage-stable binders, as long as the tire rubber is fully digested into the binder. “Since the material is totally homogeneous, the rubber is totally incorporated and digested—the solubility is above 98 percent,” says Jean Azoury, Group Manager for Paramount Petroleum Corporation. He adds, “You can store it over the season and shut down the heat on the tanks. It is very stable and does not separate.”
Azoury says there is no special equipment required for shipment or paving with terminal blended asphalt. The material is delivered to the hot mix plant by truck, mixed and shipped to the job. And no special equipment is required for paving, or odor/fume control. The TR mix is compacted like regular hot mix asphalt.
Terminal blends can be used in all paving and maintenance applications requiring crumb rubber content. They can be used in dense-graded, open-graded and gap-graded mixes. They can be used in chip seal applications, emulsions, slurry seal applications and tack coat applications. Terminal blends can be used with rubber contents as low as 5 percent and as high as 25 percent, depending on the application and the project’s requirements.
Because of the smooth, homogeneous nature of terminal blends, they can be readily used in dense-graded mixes. The wet process or field-blended asphalt rubber is typically used in gap-graded and chip seal applications.
In California, PG-TR grades are specifically targeted for use in the same applications for which PG-PMA binders are used, including dense-graded mixes for thick structural sections. “The ability to use PG-TR binders in dense-graded mixes opens up a new and very large field of application for the use of terminal blended rubberized asphalt binder,” says Goss.
PG Graded Terminal Blends
Perhaps the biggest advantage of terminal blended asphalt is that it is a fully SHRP- graded material. “It can be PG graded and manufactured to the grade needed since the rubber is fully digested into the asphalt like a PMA,” says Azoury. Typical terminal blended grades are PG64-28TR, PG70-22TR, and PG76-22TR. These grades can have varying rubber content and be as high as 25 percent by weight. Caltrans, Nevada DOT, and the Pacific Coast Conference on Asphalt Specifications have adopted the PG terminal blends and have accepted them as an alternative to PMA materials. Recently, Nevada DOT did a six-mile job with PG64-28TR as a replacement for their PG64-28NV specification.
California Assembly Bill 338
California Assembly Bill 338 was signed into law by Governor Schwarzenegger in October 2005. The bill would have originally required increasing amounts of wet process asphalt rubber to be used in HMA—20 percent of total HMA tons in 2007, 25 percent of HMA tons in 2010 and 35 percent of HMA tons in 2013. Through the work of the Asphalt Pavement Association of California, the language of the bill was changed to broaden the types of rubberized asphalt materials that could qualify and be included under the law.
While keeping the quantities of tire rubber that must be used comparable to the wording in the original bill, the final version of the law states the requirement in terms of pounds of crumb rubber modifier per metric ton of total asphalt paving materials, so that any paving material that uses tire rubber may be included in the program.
Terminal blends can be used wherever conventional asphalt mixes or asphalt surface treatments are needed. Historically, these blends may provide better resistance to reflective cracking than standard dense-graded mixes.
The primary reason for using terminal blended asphalt is that it provides significantly improved engineering properties over conventional paving grade asphalt. TR also provides the environmental benefit of using waste tires.
|Successful TR Projects
In May 2000, Caltrans constructed a test section using terminal blended asphalt rubber on Highway 1 in Laguna Beach, California. The project was a mill and overlay with terminal blend HMA placed in thicknesses of 60 to 75 mm. The mix was a 12.5mm Type G, MB-4 binder mix. R.J. Noble Construction Company placed approximately 10,000 tons of asphalt mix on the job. Valero Energy Corporation of Wilmington, California, supplied the rubberized asphalt binder. Caltrans has surveyed the project twice since construction and concluded that the section is performing very well.
In 2007, the City of Santa Cruz, California, placed the biggest chip seal project in its history, requiring approximately 580 liquid tons of terminal blended rubberized asphalt. The particular binder was a PG70-22TR and needed no special equipment for placement and compaction. The TR binder was handled just like any other asphalt binder. The PG70-22TR was delivered to the job site at a temperature of 310 to 330°F. The aggregate used for the chip seal was 3/8-inch pre-coated stone with 0.25 to 0.50 percent of PG 64-10. The application rate of the asphalt binder was 0.32 to 0.35 gallons per square yard, and the aggregate spread rate was 23 lbs to 24 lbs per square yard. The week after the chip seal was applied, the whole project was sealed with Type II slurry seal, which is standard practice for Santa Cruz.
The PG70-22TR was supplied by Paramount Petroleum of Paramount, California. The pre-coated aggregate was supplied by Granite Rock Company. Graham Construction was the contractor. Santa Cruz city officials viewed the PG70-22TR as a cost effective alternative to similar products which met or exceeded the performance characteristics of other hot applied chip seals.