Blending crumb rubber made from scrap tires into asphalt formulations to pave roads is a winning strategy – one being adopted by more and more state departments of transportation (DOTs).
Rubber modified asphalt (RMA) is a closed-loop recycle process that takes scrap tires off the road, keeps them out of landfills, and makes for lower cost, better quality roads.
Earlier this year, Florida’s DOT (FDOT) became the latest state to create new specifications that ease the way for more paving using rubberized asphalt, thereby further opening competition with conventional polymer-asphalt formulations.
Asphalt, also called bitumen, is a sticky, black and highly viscous liquid or semi-solid form of petroleum. It’s found in natural deposits or may be refined as a product. 70 percent of all asphalt/bitumen is used in road construction as a binder mixed with aggregate to create asphalt concrete.
Currently, states have either standard specification for blending in rubberized asphalt, or special provisions. Twelve states have standard specs where rubberized asphalt has been used for many years and is readily accepted. In addition, about half the states have some type of special provision in their spec books to allow its use.
There are three basic ways of putting recycled rubber into asphalt paving:
1. The asphalt-rubber (AR) binder first developed in the 1960s has an American Society for Testing and Materials, specification ASTM D6114, is defined as “a blend of paving grade asphalt cement, ground recycled tire (vulcanized) rubber and other additives, as needed, for use as binder in pavement construction. The rubber shall be blended and interacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles prior to use.”
This binder is heavily modified and has about 20 percent rubber with rubber particles 2 mm and smaller in size. It is comparable to an asphalt modified with 7 percent polymer and can be used in reduced thickness compared to regular asphalt. This binder is widely used in Arizona, California and Texas, and being used more frequently in the Northeast U.S. and in Canada.
Cost savings can be achieved with AR mixes through reduced thickness designs that have been validated in tests in several states, and in practice in California. (California has been using AR successfully for over 30 years). Generally, a two inch AR Gap Graded Mix has the same structural value as a four inch unmodified dense graded mix. The AR friction courses can be placed in 12 mm lifts on asphalt and 25 mm lifts on concrete and can last 7 to 15 years without maintenance depending upon preexisting conditions. In some cases, total asphalt cost can be reduced by 50 percent with the reduced thickness.
2. Then there’s the Polymer Switch, Performance Graded (PG) binder where rubber is 8 to 12 percent with a 0.5 mm particle size. It can be used in any type of asphalt mix. This binder is being routinely used in Florida, Louisiana, Missouri, and recently in Arizona and Texas in the place of polymer modified asphalt. Louisiana was the first state to allow the use of fine grind rubber in the place of SBS (styrene-butadiene-styrene) polymer at the bidder’s option. This binder can provide a 15 percent reduction in modified asphalt costs since recycled tire rubber has a much lower cost. Louisiana’s DOT has seen a $2 to $5 dollar savings per mix ton when rubber is used instead of polymer.
3. Finally, there’s a mix additive, often called Plant Mix. This crumb rubber is also small, 0.5 mm and smaller and can be used in almost all asphalt mixes. Georgia’s DOT currently allows the use of Plant Mix rubber in certain projects. This mix provides a lower cost than a polymer modified mix since it is made on demand using a simple metered hopper.
“The State of Florida has had a long-term commitment to the use of recycled materials, such as recycled tires,” said Jim Musselman, state bituminous materials engineer for FDOT. “Tire rubber is routinely used as a modifier for asphalt binder in pavements. In controlled amounts, this product helps to provide resistance to rutting and cracking. Under the right conditions, rubberized asphalt can minimize the impact on limited resources and help to achieve Florida’s goal of longer lasting roads and bridges.”
Musselman reported that the new PG 76-22 specification of 500-ton test sections were placed on 4 FDOT construction projects at various locations around the state. In addition, test sections were also placed at the National Center for Asphalt Technology test track and at FDOT’s Accelerated Pavement Test Facility. “Constructability and short-term performance was good. While the evaluations at both locations are still on-going, performance to date has been positive. It is our hope that overall performance of the material will improve and its usage will increase.” Musselman added.
While there are a number of ongoing projects in Florida that are using the new specification, only six of them have actually started placing the asphalt layers that include asphalt rubber. According to Musselman, no significant problems have been encountered.
“The update to Florida specifications requires storage stability so that the rubber will not separate from the asphalt,” said Doug Carlson, vice president of asphalt products at Liberty Tire Recycling.
Liberty, the largest provider of tire recycling in the U.S. reclaims more than 33 percent of the nation’s discarded tires and annually converts more than 140 million tires into raw material for sustainable products. The company operates two scrap tire recycling facilities in Florida.
“Florida’s old rubberized asphalt specification did not have any equipment requirements for the manufacturing and storage of the binder. There had been some problems with separation and it was non-homogenous when used. The rubber went to the bottom of the tank and all of asphalt would be at the top of the tank. That was because there were no equipment requirements for an agitation system during
storage,” said Carlson.
According to Carlson, the Florida specification is now a performance graded standard. It allows rubberized asphalt to be used in any type of mix, but is designed for use in mixes for heavy traffic and high pavement surface temperatures.
“There are benefits to the paving contractor,” Carlson said. “The new specification requires a suspension agent to keep the rubber suspended in the liquid so the advantage to the contractor is that their normal polymer tanks can easily be used to handle rubberized asphalt. This can be a significant cost saving since rubberized asphalt generally costs from 5 to 15 percent less than polymer asphalt. Rubber costs less than the asphalt liquid and if asphalt prices stay high, it could make sense to put rubber into regular asphalt, even if a modifier is not needed. The market will decide.”
The change in the Florida specification also allows the use of cryogenically produced crumb rubber. Cryogenic crumb rubber refers to the grinding of scrap rubber at temperatures near minus 80°C using liquid nitrogen or commercial refrigerants, whereas ambient or mechanical grinding rubber reduction happens at or above normal room temperature.
Ryan L. Alleman, sales director for coatings, asphalt and construction materials at Lehigh Technologies in Tucker, Georgia, said, “Lehigh Technologies is the world’s largest producer of micronized rubber powder (MRP) and ground tire rubber (GTR); however, we are not a tire recycler. We purchase scrap rubber in the form of chips from recyclers throughout the U.S. and further reduce it to finer powders through our patented manufacturing process.
“We rely on hard technical data to help develop the specialty chemical industries we serve, through research completed either at our applications and development center in Tucker or at industry partner labs. One of our recent studies was a joint developmental project completed at the National Center for Asphalt Technology. It assessed nearly a dozen rubber powder products that differentiated in grinding technique and particle size in both a liquid binder and in mix designs. The conclusion affirmed that particle size and surface area have a significant impact on the performance of a rubber modified asphalt binder while grinding technique proved to be insignificant. This has helped shed light on how the various properties of GTR and MRP affect an asphalt binder and has supported multiple states in the updating of their GTR specifications to allow the use of cryogenic rubber, Florida being one of them,” he stated.
Florida is a pioneering state in RMA and related product specifications and many states that are now pulling together their program look to the rubber powder experiences of states like Florida to support their program’s development. This is why we felt it was important to pull together industry and state leaders to create a study relevant to today’s activities,” concluded Alleman.
Tire manufacturers are also supporting the use of RMA. Bridgestone Corporation, the world’s largest tire-rubber company, is an active proponent.
John Sheerin, environmental director of retail operations for Bridgestone America was asked about his company’s support of rubberized asphalt and the new Florida specifications.
“Bridgestone has an initiative called Tires4ward, where we envision a waste-free tire industry. We want to make sure that for every new tire we sell in the U.S. a spent tire goes to a valuable purpose. One of the best is rubberized asphalt. This is a technology that’s been slowly developing over many years and we think that it’s a very favorable time to emphasize rubberized asphalt. Not only does the material last longer on the roads and have lower maintenance costs, but the economics are now becoming favorable on the original install costs as well. It’s getting to the point where the question is: Why wouldn’t you use rubberized asphalt?
“The new Florida specification is a significant development. Road building is very particular and Florida’s new specifications should enable road construction engineers and contractors to speak the same language. For Florida to take this step and lay it out clearly for all the engineers makes it simple and standardized for the entire state and should improve the volumes of rubberized asphalt used. That will assure that the RMA being laid down meets the high quality standards necessary to make sure that our roads are better.”
Last year Bridgestone held conferences in Tennessee and Ohio for the purpose of promoting the greater use of asphalt rubber. “The engineers and regulators from the DOTs who attended those conferences were very favorably impressed. The indications we received from those seminars was that the use of the material would continue to increase,” Sheerin said.
It appears that increased state interest in rubberized asphalt is not only a great recycling story, but also one that saves tax payer dollars while paving the way for more durable roads.
Published in the June 2014 Edition of American Recycler News