Launch Slideshow

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Technology Transfer

Technology Transfer

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    Engineers and chemists began exploring ways to add rubber to asphalt in the 1920s and came up with several processes to produce what's known generically as rubberized asphalt. In the “wet process,” crumb rubber (shown) is blended at high temperatures with liquid asphalt and other binders. “Asphalt rubber” refers to a specific material containing at least 15% ground rubber as defined in ASTM D6114-97 (2002). Photos: Rubber Pavements Association

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    Mix temperatures for asphalt rubber are slightly higher than conventional hot mixes, and workability is limited. The material should be at least 350° F for placement and fully finished before the temperature gets below 275° F. Such requirements mean that steel-wheeled rollers must be used for compaction.

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    In 2002, Arizona DOT placed 1 inch of asphalt rubber on a stretch of concrete pavement that carries 150,000 vehicles/ day. The resulting reduction in traffic noise—averaging 3 to 5 decibels—was so striking that the public clamored for more. By the end of that year, the department announced it would install the overlay on all 115 miles of the state's freeways.

SO WHY ISN'T EVERYONE USING IT?

Asphalt rubber first became a hot topic when the Intermodal Surface Transportation Act of 1991 (ISTEA) proposed to require its widespread use.

At the time, asphalt rubber was a promising technology that had seen limited application in the Southwest. The law's drafters regarded the mandate as a way to encourage the environmentally beneficial reuse of waste tires and included language prescribing specific amounts of crumb rubber to be added to asphalt mixes. Industry interests balked, claiming there had been no research to determine whether the prescribed levels of crumb rubber were technically feasible.

The mandate also ignored the work of the Strategic Highway Research Program, under way since 1987 and funded by $150 million from state governments, which was developing and validating performance specifications for asphalt pavement based on traffic loads and climatic conditions. Both public- and private-sector interests protested strongly enough to delay and finally kill the mandate, but the controversy would overshadow the potential benefits of asphalt rubber for years to come.

Besides the ISTEA fiasco, other factors contributed to the product's limited popularity. The “wet process” of producing it was invented in Arizona and was protected under patents until 1992, so the technology didn't spread as quickly as it might have otherwise.

Douglas Carlson, executive director of the Rubber Pavements Association, cites a “chicken-and-egg” issue that has affected adoption of the technology elsewhere: Authorities are reluctant to specify the material without experienced local suppliers and contractors, and suppliers are reluctant to invest in the technology without assurance that the demand justifies the expense.

KNOW ITS ADVANTAGES—AND LIMITS

Adapting hot-mix plants to make asphalt rubber requires two pieces of mobile equipment: a blender unit to hold the crumb rubber, as well as a blend tank where the rubber is mixed and saturated with oil before the mixture moves into the plant plumbing. This equipment ranges in cost from $250,000 to $1 million, compared to more than $7 million for a hot-mix plant.

Also, asphalt rubber creates a much more viscous binder than conventional hot mixes and thus a much thicker film surrounding the aggregate (about 9% binder by weight versus 5% or 6% for conventional asphalt). The higher binder-to-aggregate ratio means increased plant emissions, but nothing that exceeds permitted conditions. Similarly, fumes during placement are higher than with conventional hot mixes, but the rubber introduces no different fumes at typical temperatures, and fume levels stay within OSHA's recommended limits.

Asphalt rubber can't be applied in inclement weather; air temperatures below 65° F will prevent it from adhering, and those above 110° F will make jobsite conditions intolerable. Paving can't be conducted in the rain or emulsion put down as a tack coat, as the additional moisture will affect the material's properties.

ITS TIME HAS COME

Even though most projects have been located in the Southwest, some asphalt rubber pavements were installed beyond the region in the early 1990s, most notably in Florida. More recently, projects have been constructed in Colorado, Connecticut, Illinois, Nebraska, New Mexico, Tennessee, and Ontario.

New Jersey's first projects were 1-mile test sections installed on Interstate 95 and state Route 9 in 1994. Pleased by the performance, the state DOT installed an asphalt rubber friction course as part of a recently completed 8-mile, six-lane resurfacing project on I-95. About 80,000 old tires provided the crumb rubber for the paving mix, and the project is expected to extend the roadway's service life by 15 to 20 years. This year, the state will let contracts on major paving projects for sections of I-280 and -78, both of which will have asphalt rubber friction courses.

The Washington State DOT is testing asphalt rubber on sections of Interstates 5 and 405 as well as state Route 520 to measure its durability, the quantity and quality of noise reduction, and how the pavement performs over time.

As agencies and contractors in more states gain experience with and confidence in asphalt rubber friction courses, it's going to be even more widely used. In the process, communities will be recycling millions of scrap tires into quieter, safer, more durable roads.

— Kenneth A. Hooker is a freelance writer based in Oak Park, Ill.