What you don’t know about rubberized asphalt Click to Enlarge - A rubber mix being compacted in Puerto Rico in November 2009.
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Today, recycling tires by grinding them down to crumb rubber and mixing into asphalt formulations is finding wider acceptance by more state Departments of Transportation (DOT). And, as the price of oil goes up, so does the interest in A-R.

A-R is defined by ASTM (American Society Testing Materials) as a blend of hot paving grade asphalt cement, reclaimed tire rubber and additives where rubber content is at least 15 percent by weight of the liquid asphalt binder, and has reacted sufficiently to cause swelling of the rubber particles. Rubberized asphalt has less than 15 percent by weight rubber content. Both terms are used interchangeably in this article.

Proponents of using old automobile tires to make A-R claim that the practice is better than landfilling, and cleaner than burning the tires as a fuel. Used A-R can also be recycled endlessly by milling it off roads and adding it to new asphalt mixes. According to the Rubber Pavements Association (RPA), a 2-inch thick overlay of A-R hot mix will consume about 2,000 tires per lane mile. In the spray applied method for seal coats, about 500 tires are used in a lane mile. Approximately 18 million tires are recycled annually in paving applications.

One of the predominant uses of A-R is in Asphalt-Rubber Open Graded Friction Course (AR-OGFC) To appreciate one of AR-OGFC’s most outstanding qualities, you have to drive on it. It’s much quieter than conventional asphalt or concrete.

“A few years ago, we did a project that was a 7-mile section on Interstate 95 near our Ewing offices. We received dozens calls from drivers asking what is that stuff? It’s great, the noise is less. People driving on it can tell the difference over regular asphalt and they say, ‘Wow! What a big difference.’ It’s very unusual for us to get that kind of positive public reaction,” said Eileen Sheehy, manager of New Jersey’s DOT Bureau of Materials.

Mike Harrington, sales manager for CRM told a similar story. His company has been supplying crumb rubber for asphalt for over 20 years and is the largest crumb rubber producer in the western United States. “Several years ago, they paved about a quarter mile of Loop 101 in Phoenix with rubberized asphalt. The people who lived next to the freeway said things like: ‘What happened, it’s so quiet?’ Then, the people in the next community asked why they didn’t have it. There was such an outpouring of demand, that over the next few years the state wound up having to pave the whole thing.”

“With rubberized asphalt, we were getting about a three decibel reduction in noise over conventional asphalt, but over concrete you may get up to a ten decibel reduction. And, that’s really significant,” said Sheehy.

According to NJDOT, tires on concrete pavement generate between 100 and 110 decibels of sound, depending on the age and surface texture. On conventional asphalt the noise is in the high 90s to low 100s. Rubberized open-graded friction courses are in the 95 to 97 decibel range.

Doug Carlson, executive director of RPA put it this way – “Sound reduction largely depends on what method is used to measure the sound. There are many different ways. From feedback we have received from numerous wayside data sources, noise reductions can be in the 40 to 70 percent range, depending on the preexisting surface type and condition.”

A-R helps reduce noise emanating from tires at higher speeds. At speeds over 35 mph, the predominant noise is made by the tires. At lower speeds much of the sound comes from the engine, exhaust and rattles. The quietness of A-R is largely due to its smoothness. A Texas DOT study found that ride quality improved by 61 percent on A-R. Because this pavement lasts longer and cracks less over time, smoothness is maintained. WesTrack, a Nevada road test facility built under a grant from the Federal Highway Administration, found a 4.5 percent fuel efficiency increase riding on smoother pavements.

“What we are really trying to do is cut the noise at the source,” said Sheehy. New Jersey has completed most of the sound walls and barriers mandated by federal requirements to mitigate noise pollution. Sound walls and other types of barriers are expensive, costing between $200 to $400 dollars per linear foot. In many urban areas there is not enough space to build them. “Now we are dealing with what we consider nuisance noise – noise not high enough for mandated sound barriers, but nevertheless bothersome to residents. That’s why we put rubberized asphalt on Route 280, because of noise complaints. It’s also useful in areas where it’s very hilly, because we can’t always build sound barriers tall enough,” Sheehy said.

“The noise benefit has been the unsung hero. It’s kind of a perfect marriage between the business and the environment. Here is something that is ecological viable and technologically sound so it really turns out to be a win-win,” said CRM’s Harrington.

According to the United States Environmental Protection Agency (EPA), rubberized asphalt is the single largest consumer of ground rubber. California and Arizona use the most in highway construction, jointly accounting for over 70 percent of crumb rubber used.

“Over 90 percent of state highways in Arizona are paved with it. In California it’s huge. They don’t have good data, but of 65,000 miles of roadway I would say that about 20 percent has been rubberized at one point or another,” said Carlson. Florida uses A-R exclusively on every state highway for friction courses. Among others, Texas, Nebraska, South Carolina, New York, New Jersey, New Mexico, Maryland, Massachusetts and Puerto Rico are currently testing or using A-R. In Canada it’s being used in British Columbia, Alberta and Saskatchewan.

Naturally, each state DOT must test and approve various formulations before general use. This can take years, and in some cases decades. Like many states, New Jersey began testing rubberized asphalt in the early 90s in response to the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA)

The act suggested there would be mandates for states to use one percent rubber in asphalt in order to receive federal funds. Later, the mandate was dropped, but ISTEA had the effect of getting many states to test A-R. “We did some projects at that time to see how it would work. Two projects were quite successful using asphalt rubber in an open-graded friction course. There are benefits with the open-graded friction course. It has a very open texture so it drains very well. You don’t have as much spray off the surface in rainy conditions. Because you do not have as much water on the surface you don’t get as much hydroplaning. So it’s safer in the rain. We identified that as a very valuable asset for those areas that have high wet-weather accidents,” said Sheehy.

New Jersey’s oldest project, a short test section of Route 195, was constructed in 1992 and is scheduled to be removed and replaced this year. Like conventional asphalt, when a rubberized road reaches end of life, it can be milled up and recycled into new mixes of A-R, or into new mixes of regular asphalt. “We did a project back in 1991 where we took an asphalt rubber road and recycled it. We tested the emissions and properties of the mix and found no problems with recycling. It can go to an asphalt plant and be mixed in with unrubberized asphalt. We are confident that the percentage in the mix after reprocessing won’t make any difference at all,” said Sheehy.

New Jersey uses between 15 to 20 percent crumb rubber in the asphalt binder, which constitutes about 8 percent of the overall pavement mix. The state uses conventional asphalt plants and paving equipment, but out-of-state blending units are currently being trucked-in to make A-R. Sheehy said that New Jersey asphalt producers are waiting to see how A-R develops before investing in equipment.

The A-R on Route 195 lasted 18 years as compared to the average 12 year life expectancy of non-rubberized Open Graded Friction Course. New Jersey is planning to use more AR-OGFC because of its durability. Sheehy commented on 195: “It’s lasted longer than expected. We have been happy with that performance and believe it justifies the increased cost.”

In Phoenix, A-R is only about 5 percent more expensive per ton than conventional mixes. “An interesting thing has happened as a result of recent economics. As the price of asphalt per ton increases, the cost saving value of recycled tire rubber in the asphalt increases. The crumb rubber market has been steady and stable at about $300 dollars per ton. Back in the late 90s, liquid asphalt cost about $70 dollars a ton. Now it costs $500 a ton! By using crumb rubber there is about a 20 percent displacement of oil,” said Carlson.

Despite the higher initial cost in other states, RPA claims other significant cost savings based on several factors: longer life cycle, decreased maintenance, and in many instances half as much material is required.

“[California Department of Transportation] has a layer equivalency guideline for rubberized asphalt based on almost 30 years experience. If the conclusion is to give a roadway a 10 year service life, you can pave it with four-inches of conventional asphalt, or use 2 inches of rubberized asphalt and expect the same results,” said CRM’s Mike Harrington.

There are climate and topography issues related to the use and they can vary greatly from state to state. Each area must test and evaluate which formulations best suit their roadways and conditions. Warmer weather in Arizona, California, Texas and Florida has undoubtedly led to heavier usages there. But A-R has been tested and successfully used in many cold regions.

Arizona routinely uses a combination of gap-graded A-R hot mix and an open graded A-R friction course in the Flagstaff area at elevations over 7,000 feet with winter temperatures of 20 below zero. California has also successfully used A-R in the high Sierra Nevada Mountains. “In 1992, one of the very first California highways using rubber was at Donner Pass. At over 7,000 feet elevation and snow up to 30 feet, it has held up beautifully. It was recently repaved using rubber,” Carlson noted.

New Jersey, however, is working on the issue of snow and ice removal. They found that salt seeps into the pores of the open-grade and does not stay on the surface and continue to work. This resulted in the need for more frequent salt applications. “Unless that issue is resolved, we will probably see it only in areas that we identify as either a wet weather problem, or a noise problem that we want to mitigate. Our maintenance people are looking at different snow and ice removal techniques. We are also considering a slight change in the gradation, porous enough to work well in wet weather, but not as porous to work well in ice and snow.”

NJDOT has more asphalt rubber projects planned for 2010. “Generally, we are optimistic about rubberized asphalt. It has been performing well for us and I think we are going to be using it more,” concluded Sheehy.