Rubberized asphalt has been used for more than 20 years to resurface highways and city streets in Arizona when pavement surfaces reach their normal life expectancy. While it helped reduce the disposal of used tires, it recently has been recognized for its reduction of traffic noise.
1. Description of Rubberized Asphalt
Rubberized asphalt consists of regular asphalt paving mixed with “crumb rubber” -- ground, used tires that would otherwise be discarded or take up space in landfills. Used tires are processed by separating the casings, fabric and steel. The extracted rubber then is ground to the consistency of ground coffee. Rubberized asphalt has the benefit of being smoother and quieter. Noise readings have shown the rubberized asphalt generally reduces tire noise by an average of 4 decibels.
Approximately 1,500 tires are used for every lane-mile of rubberized paving. This can put a major dent in the 2 million used tires are that are generated annually in Maricopa County. Those old tires usually tires end up in landfills or in storage. Landfill space is at a premium, while tires in storage are a great fire threat.
2. Rubberized Asphalt is Temperature Sensitive
Rubberized asphalt cannot be applied during cold weather or very hot weather. The concrete pavement surface needs to be between 85 and 145 degrees Fahrenheit for the material to adhere properly. So rubberized asphalt can only be applied in the Spring and Fall in the Phoenix area – from March 15th to May 31st, and from September 1st to November 15th. Prior to application contractors must repair pavement cracks, chips and joints and prepare the concrete surface for the rubberized asphalt overlay.
3. History of Rubberized Asphalt
ADOT has used rubberized asphalt as a “pavement preservation” strategy on major highways throughout the state. But it was the City of Phoenix that pioneered the use of the product in the mid 1960s – almost 40 years ago. The earliest use of rubberized asphalt by the city was in 1964, when it was incorporated into the “chip seal” program for city streets. A rubberized asphalt chip seal – which used a mixture of rubberized asphalt and gravel – was applied to Indian School Road from Central Avenue to 7th Street in 1971 as a temporary measure. However, it performed so well that the street was not reconstructed until 20 years later in 1992. Despite the success of the chip seal program, it was discontinued in 1989 because of potential damage to cars by loose chips or gravel. In 1989, the city began using an asphalt rubber hot mix to add a one-inch overlay to prolong the life of streets. This product was shown to have a number of advantages:
• It does not reflect cracks from the existing pavement;
• It is more durable and skid-resistant than conventional asphalt; and,
• It reduces traffic noise and provides a smooth, quiet ride.
Noise Tests on Chip seal and Asphalt Rubber pavements on 7th Street by the City of Phoenix showed a decrease of about 10 decibels, or about 90% reduction in noise level. Research shows reduction in noise levels of 50 to 75% is commonly attained.
During the 1990s, the city resurfaced more than 200 miles of streets with 450,000 tons of rubberized asphalt, which used about 1.1 million old tires. The city reported that rubberized asphalt place on Dobbins Road in 1989 has performed without maintenance for 14 years and has an estimated life span of up to 18 years.
ADOT also is considered a pioneer in the use of rubberized asphalt in paving projects. More than 4.2 million tons of rubberized asphalt has been used on Arizona highways since 1988, at a cost of some $225 million. Those projects have resulted in the recycling of about 15 million old tires.
The Quiet Pavement program was developed by ADOT in cooperation with the Maricopa Association of Governments (MAG) – the regional transportation planning body – and area cities. It will be completed over a three-year period and paid for by using $34 million from other MAG regional transportation funds and projects. The program will not delay the planned completion of the Regional Freeway System by the projected date of 2007.