No Sound Walls Needed 

The Alta Arden Expressway is a four lane urban arterial street owned and maintained by the County of Sacramento, California. In 1993 when the pavement was resurfaced with an inch and a half (1 1/ 2 ") Asphalt-Rubber Gap-graded hot mix, the expressway carried approximately 18,500 vehicles per day. Noise measurements were taken one month prior to the overlay and one month after, followed by another study at 16 months. The final measure-ments were taken in 1999 at six (6) years into the pavement life. The ADT had increased by 2% during the six years. A conventional asphalt concrete pavement on Bond Road in Sacramento was chosen as a "control." As with the Alta Arden pavement, meas-urements were taken at a month prior and one month after the conventional overlay. The third and final measurement of this conventional asphalt pavement was taken at four years as it measured "0 dB" at that time period. A third Asphalt-Rubber pavement on Antelope Road was included in the study, however a sixteen (16) month period elapsed between the "before" and "after"measurements. The Asphalt-Rubber overlay was placed ten months after the original noise reading and the "after" tests were conducted six months later. The final measurements were taken in September, 1999. The change in noise levels at six months was ­4 dB and at five years, the pavement had a 3 dB reduction. Theron Roschen, Sr. Pavement Design Engineer for Sacramento County Public Works summarizes his account of the Noise Study in the article below. Roschen is also the Program Director for the Northern California Rubberized Asphalt Concrete Technology Center which is funded by the California Integrated Waste Management Board with Tire Disposal fees. Listed below are the Noise Test Results from these two pavements.
 Roadway Pavement Type Duration of Time Elapsed After Paving Change in Noise Levels, dB Leq
Alta Arden Expressway Asphalt-Rubber GG 1 month -6 dB
    16 months -5 dB
    6 years -5 dB
       
Bond Road Conventional Asphalt 1 month -2 dB
    4 years 0dB

Asphalt-Rubber Saves Money in Many Ways
by Theron Roschen, PE, Principal Civil Engineer,
Sacramento County Department of Transportation, Design Services
(916) 874-7225 · E-Mail roschent@SacCounty.net

  Sound walls adjacent to local road-ways are unsightly impediments that cut through the community, blocking the open view of residents and motorists alike. Sound walls are an attractive nuisance for graffiti and are costly in their construction and maintenance. In addition, their effectiveness is significantly reduced when a driveway must breach the wall to gain access from an adjacent residence.

  Sacramento County has recently
completed a six-year noise study that compared pre- and post-overlay traffic
noise on Asphalt-Rubber pavement overlays to conventional pavement. The study showed a sustained noise reduction that averaged 4 decibels for the Asphalt-Rubber pavement with no appreciable reduction for conventional asphalt overlays after just 16 months. The use of Asphalt-Rubber as a mitigation measure in environmental documents has been successfully used in Sacramento County and avoided the cost and visual blight of a sound wall on our road projects.

  Surprisingly, this is not the primary motivation for the County to use Asphalt-Rubber. Beyond the recycling and noise benefits, our primary choice to use Asphalt-Rubber is due the reduced overall thickness equivalency that is recognized by the California DOT, and the modified engineering properties that extend the life-cycle of the pavement when compared to conventional asphalt. No minor detail in these days of dwindling maintenance budgets. The primary noise component on local roadways is from the vehicle tire friction known as "tire wash". The noise reducing qualities of rubberized
asphalt are due in large part to the additional porosity increased film
thickness on the aggregate, and resiliency of the pavement over time, which results from the use of the rubber in the
asphalt binder.

The product used 18% by weight of recycled crumb tires in the liquid binder. The enhanced properties of the liquid binder introduces additional sound absorption to the pavement, which results in reduced reflection of sound from the pavement into the community. As a result, tire noise is reduced, but engine and exhaust noise is not affected by the rubberized surface.

   Therefore, a roadway containing primarily automobile traffic would be expected to exhibit the greatest decreases in traffic noise following paving with rubberized asphalt. This assertion has been borne out by the Sacramento County study.

   Noise studies of the benefits of porous and rubberized roadway treat-ments have been conducted in other areas, such as Phoenix and Tucson, Arizona, and in Orange County and Thousand Oaks, California, each reached similar conclusions regarding the noise-reducing benefits of the pavement. In addition, studies in Belgium, France and Austria have yielded similar noise reduction. The Sacramento County study is available
through the RPA web site and at www.rubberizedasphalt.org.

   A 10-year follow up study is currently underway in the city of Thousand Oaks, one of California's major municipal users of Asphalt-Rubber, and will be available this fall.

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