(This research was conducting by Arizona State University for the Arizona Department of Transportation in April 2006)
Since 1990, it has been the policy of the State of Arizona that the recycling and reuse of waste tires are the highest priority. The Arizona Department of Transportation (ADOT) has long supported the use of recycled waste tire rubber in asphalt rubber hot mix. AR mixtures have been shown to perform successfully and have several added benefits such
as the reduction of highway noise, providing better surface drainage characteristics to enhance visibility and skid on wet pavement surfaces. Furthermore, some aspects of life cycle costs have also been conducted and demonstrated the potential impacts on maintenance and rehabilitation savings to ADOT. Joint ASU/ADOT research activities
related to Asphalt Rubber (AR) mixtures started in July 2001 and are continuing. In this work we test the hypothesis that AR-ACFC road surface layer results in less tire wear than PCC road surface layer.
Tire wear contributes to atmospheric particulate matter (PM) which is regulated by the United States Environmental Protection Agency (US EPA) because PM has been shown to affect human health. PM is classified by the size of the particles; PM10 and PM2.5 include particles with diameters smaller than 10 and 2.5 μm, respectively. PM2.5 has
been shown to contribute to morbidity and mortality (Dockery et al., 1993; Pope et al., 1995; Katsouyanni et al., 1997; Krewski et al., 2000). This epidemiological research has found consistent and coherent associations between outdoor air quality and health outcomes including respiratory symptoms, reduced lung function, chronic bronchitis, and
mortality (Bates, 1992). The PM10 fraction includes particles that are respirable, and so of concern for human exposure.
Vehicle emissions are a significant source of both PM2.5 and PM10. Vehicle fleet emissions per mile traveled have been reduced significantly in the last 30 years as a result of improved engine operation and tailpipe controls; this downward trend is expected to continue into the future and is an important means to reduce PM. The main focus of
these reductions has been on tailpipe emissions; however, “zero emission” vehicles will continue to generate PM from tire wear, road wear, brake wear, and re-suspended road dust. These non-tailpipe emissions will become a relatively more important component of PM emissions but are difficult to characterize. In this proposal work, we apply our
existing aerosol measurement expertise (Allen et al., 1996; Allen et al., 2001) to evaluate tire wear emissions from the vehicle fleet using the Deck Park highway tunnel in Phoenix, AZ.
The amount of rubber loss was estimated to average approximately 90 mg/km (Dannis, 1974) which corresponds to 1.3 million metric tons per year for the entire US (Reddy and Quinn, 1997). Tire wear particles are generated during rolling shear of the tire tread against the road surface. Average tire tread wear rate for single passenger tire is between 6 and 900 mg/km, depending on the road surface type (e.g., asphalt vs. concrete), driving conditions (acceleration, abrupt deceleration, speeding, etc) and tire conditions (tire pressure, vehicle load, retread vs. new, etc.). Tire wear emissions (TIRE) are estimated in the EPA MOBILE 6.1 model as:
Tire Wear Emissions 3
TIRE = 0.002 * PSTIRE * WHEELS where TIRE has units g/mi, PSTIRE is the fraction of particles smaller than a cutoff size, and WHEELS is the number of wheels on a vehicle (EPA, 2003). For PM10, PSTIRE is 1.0; for PM0.1, PSTIRE is 0.01. Using this formula, a passenger vehicle is estimated to emit 13 mg/km of PM10 and 0.13 mg/km PM0.1. The MOBILE 6.1 emission estimates are used for air quality modeling, however these factors have not been verified
experimentally for existing or new pavement surfaces.
We hypothesize that AR-ACFC road surface layer results in significantly less tire wear than PCC road surface layer. Reduced tire wear would result in lower vehicle operating costs and lower particulate matter (PM) emissions from vehicle traffic. In the present research, we measured the rate tire-wear marker compounds in PM emissions at the Deck Park Tunnel Highway on Interstate 10. The Deck Park Tunnel highway surface was PCC until June, 2004, when it was resurfaced with an AR-ACFC. This research takes advantage of a rare opportunity to sample tire wear emissions at the tunnel before and after the AR-ACFC overlay.
Measured tire wear emission rates developed here may then be used by ADOT as inputs to federally-mandated air quality models for the Phoenix airshed. If, as hypothesized, resurfacing with the AR-ACFC reduces tire wear emissions, this additional benefit of AR may be incorporated in ADOT air quality planning.