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WSDOT has been evaluating new types of pavements that might reduce freeway noise at the source - from tires as they roll across the pavement surface. These new pavements are:
Test installations of open-graded friction course pavements were evaluated on I-5 in Lynnwood, SR 520 near Medina and I-405 in Bellevue.
The Quieter Pavement test sites on I-5, SR 520 and I-405 were three of the most tested and documented quieter pavements in the world. WSDOT received an award for excellence in Quieter Pavements Research from the Rubber Pavements Association (RPA) in recognition of WSDOT’s contribution to understanding the acoustics and performance of quieter pavements. RPA is a non-profit industry association of manufacturers, contractors, consultants, testing laboratories, suppliers, government organizations and individuals that encourage the use of asphalt pavements containing recycled tire rubber.
Noise levels from the OGFC sections were compared to those of standard hot mix asphalt (HMA) pavements built at the same time. HMA is the standard for WSDOT asphalt pavement so it is used as the control pavement to compare against. If the OGFC section was more than 3 A-weighted decibels (dBA) quieter than the control section it was considered to be an audible difference: (detectable by most human ears). If the difference was less than 3 dBA the OGFC was not considered to be providing any noise reduction.
Most of the OGFC pavement test sections were audibly quieter when they were first built. None of the sections remained quieter for a significant period of time. For additional details, view:
A host of factors influence the noise generated by tire and pavement interaction including:
In general, asphalt pavements tend to be quieter than concrete pavements and OGFC pavements, initially, can be the quietest form of asphalt pavement. OGFC pavements are the most common type of “quieter pavement,” which is why WSDOT tried them.
For concrete pavements, work has focused on using alternative methods of finishing wet concrete to construct new textures that produce less tire-pavement noise. One such method is the result of concrete industry research that developed a grinding method for existing pavements and produced the quietest concrete pavement ever measured: Next Generation Concrete Surface (NGCS). WSDOT constructed and evaluated two NGCS sites in Washington.
The On-Board Sound Intensity (OBSI) measurement method was used and measurements were taken approximately every month. OBSI is the best method for measuring the acoustic performance of pavements because the microphones are mounted very close to the tire pavement interface.
WSDOT uses three criteria to rate pavement performance: smoothness, structural condition and rutting. Rutting due to raveling has been the major downfall of the OGFC quieter pavement sections.
The map below shows the locations of all of the quieter pavement trial installations.
All of the tested sections were overlays and included sections of conventional hot-mix asphalt (HMA), OGFC modified with asphalt rubber binder (OGFC-AR), and OGFC modified with polymer-modified binder (OGFC-SBS). The design for the OGFC pavements used at all three sites matched as close as possible the design of pavements used in Arizona, one of the leaders of the states using quieter pavements.
Differences include:
Historically, noise barriers have been the most effective method for reducing traffic noise. Noise barriers include noise walls and earthen berms that separate traffic noise from adjacent properties. Typical reduction is 5 to 10 decibels, with 10 decibels being about half the perceived noise level. While noise barriers can be effective, they can also be expensive to install and are not constructible or effective in all locations.
States like Arizona and California are using OGFC pavements to reduce noise levels on their urban roadways. The Arizona Department of Transportation (ADOT) provided WSDOT with the mix design for the OGFCs.
OGFCs are potentially quieter than conventional asphalt pavement because they are designed to have tiny air holes, or voids, throughout their entire depth. The air voids absorb and dissipate the sound generated by the tires on the pavement surface. The OGFC test sections also use a different binder than conventional asphalt. Each of WSDOT’s locations includes a section using asphalt rubber binder (OGFC-AR) and another using polymer modified binder (OGFC-SBS).
Different surface texturing of concrete pavements can affect the noise performance, too. WSDOT evaluated several types of new concrete surface textures for noise and durability:
