Many times, understanding of an issue comes by hearing about it again and again. So it may be with the tire/pavement issue.
A lot of hearing — and speaking — occurred in 2004 and earlier this year at The Tire/Pavement Noise Strategic Planning Workshop in Indiana. The first workshop was one of the major introductions to the subject of noise-reducing pavements in the United States. Pavement engineers, noise engineers and environmental engineers came from Europe and around the United States to hear presentations and ask questions about how pavements can reduce road noise.
Since then, we have heard a lot more, both from U.S. and European sources. And it seems like some patterns and guidelines are emerging. A number of experts have made presentations and several European and U.S. studies have been published.
The Federal Highway Administration’s (FHWA) Office of Pavement Technology (HIPT) has an active program encompassing research, development and technology transfer in the area of tire/road noise. HIPT is actively investigating refinements to current asphalt and concrete pavement technology to optimize their tire/road performance.
“The technologies being investigated include thin, gap-graded asphalt surfaces, porous asphalt, alternative materials and improved concrete textures,” says FHWA spokesman Doug Hecox. This work, which began in 2005, is being performed at the National Center for Asphalt Technology (NCAT), Purdue University, Iowa State University and with private contractors.
FHWA is also working with an Expert Task Group to develop a draft provisional standard test method for measurement of tire/pavement noise close to the tire. This test method is being considered for adoption by the American Association of State Highway Transportation Officials (AASHTO) Subcommittee on Materials.
How Much and How Long?
Also, the DOT’s Volpe Center is studying the durability of noise-reducing pavements, how much noise reduction they provide, and how to include pavement effects in noise predictions. “We want to know ‘how much?’ and ‘how long?’”, says Judy Rochat, Physical Scientist at the Volpe Center. “How much? means how much noise reduction occurs on a particular pavement, and How long? means how long will the noise-reducing pavement last.”
Rochat says that Arizona DOT’s (ADOT) Quiet Pavement Pilot Program (QPPP) is a pivotal program because of the agency’s noise-reducing work on the Phoenix beltway. ADOT is currently working with FHWA on its noise abatement program.
California Department of Transportation (Caltrans) has also been heavily involved in asphalt pavement research. That agency wants to know what happens over time to the noise abatement qualities of pavement, and they also want to know how long these noise-reducing pavements will last. Caltrans is also doing a cold weather study on noise-reducing pavements near Truckee, California.
The QPPP in Arizona and tire/pavement noise research in California and other states will help the Volpe Center and FHWA to understand the effects of pavements and how they might be included in noise predictions.
Quiet Pavement in Europe
Quiet pavement has a longer history in Europe and its experience is proving helpful in evaluating the benefit of quiet pavements. Highway pavement noise has been studied in Europe for more than two decades, and policies are in place to reduce noise by using quieter pavements.
The current trend in European countries paving with asphalt is to use the thin-surface, gap-graded mixes with small aggregate in urban areas that are subject to severe winter snow and ice accumulation. More porous gap-graded asphalt surfaces are used on rural and high-speed roads with moderate winter conditions.
The focus of the European effort is found in three quiet pavement technologies:
- Thin-surfaced, negatively textured, gap-graded asphalt mixes (such as NovaChip, microsurfacing and stone matrix asphalt (SMA).
- Single and double layer, highly porous asphalt mixes with more than 18 percent voids, and
- Exposed aggregate concrete pavement
“Quiet Pavements” Team
In May 2004, a U.S. “quiet pavements” team of state, federal, academic and highway industry representatives toured five European countries for 17 days and, along with a comprehensive study of research done on noise-reducing pavements, published a FHWA report on noise-reducing pavements in Europe.
Some findings made by the team about quiet pavement systems in Europe were:
- Two-layer porous asphalt (TLPA) on high-speed highways produced exceptionally quiet pavements.
- Porous pavements should not be placed in urban areas where the operating speed drops below 45 miles per hour (mph) because highly porous mixes tend to clog under slow traffic.
- A reduction in aggregate size in the wearing surface mix will normally result in immediate noise-reducing properties of mixes.
- Aggregate sizes in Europe for quiet surface mixes are 0/4 millimeters to 0/10 millimeters. But most U.S. DOTs use Superpave aggregate gradings of 19 mm, 12.5mm and 9.5 mm. [A reduction in aggregate size gradings to the next smallest size produces a noise reduction of 1 to 3 decibels.]
- Thin-textured surfaces using a small aggregate size worked well for urban or low-speed sections. To achieve noise reduction, the pavement was always negatively textured (containing holes or voids). Positively textured pavements such as chip seals increased noise.
European organizations are currently encouraging an extensive amount of research on quiet pavement technology. Some of these include:
- Roads to the Future
- Silent Roads for Urban and Extra-Urban Use
- Program of Research, Experimentation and Innovation in Land Transport
- Sustainable Road Surfaces for Traffic Noise Control
Recent U.S. studies are confirming some of the European findings. Arizona, California, Florida and Texas are among the lead states investigating the noise-reducing properties of gap-graded or porous mixes.
The Arizona DOT uses asphalt-rubber friction courses (ARFC) as a noise mitigation strategy. ARFC’s pavement life is typically 8 to 12 years compared to 6 to 9 years for an asphalt concrete friction course (ACFC). Decibel reduction on an ARFC pavement is typically 4 dB(A) lower than the best Portland cement concrete (PCC) pavement.
Probably the longest running quiet pavement study in the nation is on I-80 at Davis, California. That study and others are comparing dense-graded asphalt pavement with open-graded and rubber-asphalt. After eight years, the I-80 pavement has provided a 4.3 dB(A) noise reduction when compared with the original asphalt pavement.
Bruce Rymer, Senior Engineer for Caltrans, says that open-graded or rubberized asphalt pavements have shown the best performance on the roads tested in California. Although Rymer adds that variations on the different types of pavement tested can be as much as 2 to 3 dB(A)s.
Arkansas and Kansas Surveys
NCAT conducted a noise survey of several sections of highway in Arkansas—seven sections of I-40, one section of I-430, two sections of I-440, one section of SR 440, and one section of I-30. The average tire/pavement noise for the HMA sections was 97.2 dB(A) and the average tire/pavement noise for the PCCP sections was 101.5 dB(A).
A noise survey by NCAT of a section of I-435 in Kansas showed that the transverse-tined PCC pavements were considerably noisier than the hot mix asphalt and that the placement of an HMA overlay on this section of I-435 reduced the noise level on that section by more than 10 dB(A).
New Jersey Study
A total of 42 pavement sections, varying from HMA to PCC, were tested by NCAT in New Jersey using the Close Proximity Method (close to the tire) to evaluate the effect of pavement surface type on tire/pavement noise. Comparisons for all of the pavement sections were conducted at 60 mph. The tests found:
- That HMA surfaces generated lower noise levels than the PCC surfaces. If the PCC surfaces were diamond-ground, however, noise levels were comparable to the HMA surfaces. The average tire/pavement noise level for the HMA materials was 98.5dB(A), and the average noise level for the PCC materials was 102.6 dB(A).
- That the nominal aggregate size of the HMA materials had an effect on pavement noise. The dense-graded aggregate (DGA) mixes showed that 12.5mm Superpave mixes produced less noise than the 19mm Superpave mixes. And the 9.5mm nominal aggregate-size SMA mix had lower noise values than the 12.5mm nominal aggregate size SMA mix.
- That the OGFC asphalt surfaces modified with crumb rubber (CR) had lower noise levels than OGFC mixes without CR. However, the CR OGFC mixes also contained finer aggregate gradations than the traditional OGFC mixes used in New Jersey.
- That measured noise levels at 55, 60 and 65 mph generated noise increases of 0.18 dB(A) per mph. When broken down by surface material type, the HMA materials had a noise gradient of 0.19 dB(A) per mph.
NovaChip, Microsurfacing and SMA
In general, both NovaChip and micro-surfacing provide good ride quality, and they have similar noise levels. However, during their pavement life, tire/pavement noise increases.
The New Jersey study found that the approximate size of the SMA mix has a large impact on tire/pavement noise. The 9.5mm SMA mix had an average sound pressure value of 98 db(A), while the 12.5mm mix had an average of 100 db(A). This means that the 12.5mm mix had 37 percent more noise.
As highway agencies in the United States and Europe continue to study noise-reducing pavements and how to make them more effective, asphalt pavement designers and contractors will continue to find new ways to make roads quieter.
For more information about noise-reducing pavements, visit quietpavement.com.