In recent years, tire-pavement noise has become a source of public annoyance in both the United States and Europe. In 1985, Maricopa County, Arizona, passed a new tax to fund a multi-million-dollar road construction program for the Phoenix metropolitan area. As construction began, the impact of freeway noise on the public was not a big issue, says James Delton, state materials engineer, Arizona Department of Transportation. If certain noise levels were exceeded, noise barrier walls were built.

As the program matured, however, the public became much more sensitive to the noise issue. "The heights of required sound walls became very tall, which translates to very expensive," Delton said. His remarks and others presented here were delivered at Quiet Asphalt 2005, a conference held last November at Purdue University in Lafayette, Indiana. Sponsors of the conference included the Asphalt Pavement Alliance and the Federal Highway Administration.

ADOT officials decided to control freeway noise at the source, rather than by increasing the distance between pavement and neighborhoods, or by building expensive sound walls. The result is the Arizona Quiet Pavement Pilot Program, which includes a 10-year, multi-million-dollar research program endorsed by the FHWA.

Ultimately, ADOT will spend more than $70 million to resurface Phoenix-area concrete pavements with an asphalt-rubber friction course — a material that is generally quieter than concrete under traffic. In one test section, the asphalt-rubber friction course mix achieved an 11-decibel reduction at the tire, compared to bare concrete. The ARFC has a top-sized aggregate of 0.357 inch and 9 to 9.5% asphalt binder. When the surface is used on an asphalt pavement, it is applied 0.5-inch thick, and on concrete pavements it goes down 1-inch thick, Delton said.

European issue

In Europe, road noise causes ill health effects, including high blood pressure, said Gert Ahe, director of the Danish Road Institute. Pavement noise causes reduced housing values, and in many households, the noise exceeds the threshold of annoyance. While noise is an unsolved problem on existing European roads, noise planning is integrated into the design of new roads and housing developments. And several European countries have begun following the European Union Noise Directive, which calls for a 10-year plan with measures intended to mitigate the problem.

For the European Scan Tour, a group of industry and governmental officials studied pavement design and noise levels in Denmark, the Netherlands, France, Italy, and the United Kingdom, said David Newcomb, vice president of research and technology at the National Asphalt Pavement Association. "In every country we visited, road noise is a significant annoyance," said Newcomb. "Every country had maximum or target noise levels on pavements. All of them are working to meet the EU Directive, and everyone has noise legislation or a policy in place. Every country we visited is using quiet pavements."

In this country, the National Center for Asphalt Technology has embarked on a multi-year research program with two goals, said Douglas Hanson, senior materials engineer with AMEC, an international consulting firm. NCAT wants to develop technology to design a low-noise pavement surface, and to be able to use that technology to provide economical solutions to highway design problems without arbitrary rules, Hanson said
NCAT developed a close proximity noise trailer, which places microphones close to a test tire and measures the sound that emanates at highway speeds. Hanson has tested noise levels on 250 different hot-mix asphalt sections and 70 concrete pavements. Pavements have been tested in 18 states, including California, Colorado, Arkansas, New Jersey, Ohio, and Texas.

Average noise levels of hot-mix asphalt, as measured close to the tire by the NCAT trailer, read as follows: (If the noise was measured by the far field method away from the tire, the noise levels would be much lower

Dense graded HMA, 97 dB(A), or decibels, a measure of sound amplitude.
Stone matrix asphalt, 97 dB(A).
Fine open-graded friction courses, 92 dB(A).
Coarse open-graded friction courses, 95 dB(A).
"In general, the finer the gradation used in an HMA mix, the quieter is the pavement," Hanson told attendees at Quiet Asphalt 2005. He said it is possible to build a low-noise HMA pavement surface, and that pavements have a noise signature — frequency in hertz plotted verses noise level in decibels. "Plus, the noise level of a pavement will increase with time, but we don’t know by how much," said Hanson.

Wide ranges of noise

Tire-pavement noise varies over a significant range in each major pavement category — dense-graded asphalt, open-graded asphalt, rubberized asphalt, or concrete — said Paul Donavan of Illingworth & Rodkin, a Petaluma, California-based noise authority. The pavement texture can reduce tire pavement noise by up to 8 to 10 decibels depending on existing and final conditions. Donavan said that as a group, open-graded or rubberized asphalt surfaces show the best noise performance in California and Arizona.

Research in Europe, Donavan said, shows tha

Highly porous, two-layer asphalt construction can provide slightly better noise performance than other pavement types.
Stone matrix asphalt surfaces provide a similar range of performance to dense-graded asphalt surfaces.
Constructions of the same specification can produce tire pavement noise with up to 2 decibels of variation.
When comparing the quietest pavements, a European double-layer porous asphalt with a fine-graded aggregate on the surface posted the quietest performance, at 94.5 dB(A), Donavan said. Other quiet pavements ranged between 95 dB(A) for a European porous asphalt and 97 for a rubberized asphalt in California. One rubberized asphalt pavement from Arizona registered a reading of 95.5 dB(A).

Hanson found that dense-graded HMA ranged between 93 to 99 dB(A), with an average of 97 dB(A). Stone matrix asphalt had about the same average, and ranged between 95.5 dB(A) and 100.5 dB(A).

Open-graded friction courses absorb part of the sound energy, which makes them quieter, Hanson said. What’s more, there is a direct relationship between air voids in the mix and the noise level produced by an OGFC. At about 13% voids, noise levels reach 98 dB(A), and the noise plot slopes down to 95 dB(A) at 20% voids.

"If I had to design a low-noise HMA pavement now, I’d consider the following," said Hanso

An open-graded mix with more than 15% air voids, and try for 20% or more; 90 to 100% passing the 0,375-inch sieve; use of a polymer-modified asphalt with fibers to allow 8 to 9% binder in the mix.
A very-fine-graded SMA mix.
A two-layer concept with 0.75 inch of fine-graded OGFC overlaying 1.5 inches of coarse-graded OGFC.
Durability in Arizona

Under its Quiet Pavement Pilot Program, Arizona can use its asphalt-rubber friction course, as a noise mitigation strategy and receive federal funding for highway construction. ADOT’s Delton said several rubberized asphalt overlays on concrete are about a decade old and show excellent performance when used that way. And he said that compared to a concrete top surface, an ARFC overlay provides a smoother ride. ARFC provides comparable friction levels to a concrete surface, reduces the amount of water spray, and provides increased contrast to pavement markings in wet weather, Delton said.

Furthermore, Arizona research shows that pavements with ARFC experience less rutting than overlays or inlays without rubberized asphalt. And, pavements with asphalt rubber as the wearing course cost less to maintain than neat asphalt cement surfaces, Delton said. ADOT data shows that most asphalt inlays and overlays will experience the first crack at about four years of age. But once initial cracking occurs, asphalt rubber greatly reduces the yearly increase in cracking.

The oldest section of ARFC in Arizona is an overlay of concrete pavement on Interstate 19 in Tucson. That section was the first of the modern ARFCs placed in one lift. Maintenance costs have been very low. Earlier ARFC layers were placed in three lifts.

"The I-19 section was placed in 1989, has received no major maintenance since then, and is still serving traffic today," Delton said. "Many years of performance history of ARFCs placed in Arizona have shown it to be a durable and long-lasting surface course. In virtually every performance measure, it does better than the comparable mix without rubber. This is the case when ARFC is used as an overlay on either asphalt or concrete pavements."

Participants in the European Scan Tour studied thin surface treatments in several countries; Colsoft, which uses a polymer-modified bitumen with a crumb rubber addition; single-layer porous asphalt surfaces; two-layer porous asphalt; and various concrete surfaces including exposed aggregate concrete.

Two-layer porous asphalt is in the experimental stage in Denmark, France, and Italy, said NAPA’s Newcomb. In the Netherlands, it’s in the developmental stage. A typical design in Denmark calls for a top course of 1 inch of 0.333- or 0.5-millimeter aggregate. The bottom layer consists of 1.75 inches of 0.6875-millimeter aggregate mix.

Noise reductions are significant with two-layer porous asphalts. They are 8 to 9 dB(A) quieter than conventional mixes, and 4 dB(A) quieter than single-layer porous mixes at high speeds, Newcomb said. On the downside, construction costs of two-layer mixes are 25 to 35% higher than a single-layer mix. Construction of the two-layer mix should be placed warm-on-warm. Tack coats are essential. Typical voids in the mix run greater than 20%, and typical binder percentages are 5.7 to 6%, based on aggregate weight.

Clearly, tire/pavement noise is an issue that has spread into urban areas across the country. What’s an agency to do? While the FHWA prohibits the use of pavement types or surface textures as a noise abatement measure, a Quiet Pavement Pilot Program should account for noise reduction benefits of pavement types, said FHWA’s Mark Ferroni, Highway Traffic Noise Team.

Perhaps NAPA’s Newcomb summed it up best: "Highway agencies should begin measuring the tire/pavement noise profile of their commonly used surfaces," he told attendees at Quiet Asphalt 2005. "Surfaces capable of reducing noise by 3 dBA(A) should be considered where noise is an issue."