By: John Roberts, Gerald F. Voigt, P.E., and Michael Ayers, Ph.D., Contributing Authors
For people living near a heavily trafficked highway, the issue of noise can be a concern . . . and it’s a concern shared by agencies and industry, too. Busy highways near residential areas have long been sources of noise to the surrounding residents. In the past, little or nothing was done to mitigate either the causes or effects of highway noise. As noise has become a larger issue, noise or sound walls have been used increasingly to mitigate the effects of noise pollution from highways, but these have proven to be very costly.
A truly objective analysis of noise would consider factors such as engine and exhaust stack noise, but the issue recently has focused on tire/pavement noise, and in particular on asphalt (especially open-graded friction course or OGFC) and concrete pavements.
The tire/pavement noise levels between well-designed and constructed concrete and OGFC are minor. Studies have shown only slight—barely perceptible—differences between tire/pavement noise levels of concrete and asphalt pavements.
What is all the noise about?
At the center of the tire/pavement noise controversy are the testing methodologies. The more conventional and widely accepted method for measuring vehicle noise is the “statistical pass-by method” (SPB), which measures noise where it would most likely be heard by receptors and provides readings useful for environmental impact analysis.
The “close-proximity” (CPX) method measures noise at the tire/pavement interface and is well suited for investigations of road surface influence on traffic noise, but does not provide a measurement useful for environmental impact analysis. Furthermore, this method is not used with tires designed for heavy vehicles. It is known that road-surface sound emission characteristics depend on the tire used, including whether the tire is for light or heavy vehicles. The results obtained with this method, therefore, best describe conditions when sound from light vehicles constitute the major part of traffic noise (<10% trucks).
A recent study on tire/pavement noise conducted by the National Center for Asphalt Technology (NCAT) for the Michigan DOT (MDOT) demonstrated that concrete pavements that are textured with a surface that minimizes noise generation are just as quiet as asphalt pavements. The study resulted in the decision by MDOT not to overlay the concrete pavement.
In the NCAT study, test sections included longitudinally tined, transverse tined and diamond-ground concrete, as well as stone matrix, conventional dense-graded and Superpave asphalt. Two types of tires were used: Uniroyal and MasterCraft. (It should be noted that different tire tread patterns have significantly different noise characteristics.)
The diamond-ground concrete section was the quietest of both asphalt and concrete sections, based on tests performed with the Uniroyal tire. A few heavily textured concrete sections increased the concrete pavement noise average, obscuring the quiet concrete section results.
The heavy textures on the concrete sections were mandated by the MDOT and the Federal Highway Administration (FHWA) for skid resistance and public user safety.
Even so, the tire/pavement noise of nearly all of the sections were within about 3 dBA of each other. These research findings show concrete is quiet.
It also should be noted that an excessively deep texture in a pavement is not desirable and will not enhance friction and hydroplaning characteristics.
Pass-by or close proximity?
The NCAT study used the CPX method to measure the noise levels on nine different pavement sections. The CPX method, which is not the standard measuring method recommended by the FHWA for noise levels along highways for environmental analysis, involves placing a receiver (microphone) near the tire on a trailer being pulled along at highway speeds. The CPX method is inexpensive, easily applicable in most cases, measures along the extended length of a road surface and provides an “absolute level.” Unfortunately, CPX poorly represents surface influence on truck-tire noise. Also, the associated propagation effects are not accurately represented; and the results are restricted to tire/pavement noise . . . not engine, exhaust and drive-train noise.
The standard pass-by method involves measuring the noise at ground level near the receptors (houses, buildings, etc.) along a roadway and is more representative of actual traffic cuts. The receptors are usually 25 to 50 ft from the noise generator (edge of the roadway). The farther a receptor moves away from the noise generator, the quieter the noise seems—doubling the distance from the source can reduce noise intensity by as much as 6 dBA. Therefore, slight differences in noise due to pavement surface type are far less influential at distances normally used for measuring vehicle noise along a roadway.
Tine it or grind it
So, given the challenges of addressing the concerns about tire/pavement noise, what options exist to address this issue? In terms of concrete pavement construction or rehabilitation, there are two effective means of mitigating tire/pavement noise: proper tining or diamond grinding.
Recent research shows that improved forms of tining—such as longitudinal tining—reduce noise levels. Research conducted by Marquette University measured noise, texture and friction at 57 test sites in Colorado, Iowa, Michigan, Minnesota, North Dakota and Wisconsin. Among the study’s findings:
• Longitudinally tined concrete pavements and an asphalt pavement exhibited the lowest exterior noise;
• One asphalt pavement and the longitudinally tined and random skew tined (1:6 skew) concrete pavements exhibit the lowest exterior noise. The random skewed can be easily built and eliminates discrete frequencies; and
• When comparing different pavement textures with mean texture depths of about 0.276 in. (0.7 mm), the following exterior noise reductions were observed, compared to a uniform, transversely tined concrete pavement: random transverse, 1 to 3 dBA; random skewed, 4 dBA; longitudinal, 4 to 7 dBA; open-textured asphalt, 5 dBA. (Random transverse or random skewed means the teeth on the concrete rake are spaced at random intervals.)
To mitigate the noise factor, a number of state DOTs have recently shifted away from uniformly spaced transverse tining of concrete pavements. A survey of states by ACPA reveals that nine states have either changed to longitudinal tining or are considering doing so.
All pavements produce noise, and equally important is the fact that all pavements eventually will need to be resurfaced, restored or reconstructed. Unfortunately, when pavement rehabilitation is performed prematurely, it represents a huge waste of taxpayer money that otherwise could be used to address serious safety and road-user delay issues. Diamond grinding of pavements has been shown as an effective means of not only restoring or improving the original surface characteristics of the pavement when constructed, but also reducing noise.
In another research project, Marquette University researchers showed that diamond-ground pavements exhibited no discrete frequencies, and compared to transverse tining, lowered noise levels by about 3 dBA. The study also reported that diamond grinding, if deep enough to remove most of a uniform transverse texture, can be considered for existing concrete pavements with excessive whine.
Also, a report by the Arizona DOT underscores how diamond grinding can be used to address tire/pavement noise associated with concrete pavements. The report detailed a test proj-ect to compare the effectiveness of diamond grinding on reducing traffic-generated noise characteristics. The project involved four sections of Rte. 202 near its intersection with I-10. The project resulted in decibel readings being reduced to as low as 95.5 dBA, as measured using the CPX method. The report stated that as a result of the diamond-grinding, “the high frequency pure tone noise, commonly known as tire whine, has been significantly reduced.”
The report also found, “the use of pavement grinding as a traffic noise abatement could be beneficial for both reducing tire pavement noise levels and muting the tire whine pure tone sound of the older concrete pavement transverse tining texture.” Other research also confirms the tire/pavement noise reduction possible with diamond grinding. Research conducted by Parsons Brinckerhoff for the Utah DOT showed a 1 to 5 dBA reduction in tire/pavement noise due to pavement grinding and showed post-grinding noise levels in the range of 76.2 to 79.2 dBA, as measured using the SPB method. Research funded by the New York State Thruway Authority and the FHWA showed diamond-ground pavements to be 2 to 5 dBA quieter than transverse-tined surfaces. The research also showed greater wet-weather skid resistance, and after one year showed the diamond ground concrete’s skid resistance to be “superior.”
In addition to the research that substantiates that properly textured concrete pavements are just as quiet as asphalt pavements, it’s important to note that a number of resources are available from the American Concrete Pavement Association (ACPA) and the International Grooving & Grinding Association to address surface texturing and noise questions.
Safety should never be silent
A wide range of research substantiates that concrete pavements generally provide superior skid resistance and durability. There also are stated policies that generally advise a common sense approach to address noise, emphasizing a balanced approach that does not trade off safety or performance.
Federal and state transportation agencies, through policies and official positions, have generally acknowledged the relatively small amount of noise reduction associated with surface type and have urged specifiers not to trade off safety. In a June 1995 policy and guidance statement, FHWA wrote: “While it is true that noise levels do vary with changes in pavements and tires, it is not clear that these variations are substantial when compared to the noise from exhaust and engines, especially when there are a large number of trucks on the highway.”
In its synthesis 268, the NCHRP reported: “It is the official policy of the FHWA, and in the opinion of the American Association of State Highway & Transportation Officials, that a small amount of noise reduction is not worth sacrificing safety and durability. This means that the practicing highway design engineer must try to find a ‘happy medium’ between noise control and maintaining a high level of safety.”
FHWA’s environmental policy also states that “unless definite knowledge is available on the pavement type and condition and its noise generating characteristics, no adjustments should be made for pavement type in the prediction of highway traffic noise levels . . . The use of specific pavement types or surface textures must not be considered as a noise abatement measure.” With sound policies and solid research firmly in place, it raises the question why surface texture continues to be a subject of debate, particularly when the proposed solutions require so much maintenance and repair.
About The Author: Roberts is executive director of the International Grooving & Grinding Association, Coxsackie, N.Y. Voigt is senior vice president, engineering services, and chief operating officer for the American Concrete Pavement Association, Skokie, Ill. Ayers is dir
