There are two immutable facts about urban noise. First, it comes from several sources and, second, it is the subject on the minds of a range of people, including residents who must contend with urban noise; federal, state and local agencies of jurisdiction; other public officials; industry; academia; and others.
For its part, the concrete pavement industry is working to address one form of noise—sound at the tire/pavement interface—commonly called tire/pavement noise.
To address the issue, it’s important to draw a distinction between pavement smoothness and pavement texture. Both may be factors in noise, but are often confused. The fundamental differences are these:
Pavement smoothness is the characterization of the evenness of the pavement over a given distance. Expressed another way, a smooth pavement is one that features as few bumps in the road as possible. Uneven spots (hence a rough pavement) can occur in a pavement for many reasons, but typically occur because of the:
- Natural shifting that occurs beneath the pavement surface over time;
- Wear and tear on the pavements over time; and
- Workmanship or quality issues in construction, which also may become more pronounced over time.
Pavement texture is a characterization of the surface of the pavement. Texturing—whether designed and built into a pavement or imparted by equipment onto an existing, hardened concrete pavement—can improve skid resistance, surface drainage and, of course, sound qualities.
Inevitably, many discussions about tire/pavement noise turn to the state of Arizona, where in Maricopa County (the county surrounding Phoenix), rubberized asphalt overlays were placed over some concrete pavement highway sections in response to consumer complaints.
“What’s often overlooked is that the rubberized asphalt overlays in Arizona represent a pilot program conducted to consider pavement type as a noise mitigation strategy,” said Larry Scofield, P.E., director of environmental engineering for the American Concrete Pavement Association (ACPA). Prior to joining ACPA, Scofield worked almost 30 years with the Arizona Department of Transportation (ADOT), including 20 years as the manager/researcher for the Arizona Transportation Research Center.
Scofield is quick to add that there are a number of factors that make the Arizona experiment unique. Among them are:
- Atmospheric conditions: There can be a 10 dB(A) shift 1,500 ft away from the roadway, he said, which is a function of thermal gradients occurring from temperature inversion, a common phenomenon in Maricopa County. Temperature inversion, Scofield explained, is a meteorological phenomenon where air temperature increases with height.
- Original pavement designs: Most of the concrete highways in the area are new and have short slab lengths, making them conducive to overlays, Scofield said. “These freeways were textured with a transverse texture, which we now know is relatively loud. But at the time this texturing method was specified tire/pavement noise was not part of the equation.”
Tining and turf
Like the pavements in Maricopa County, the secret to a relatively loud or quiet pavement often lies in the pavement’s texture.
Since the late 1970s, the Federal Highway Administration (FHWA) has required concrete pavement surfaces being used on federally funded highway applications to be textured.
Many states turned to transverse tining, and in many applications transverse tining yielded a safe but relatively noisy surface texture. Looking forward, FHWA has released a new technical advisory on surface texturing that will replace their current advisory, which provides more flexibility to states to use longitudinal and other proven textures.
There are other approaches currently available.
“We’re opening up the industry to new ideas,” said Steve Waalkes, P.E., ACPA’s managing director of technical services. “There are alternative textures that are just as safe as transverse tining.”
Evidence of this is available in Minnesota, where artificial turf drag is used in the longitudinal direction.
“All in all, if Arizona had moved to longitudinal tining on concrete pavement years ago, Phoenix’s noise concern probably would not even exist,” Scofield said. Another example is diamond grinding, which can dramatically improve both smoothness and sound qualities, he said. “Diamond grinding is used successfully to restore the smoothness and texture of existing pavements, and in some cases to enhance the qualities of new pavements.”
A number of tire/pavement noise research and testing initiatives are currently under way. These include:Field experiments
The Center for Portland Cement Concrete Pavement Technology, FHWA and ACPA are working together on a far-reaching research project to evaluate pavement surface characteristics.
The research seeks to understand the relationship between noise and pavement texturing (and grinding for imparting texture); to evaluate the noise/texture/time relationship; and to develop construction techniques that are repeatable and cost effective.
The testing will evaluate new construction to measure noise, texture, smoothness and friction. The experiments also will evaluate in-service pavements to measure noise, texture, smoothness and friction periodically. A third phase of the testing will measure noise and texture for the purposes of establishing an inventory of pavements and studying noise-texture relationships.Laboratory testing
At Purdue University, and with support from the ACPA and its affiliate, the International Grooving and Grinding Association, testing is being conducted using a custom-built grinding head, a key component of the tire/pavement test apparatus (TPTA).
“Using the Purdue TPTA, it becomes more cost effective and efficient to vary the factors controllable by a grinding contractor and to eliminate the traffic and environmental issues,” Scofield said.
The initial test series will evaluate the effect of spacer width, blade width and groove depth on the noise generation characteristics of the resulting texture. The test plan also will evaluate the effect of pavement joints; groove effects; and tining and innovative surfaces.
Scofield explained that grinding experiments traditionally have been conducted in the field, but these evaluations can be challenging because of the time and expense of the requisite blade or spacer changes. Also, field experiments are subject to the prevailing environmental conditions and traffic control requirements.
The TPTA evaluations, being in a controlled environment, will allow quick changes and will not be subject to environmental or traffic control factors.
TPTA testing is conducted at 0 to 30 mph, Scofield said, adding, “the equipment uses two test tires and also features environmental controls.” The TPTA apparatus is capable of performing sound intensity (SI) or close proximity (CPX) testing. The special grinding unit, designed by Terry Kramer of Diamond Surface Inc. and constructed by Steve Garrison, Web Burnett and Alex Ugalde of Diamond B, can grind two 8-in. wheel paths at the same time. This allows 12 test sections to be constructed on the TPTA for one series of tests. By limiting a wheel path to only 8 in. it becomes less expensive to change out blades and spacers.
“We are optimistic the TPTA evaluations at Purdue will provide excellent quality results in a controlled setting,” Scofield said, adding, “We also plan to correlate with field data taken from whisper-ground pavement sections.” At press time, the first series of tests were about to be completed.Sound intensity testing
ACPA has invested in an apparatus to conduct tire/pavement noise testing. Scofield noted that the association is conducting limited evaluations for agencies of jurisdiction using a well-defined and consistent protocol that employs a standard test vehicle and specific brand and type of tires. Scofield added that these data are being collected and correlated against other research results. Other field testing
ACPA has conducted or participated in field experiments in Arizona, Kansas and California. Scofield noted that these field evaluations are yielding some surprising and positive results that underscore the benefits of existing technologies used to impart surface characteristics.
A number of other noteworthy initiatives also will prove useful to the ongoing research and evaluation of surface characteristics and noise, Scofield said, including:
- NCHRP research projects:
NCHRP 1-43: “A guide for pavement friction” initiated in June 2003 with a final report expected in the fall of 2005;
NCHRP 1-44: “Measuring tire/pavement noise at the source” is a research project whose anticipated timeline is fall 2007. This project will evaluate the near-field measurement systems and recommend the preferred test method. This study also will evaluate the use of the measurement system on both passenger cars and commercial trucks;
NCHRP 10-67: “Texturing of concrete pavements” is a research project whose timeline is the winter of 2007. This project will develop parameters for selecting surface characteristics specific to the roadway application and environment; and
NCHRP 8-56: “Truck noise source mapping” is a research project whose timeline is the summer of 2008. This project will focus on truck noise source issues and this is important in understanding the complete traffic stream.
- FHWA Expert Task Group (ETG) on SI Standardization: The ETG is considering a provisional standard to be released in August 2005. The purpose is to develop a procedure for conducting SI testing.
- Caltrans’ NITE Program (Noise Intensity Testing in Europe): 66 pavement sections in Europe were tested using the same equipment as used to measure pavements in the U.S.