Progress Reported on Quiet Concrete Pavement
Field tests show that the Next Generation Concrete Surface is competitive with the quietest pavements available.
In the past decade or so, environmental noise has become a major issue in many parts of the world, including the United States. Stakeholders ranging from the general public to the roadbuilding industry have expressed concerns over highway noise. Recognizing that highways are a factor in urban noise, the concrete pavement industry set out to research and develop quieter pavements without compromising performance, durability, safety or other inherent benefits of concrete.
In recent years, the concrete pavement industry has developed a Next Generation Concrete Surface (NGCS). With field tests indicating sound levels of 99 to 101 decibels, these diamond-ground surfaces are producing results that are very competitive with the quietest pavements available, says Larry Scofield, P.E., Director of Pavement Innovation for the American Concrete Pavement Association (ACPA).
Sound testing is conducted using the On Board Sound Intensity (OBSI) method, which allows assessments of acoustic performance of pavements over time. The method was developed by General Motors and introduced to the highway community by the California Department of Transportation. More about the NGCS follows:
Research showed that much of the perceived problem with concrete pavement’s noise results from a pure tone, or “whine” that occurs when noise with a certain discrete frequency emanates from the tire/pavement interface. Almost always, those frequencies were set with concrete pavement by uniformly-spaced transverse tining.
In an effort to improve pavement safety, the FHWA in the late 1970s had mandated transverse tining, and many states complied. Then in 2005, FHWA dropped the requirement for transverse tining, and opened the door to other concrete pavement texture treatments. California, for example, had used longitudinal tining for more than 30 years and reported few if any problems with it.
Recognizing the worldwide interest in quiet pavements, in 2004 ACPA, with support from the cement industry, developed a program to research the noise characteristics of concrete pavements. A primary objective was the evaluation and development of a quieter concrete pavement surface. Also providing support for the program were the International Grooving and Grinding Association (IGGA) and several of its members.
Purdue University’s Ray W. Herrick Laboratories conducted the research using its Tire Pavement Test Apparatus (TPTA). The machine consists of a 38,000-pound, 12-foot-diameter drum that makes it possible to test numerous pavement textures and compositions in combination with various tire designs. Six curved test sections of pavement fit together to form a circle around the vertical edge of the drum. Two tires, mounted on opposite ends of a beam, are then rolled over the test samples while microphones and other sensors record data. The TPTA has been described as a “noise microscope” for tire-pavement noise. Tire speeds of up to 30 mph can be tested.
Scofield says the Purdue diamond grinding research was based on theories that the blade- and/or spacer-widths might be the keys to a more quiet pavement surface. But after evaluating the range of blade and spacer widths requested by the industry, Purdue reported that no unique relationship could be found between sound levels and spacer width, blade width and spacer/blade configuration. Instead, it appeared that the controlling variable – where sound was concerned – was the variability in the fin profile height resulting from the grinding process. The fins are the tiny vertical ridges that appear on a diamond-ground concrete surface.
Textures with low variability were quieter than textures with high variability. In conventional diamond grinding, the resulting fin variability is influenced by the blade/spacer configuration, the concrete mixture, aggregate type, pavement condition, equipment set-up and more. Because the fin variability occurs in the field, it is difficult to adjust in a laboratory setting. Researchers decided to grind a pavement smooth, then impart additional texture by grooving, which provided an exact fin profile that could be controlled and predicted.
A conventional diamond-ground surface produces texture in the upward or positive direction, while the texture tested at Purdue produces texture in the downward or negative direction. “The texture, which later was called the Next Generation Concrete Surface (NGCS), was desirable from the standpoint that it was more of a ‘manufactured surface’ and thus could be controlled as necessary on an experimental basis,” says Scofield.