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Innovations In Road Building

NOISE-REDUCING ASPHALT

Tarakanova A.

Vladimir State University named after A. and N. Stoletovs

Vladimir, Russia

Introduction

reduced noise highway traffic tire

Highway traffic noise is an increasingly important issue.As traffic volumes increase, noise levels increase. Simultaneously many communities are seeking to improve quality of life by reducing environmental noise. Thus, highway traffic noise has become an element of the tension between economic development an quality of life.

At freeway speeds for well maintained vehicles, the primary source of traffic noise is tire/pavement noise. Thus, the tire/pavement interface is the primary target of studies to reduce traffic noise. Quieter pavement has been demonstrated in several communities in the U.S. and has been extensively studied in Europe, Japan, and Australia. Based on these case studies, there is evidence that it is possible to construct and maintain pavements that are quieter than typical pavements that are also safe, durable, and cost effective.

Noise from highway traffic is an environmental problem in both metropolitan and rural areas. The most significant impact of traffic noise is the annoyance it causes to humans and the associated negative effects this annoyance has on the quality of life. However in addition to annoyance, traffic noise may also impact health , create difficulty with speech communication, suppress real estate values, and cause the stagnation of economic expansion due to public resistance to expanded highway capacity.

Without a significant strategy for traffic noise reduction, the type of conflict between economic development and environmental concerns that has essentially stopped airport expansion for the last twenty five year will impact highway expansion. It is essential that reduced noise highway alternativesbe identified and utilized to minimize the impact of traffic noise.

Highway traffic noise is generated by four subsources of highway vehicles: engine/drivetrain noise,exhaust noise, aerodynamic noise, and tire/pavement interaction noise. For properly maintained automobiles, tire/pavement interaction is the dominant sub-source at speeds above approximately 50 kph (30 mph) .For properly maintained trucks that are not using engine compression brakes, the tire/pavement interface noise is similarly dominant but at a slightly higher speed. Pavements that produce less noise for the tire/pavement interface sub-source are an important strategicsolution necessary to address future highway noise problems

Relevant topics in sound and acoustics: noise and sound

Sound results from small, fast pressure variations in a fluid medium. In the case of tire/pavement noise,the medium is air. Thus, any event resulting from tire/pavement interaction that creates a pressure variation in air will create sound. Acoustics is thescience of sound and is the broad discipline of the study of the generation, propagation, and reception of sound in all aspects. Noise is defined as unwantedsound.

Typically sound is created by a vibrating surface, in the manner of a speaker cone, or by aerodynamic forces, in the manner of an air disturbance (e.g., air being moved by a fan). Vibrating sources are typically referred to as mechanical sources of sound. On the tire, the tread blocks and tire carcass vibrate and create sound directly. Vibrations are also passed to the pavement, wheel, vehicle body, and other structures which vibrate at frequencies in the range of hearing, creating additional sound radiation. Sources that create pressure perturbations using fluid forces are referred to as aerodynamic sources. The dynamic pumping of air in and out of the tread passages of a tire create sound. In addition, the turbulence around the tire and vehicle create sound.

Sound can be either desirable or undesirable. Music is an example of desirable sound. Sound generated by tire/pavement interaction is undesirable and will be referred to as tire/pavement noise. Depending on amplitude and duration, noise may have a range of effect on humans. At high levels for sustainedperiods of time, noise can cause hearing loss andadverse health effects such as high blood pressureand hypertension. At more moderate amplitudes,noise can cause speech interference, sleepdisturbance, annoyance, and a loss of the quality oflife.

Tire and pavement noise generation

Tire/pavement noise is generated by several subsources. The material presented here is a summaryof the material presented in a very comprehensive form. The figures in this section are from that reference and are reprinted with the permission of the authors. At the tire/pavement interface, several mechanisms create energy which is eventually radiated as sound. These will be referred to as source generation mechanisms. There are also characteristics of the tire/pavement interface that cause that energy to be converted to sound and radiated efficiently. These characteristics will be referred to as sound enhancement mechanisms.

Source generation mechanisms: Tread impact

The tire tread blocks travel around the tire as the tire turns. At the entrance of the interface between the tire and pavement (referred to as the contact patch) an impact occurs as the tread hits the pavement. The tread impact can be compared to a small rubber hammer hitting the pavement. This impact causes vibration of the tire carcass. If both the tread block and the pavement can be made resilient, the energy created by this impact can be reduced. Randomization of the pavement texture and the tread pattern reduces the repetitiveness of this impact and can change the character of the sound to reduce the annoyance of the sound.

Within the contact patch, the passages and grooves in the tire are compressed and distorted. The air entrained in these passages will be compressed and pumped in and out of the passages. Because of air compression effects and air pumping, aerodynamically generated sound is created. This phenomena is similar to sound created by clapping hands.

Source generation mechanisms: Slip stick

Within the contact patch the tread blocks transfer tractive forces from the tire to the pavement for acceleration or braking. In addition, due to the distortion of the tire carcass in the contact patch, the tread block/pavement interface experiences significant horizontal forces. If these horizontal forces exceed the limits of friction, the tread block will slip briefly and then re-stick to the pavement.. This action of slipping and sticking can happen quite rapidly and will generate both noise and vibration. This phenomenon is observed in the gymnasium when athletic shoes squeak on a playing floor.

In many cases the energy created at the tire/pavement interface is not radiated efficiently. The tread blocks are small and would not beefficient radiators in isolation of the remainder of the tire/pavement system. Also, the air pumping alone would not be a significant source of energy.

Several aspects of the tire/pavement system significantly enhance the radiated noise.

Reduced noise pavement principles

The noise levels of various pavement types reported in the literature normalized to 55 mph at 50 ft. from the center line of the roadway for PCC pavements for asphalt pavements. These data are a compilation of results reported in papers published since 1996. In the figures the average, maximum and minimum reported levels for any particular pavement are shown. Normalization of this data was done by correcting for speed based on the trends for reference energy mean emission levels used in the Traffic Noise Model [TNM] and correcting for distance using hard site propagation for a line source. Data were not used if sufficient detail was not supplied to make a speed and distance determination or if other variables were a major concern.

The significant characteristics of these data are:

1. Within any given pavement type there is considerable variation that is believed to be due to either uncontrolled or unreported pavement construction and design variations. It will be important to understand and control these variations in the future.

2. In general, open graded asphalt pavement with small aggregate size is the quietest pavement type.

3. There can be as much as a 9 dB(A) difference for a single pavement type.

4. There can be as much as a 14 dB(A) difference between the noisiest and quietest pavement under similar conditions. Reduced noise pavements have been built in field test. In 1998, 26 states reported that noise from various pavement types had been investigated while 29 states reported that changes in standard pavements would be considered for noise control (WAYS). Several countries in Europe and Asia are routinely building reduced noise pavement as part of their national strategy to reduce traffic noise. From this work, certain principles have emerged that appear to be generally true.

Porous pavements are constructed by reducing the amount of small aggregate used in the pavement such that the pavement cannot be tightly compacted. Porosity occurs in a packed pavement when only large aggregate is used. In general, porous pavement reduces tire/pavement interaction noise above 1000 Hz. Porosity levels on most installations have been approximately 18-25%. Porosity reduces the strength of the air pumping source mechanism by preventing air compression and reduces the enhancement potential of the horn, organ pipe, and Helmholtz resonator mechanisms.

To circumvent the concerns about porous pavement in low speed applications or where icing conditions are prevalent, Europeans have been developing asphalt pavements with lower porosity with small, high quality aggregate. The aggregate typically has top sizes of either 10 mm or 6 mm. Porosity is achieved by using aggregate with certain gap-graded size distribution such that the finished pavement will have a porosity that handles water and grit appropriately. The aggregate will be graded such that very little material in the size range 2-4 mm will be used for 6 mm top sizes or very little 4-6 mm material will be used for overlays with 10 mm top sizes. Overlay thicknesses are from 15 to 25 mm depending on the size of the aggregate used. The small aggregate size tends to produce smooth pavement with very little characteristic texture greater than 20 mm. The porosity, while relatively low, is still effective in reducing high frequency noise.

Texture is important on all pavements to enhance wet weather friction. Negative texture with characteristic lengths less than 10 mm also tends to reduce noise. However, texture of other sizes and type tends to increase noise, sometimes significantly. Local texture depth of approximately 3 mm (which translates to average texture depths slightly greater than 1mm) appears to be sufficient to achieve noise reduction effects.

Conclusions

Quiet pavement that is safe, durable (both for sound control and wear) and economical has been demonstrated to be possible with current technology. However, the variation of available materials and construction techniques among the states mean that there is no general guideline available yet for design of quiet pavement. The designer will also need to specify pavement with good surface friction numbers, desirable splash/spray characteristics, and good wear characteristics at reasonable cost.

Many of the issues of quiet pavement are only partially understood. As effort on the various aspects of quiet pavement proceed, a better understanding of the problem will evolve which will allow more accurate prediction of sound levels, development of lower-noise pavement designs, and direct measurement of the properties affecting noise reductions. The FHWA is encouraging these efforts by states with the Quiet Pavements Pilot Program. At present one state is participating (Arizona) but more are expected in the near future. Other states have quiet pavement programs independent of the FHWA pilot program. Significant effort is ongoing in Europe and Japan. Over the next 5-10 years the body of knowledge about reduced noise pavemet will grow substantially such that implementation of quiet pavement will be routine.

References

1. Donavan, P.R. and B. Rymer . Highway Pavements for Tire/Road Noise Generation, -2003.

2. Paul R. Donavan.Vehicle Exterior Noise,”Handbook of Noise and Vibration Control. - Editor, Malcolm Crocker, John Wiley and Sons, in press. Hibbs. - 2002.

3. B.O. and R.M. Larson. Tire Pavement Noise and Safety Performance. -1996.

4. Irwin, J.D. and E.R. Graf. Industrial Noise Control. - 1979.

5. Krupa, Gregg. Residents Near I-275 Want Relief from Noise. The Detroit News, Detroit, Michigan, August 5, 2003.

6. Sandberg, U. A Road Surface for Reduction of Tire Noise Emissions. Proceedings of InterNoise. - "M-Technologies" September 11-13, 1979.

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