Absorption properties of the noise barriers made of scrap tires.

Misik, Ladislav ; Radvanska, Agata

1. INTRODUCTION

Vehicles in motion produce the friction between the vehicle's body and the air touching the vehicle. Such friction renders an aerodynamics effect that noise will be generated because of the gradient in the air pressure field induced by the friction. This pressure field will propagate to generate noise that can be heard at significant distances. Additionally, the contact of grooved tires on pavement surfaces occurring at high speeds creates a substantial sound pressure field as well as engine operations and exhausts systems. The acoustic spectrum of traffic noise, originated by moving vehicles, is of multiple frequencies. The majority of the spectrum falls within the frequency range of 250 Hertz and 4000 Hertz (Lapcik, 1998). The noise within this frequency range can be easily heard by the human ear, and can cause great discomfort. To control the propagation of this traffic noise, common practice is to build noise barriers along highways so that noise will be contained and absorbed within barriers, and will not propagate to any significant distance.

2. MATERIAL OF NOISE BARRIERS

The most of highway noise barriers are built with pre-cast concrete or concrete blocks or slabs. The study shows that these barriers are of very high acoustic reflectivity (95% and above) (Bulletin of the Acoustical and Insulation Materials Association, 1974) and of low sound absorption for the frequency band of highway noise between 250 Hertz and 4000 Hertz. Thus the effectiveness of concrete noise barriers in controlling vehicle noise is not satisfactory. With the drastic increase in highway traffic in the last two decades, the effort to develop new and better noise-reduction barriers for highways as well as airport and other applications has been intensified. It is predictable that such intensification will continue because noise poses an increasingly environmental threat. In recent years, some notable progress has been made in this respect. The polycarbonate noise reduction panels were developed in the U.S.A. The polycarbonate plastic has been tested to shelter a jet engine in New York airport (The Wall Journal, 1997). Another development is the noise barrier system made from lightweight hollow panels (Carsonite) made of tongue-and-groove planks of reinforced composite material filled with crumbed tire rubber. Traditional noise barrier walls have a flat surface. Now new designs are experimented with non-flat surface textures (Zhu, Carlson, 1999). These newly developed noise barriers exhibit a much better performance than concrete with respect to the capability of sound absorption and transmission loss, but the noise reduction is not the only criterion. In fact, there are other crucial criteria in constructing noise barriers. These criteria include cost effectiveness, technology maturity, durability, low cost and convenience in installation, and in maintenance and repair, and last but not least its aesthetics. The conventional concrete noise barriers meet those criteria very favorably. Polycarbonate plastic or composite noise barriers are very costly, and much less competitive in those criteria in comparison to concrete ones. This is why so far the progress made in replacing concrete noise barriers with aforementioned new noise reduction materials is very limited.

2. PARAMETERS OF NOISE BARRIERS

The most critical parameter in characterizing the capability of a material of how well it can absorb sound or noise is called the acoustical absorption coefficient (AAC). A sound wave carries certain amount of the energy called sound energy.

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When a sound wave hit a material, portion of the sound energy will be reflected or "bounced" back. From the noise reduction point of view, the lesser sound energy being bounced back, the better the effect of noise reduction. A value AAC=0 means sound energy being reflected completely, and a value AAC=1 means that all the sound energy is absorbed by the material, which is the best in noise reduction. A crumb rubber based noise barrier panel, manufactured as a mixture of 80% crumb rubber and 20% of bonding agent with a mixed size of rubber particles using the second and third spray devices (Fig. 1) has the acoustical absorption coefficient versus frequency was obtained and is plotted in Fig. 2, compared to the acoustic absorption coefficient for concrete noise barriers. It can be seen that the crumb rubber based specimen shows superiority in acoustical absorption (Zhu & Carlson, 1999, a).

3. PROPERTIES OF NOISE BARRIER

Rubber is notorious for its high flammability and the dense smoke which is produced when it burns. The noise barrier made from this material could ignite as a result of such incidents as grass fires, accidents, or vandalism. To reduce rubber's susceptibility to these concerns, flame and smoke retardants are available that can be added to the mixture during the manufacturing process. Recycled rubber tire material has been found to be nontoxic under leachate testing. However, additives, such as binders, retardants, coatings, and coloring, included in the mix to form and enhance the material, can create potential toxicity problems. These additives are, in some cases, proprietary with the specific formulations kept in confidence by the manufacturer. Rubber material, on its own, does not have sufficient rigidity to be considered as a structural component of a noise barrier panel. Therefore, bonding agents must provide adequate stiffness to enable the panels to be considered strong enough to withstand wind loading, or the rubber material must be firmly attached to a suitable stiffener, such as channel backings, cores, or casings. The structural strength of the panel must be verified through load testing on a production panel. Rubber and some binders tend to oxidize over time when exposed to the elements. They may also be susceptible to certain chemical or petroleum products. This increases the potential of premature disintegration of the panels. If concrete is used as a binder, concrete modifiers and special treatment of the crumb rubber are required before they will bond properly to each other. This is particularly important when these panels are exposed to salt, cold weather, and flexing for a long period of time. To optimize the bond between the rubber crumb particles, it is necessary to ensure that the rubber crumb is new or has been protected from the elements. The binders used should be stable under prolonged exposure to ultraviolet light. The manufacturing process should ensure that each rubber particle is completely encapsulated by the binder. If cement is used, the rubber surface should be treated or impregnated with a bonding agent compatible with both the rubber and the concrete. Or, the concrete should contain modifiers that will allow it to firmly bond to the rubber and be able to stand the test of time. Some coatings have a tendency to oxidize prematurely, particularly when used in conjunction with certain pigments. If the surfaces of the noise barrier panels are being manufactured to be sound absorptive, the coatings may clog the surface openings thereby reducing the Noise Reduction Coefficient (NRC). The recyclability of the final product may have been reduced drastically by the type of additives needed to alter the physical properties of the panel so that it can meet the various fundamental requirements for an effective, safe, and durable noise barrier product.

4. TREATMENT OF NOISE BARRIER

Barriers constructed of recycled rubber materials are limited to shapes obtainable through molding of their components. The surface texture of such panels is also influenced to some degree by the density and porosity of the rubber (Fig. 3). (Zhu & Carlson, 1999, b)

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5. CONCLUSION

The rubber tire has great potential to be further developed into a practical absorption material for reducing transportation noise.

Most importantly, it shows the possibility to transforming the problem of waste pollution from traffic vehicles into the advantages of solving the related noise pollution from the traffic vehicles themselves. Traditional sound barriers technology is often expensive to install, and made from concrete or wood materials, limiting traditional noise barriers as a solution to noise pollution. The above mentioned technologies use a recycled rubber compound to form a panel mixed with additives or as the center in a composite channel design. Installing such noise barriers is inexpensive and straightforward. The technologies are durable and outperform traditional noise barrier materials. Using scrap tires in the sound barrier is more energy efficient than using the tires for combustion fuel or placing them in landfills. This sound barrier technology effectively reduces both noise and waste pollution in an innovative way--it removes scrap tires from the waste stream entirely.

ACKNOWLEDGEMENTS

The author would like to acknowledge the support of Scientific Grant Agency of the Ministry of Education of Slovak Republic, Commission of mechanical engineering, metallurgy and material engineering, for their contribution to project 1/3174/06.

6. REFERENCES

Albany Airport to Test Noise Abatement Product. (1997). The Wall Journal, Vol. 31, September-October/1997, pp. 5-6.

Bulletin of the Acoustical and Insulation Materials Association, Park Ridge, Illinois, 1974.

Lapcik, L. (1998). Recycled Polymer Based Noise Absorbing Composite Materials, Proceedings of 5th International Conference on Composites Engineering, July 1998, pp.515, Las Vegas, USA, ISBN 0-7333-0504-0

Noise Barrier Materials and Surface Treatments, Federal Highway Administration, U.S. Department of Transportation, Available from: http://www.fhwa.dot.gov/environment/noise/5.htm Accessed: 2007-04-12

Zhu, H. & Carlson, D.D. (1999). A Spray Based Crumb Rubber Technology in Highway Noise Reduction Application, Rubber Pavements Association, 1999, Available from: http://www.rubberpavements.org/library/spray_based/imag es/fig1.jpg Accessed: 2007-01-11

Zhu, H., & Carlson, D.D. (1999). A Spray Based Crumb Rubber Technology in Highway Noise Reduction Application, Rubber Pavements Association, 1999, Available from: http://www.rubberpavements.org/library/spray_based/noise bar.html Accessed: 2007-02-28