The application of recycled rubber components as noise barriers  

Project Title: The Application of Recycled Rubber Components as Noise Barriers

Applicant: The Hong Kong Polytechnic University

Total Approved Grant: $149,452.39

Duration: August 1999 - August 2000

Project Status/Remarks: Completed


(1) To develop the design method for optimizing the acoustic performance of recycled rubber components from tire sections dumped in landfills.

(2) To develop and demonstrate the technique of applying the rubber acoustic absorbers in barriers for improving traffic and train noise attenuation.

Summary of the Findings/Outcomes:

After assessing the noise absorption performance of the recycled tyre and understanding how waste tyres can be handled currently; the feasibility of using recycled tyre as noise absorption material will be discussed. It depends on the successfulness of the three aspects: noise absorption ability, cost and availability of waste tyre and the advantages over customary absorbers. Noise absorption ability is the basic, but most important factor to account for is its feasibility. No further discussion is needed if waste tyres are not noise absorptive. As has been proved in chapter 4, the two types: powder and whole have their specific characteristics towards sound. Powder tyres undergo porous absorber mechanism and absorbs high frequency sound, while whole tyres provide air gaps and enhances sound absorption at low frequencies. A proper combination of these two types of tyres would give satisfactory sound absorption for traffic noise. On the contrary, varying air gap and thickness can shift the performance to a required range. For instance, minimizing air gaps would enhance absorption at high frequencies for train noise. If inexpensive sorting equipment is available, then the size of powder tyre can be classified into different ranges and further experiments can be carried out to determine which range has the most connected pores. On major drawback of the powder tyre is that it is not suitable as surface finishes, but perforated plates can be installed as supplementary covering materials. They have little effect if thin, with many holes of a small diameter and a perforation ration greater than 30%. When the perforation ration is smaller, the absorption coefficient decreases at high frequencies. Then, depending upon the air gap behind the plate ?V the absorber has characteristics more like those of a resonant absorber. To conclude, rubber tyres provide noise absorption and has great flexibility in modification for specific needs.

The worth of investigating a new material depends on its advantages over existing products; that is why the advantages of tyres over conventional noise absorptive materials were chosen as one of the factors to assess this feasibility. The major disadvantage of conventional noise absorptive materials is its structural weakness and fibrous nature which weakens its endurance against wind, rainwater and collisions. For instance, the fiberglass would absorb rainwater and the usual method to prevent this failure is to introduce a protective plastic sheet. However, the on-site construction of noise barriers would increase the thickness of the plastic sheet and pack the fiberglass as tight as possible, in order to avoid water penetration. This would change the original intended noise absorptive ability from the laboratory. Using tyres as noise absorptive material in the noise barrier would maintain the noise absorptive ability after prolonged exposure to an outdoor environment, because rubber tyres are is inert and non-fibrous. In addition, high mass of tyres do not only enhance sound insulation, but also contributes to structural dampening ?V decreasing the natural resonant frequency (hopefully not harmful to human hearing). Moreover, its high elasticity can absorb shock energy. Besides reuse as a noise barrier along highways, recycled tyres will be excellent for noisy construction and industrial areas where wet pavements, high temperature and collisions may encounter. One may think that noise barriers made of tyre may be dangerous, since it may ignite due to car collisions. This will generate an untolerable odor, black smoke and dust that is highly dangerous to any person nearby. Nevertheless, this worry is not of significant concern. One of the characteristics of rubber tyres is its high resistant to heat; otherwise, tyres would cause or spark into flames every time whenever vehicles brake. Recycled tyres have also been used in paving highways in USA and as shock bumpers in all car-racing tracks. These two usages would expose tyres to high risks of fire, but no complaints nor reports have eve been reported. So, there is no need to worry about that the possibility of tyre noise barriers catching on fire from car collision impacts. Even if severe collisions do occur, the burning diesel and tyres of the vehicle are grater hazard before the dangers of the noise barrier becomes a significant cause for concern.

In addition, a new noise barrier system made from fiberglass-reinforced polymer composites filled with recycled tyre waste has been on sale in USA. This system is already classified as a Class 1 Building Material (defined by the Uniform Building Code), is suitable for indoor use. Nevertheless, in order to prevent any possible fire hazards, a metal iron perforated sheet can be installed at the front of the noise barrier. The high heat conductivity of the metal sheet can divert heat energy to the entire panel in order to minimize the possibility of tyres catching on fire.

Using recycle rubber tyre as noise absorption material has been proved to be satisfactory; tyre, as a die-hard waste, has now found an extra way for reuse. Intact tyre can be filled into the noise barrier to provide air gap for the powder tyre and give sufficient reduction of sound between 250Hz and 500Hz which is the dominant frequencies of traffic noise. From the results of application to noise enclosures and barriers, it can be seen that the panel with tyre components as absorption material can effectively reduce noise reflection. On the other hand, powder tyre cost very little intact tyre even cost nothing. Compared to fragile conventional absorption material that is easy damaged, absorptive material made of waste type is economical, highly durable and effective without any adverse effects. Conclusively, the project demonstrate that rubber tyre 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.