Introducing and developing the spray technology in manufacturing crumb rubber panels for the application in highway noise reduction is one major area of the progress made in this study. This article devotes most of its analyses to address the technical aspect of the necessity and feasibility in developing such a technology. Although much progress has been made in this respect, a few tasks are needed to further advance this technology.

The first one is the panel design. Current specimens are made all with a flat surface and an about 0.4 acoustical absorption can be expectedly achieved for such a flat-surface design. The effort is needed to continue to improve the acoustical absorption. Two possibilities may be tried. One is the surface design with a rough surface texture, for example, a popcorn type of surface texture. The other one is a contoured surfacing design. In addition, from what Figure-3 shows is that the flat rubber panel can do better in the range of high frequencies with respect to acoustical absorption, now the focus will be placed to the low end of the frequency band. Hopefully, these two surface configurations will help to improve the noise reduction capability, particularly with the low band frequencies. Both analytical and experimental methods will be used in the two types of new surface design. The experimental method is conducting the tests of ASTM C423-90a on the panel specimens. However, because the testing fee for ASTM C423-90a is quite high, the analytical method will be more emphasized to evaluate the acoustical absorption nature of the two surface textures so only a limited number of and well defined ASTM C423-90a tests will be needed to achieve the goal.

The second task is to develop a mix design with respect to the size distribution of rubber particles. Though the idea is known that using multi-sized rubber particles in the panels can achieve a better sound absorption effect, the quantification of such size gradation is still not clear. Particularly, the question is unanswered whether or not if more large rubber particles are used, the acoustical absorption at the low band frequencies will be improved. The information obtained from the current study is inadequate in this respect, and more investigations are needed to develop an adequate understanding and quantification with the size gradation versus absorption.

The third task is to continue to "maturate" the development of the spray device and process technology. One element in the task is to establish a guidance of the spray process so the reliable and repeatable spray operation can be achieved. The other element is to integrate the four spray devices so far studied and to select/develop a spray device that can best accommodate the need for the current application. The third element is to develop a mobile spray unit that can be taken to highway locations. It is seen that most work done so far in this study is basically research and development (R&D) oriented. To move from R&D to the industrial application is both a quantum leap and a big challenge. In this respect, though we have a good knowledge on four spray devices tested in this study, there is much work to be done in designing and building a spray unit that can handle a large volume of the mix. This is the last element in this task.

The current study mainly focuses on the subjects that are related to the crumb rubber technology in its highway noise reduction application. However, any serious technology and product development must contain a cost analysis, and here a brief cost analysis is given. As a commodity, crumb rubber is about $0.2 to $0.3 per pound, and acrylic polymer is also at this price range, or a little bit higher. Regular spray devices do not require millions of dollars of investment. A typical industrial capacity spray gun costs $100k level or less. Therefore, though these cost estimates do not constitute a complete cost analysis, they may provide an overall sketch of what might be expected for financing this technology-product development.

This article presents a quite detailed study on the recent development of the crumb rubber technology in its highway noise reduction application. It can be seen that developing such a technology requires the effort in many different issues and subjects. Based on what is presented in this article, though facing many challenges, it appears that the future of this technology is promising and significant, because the demand for this technology will be high and it can address two environmental problems simultaneously.