It appears that crumb rubber is a worthy material for its noise barrier applications. To achieve this noise barrier application objective, it is necessary to find out the technology to "manufacture" crumb rubber panels. Two manufacturing technologies can be used to reach this objective. One is to use the molding technology in which crumb rubber is placed in a mold mixed with a bonding agent. Then temperature and pressure will be exerted on the mold. Consolidation will take place in the mix of rubber particles with a bonding agent, and a molded solid type of panel will be made.
The molding method in making panels is a mature technology. However, it is not suitable for this application because of the following major reasons. The first reason is that the panels made by the molding technology usually are produced with a flat surface. To accomplish a better sound absorption characteristic, a flat surface is not the best choice. In fact, the recent design of noise barriers tends to have a contoured surface texture. As such, molding can not easily meet such a wavy contour requirement for the noise barrier design.
The second reason resides in the fact that making molds is time-consuming and expensive. To make a non-flat mold requires even more time and is more expensive. The design development of contoured noise-panel surfaces is still in its evolution stage, and many new contour design patterns are yet to come. Thus, the necessity in making different molds to accommodate the forthcoming plausible design changes makes the employment of the molding technology unattractive, if not impractical.
Molded panels are usually made in a solid state with low porosity because the molding process requires the exertion of high pressure. Typical molded parts with a good quality can have no higher than a few percent of air voids. For the noise reduction purpose, a higher porosity is preferable as aforementioned. This is the third reason.
The last major reason is that after almost a century's construction, there are thousands and thousands of miles of existing concrete highway noise barriers. The focus for highway noise reduction from the view of the transportation department in many states is rather on what can be done to improve the noise reduction capability for existing noise barriers than on the construction of better sound absorption new noise barriers. This is primarily because not many new highways and noise barriers are under construction or planned. It appears that the molding technology can help little in this regard.
In addition to those four major reasons, there are other factors that inhibit the use of the molding technology for the highway noise reduction application, for example, high cost in investing mold equipment, but they will not be detailed here.
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