Annually, more than a quarter billion of automobile tires are disposed in the U.S.A., and most of them are dumped in landfills[1]. What to do with the disposed tires becomes a problem for environmental protection more and more each day. It appears that the best way to solve this problem is to re-use the scrap tires in production of various products. The first step moving toward the reuse of scrap tires is to re-process (shred) them into small pieces with the fiber and metal removed, which is called crumb rubber. Crumb rubber can be used in a wide variety of rubber material applications. The industrial-scale based technology for re-processing scrap tires into crumb rubber started in the 1950's and became mature in the 1970's. At present in the U.S.A., a few dozen of re-processing plants are in operation, and they provide crumb rubber with different size gradations as a commercial commodity. Now the issue becomes how to "consume" crumb rubber.

Currently the biggest customer for consuming crumb rubber is the asphalt pavement industry. Adding crumb rubber into asphalt forms crumb rubber modified asphalt (CRMA). This rubberized asphalt then can be used as part of the hot mix asphalt (HMA) to build roads, highways, and other facilities like parking lots, etc. However, CRMA costs more as much as 30% in some cases on a ton-per-ton basis in comparison with "conventional" asphalt pavement that is without any additives. In addition, the demand for asphalt is quite flat in recent years because few new roads and highways are built, and the volume of asphalt needed in repairing roads and pavements is limited. Also, the market demand for CRMA rests solely in the hands of materials engineers in each independent state, county or municipal paving agency. If a paving agency does not use CRMA, then the market in that region would be non-existent. As such, the whole amount of crumb rubber used in CRMA can only take a small percentage out of the total stock of scrap tires under current market conditions. Currently, only about 10 percent of the scrap tires in the USA are converted into crumb [2]. A comprehensive study on the current progress of rubberized asphalt pavement can be found in the article prepared by Carlson and Zhu[3].

Other products that consume crumb rubber include garden hoses, floor mats, horse trails, playground chips, roofing materials and others. In a number of states, scrap tires are burned for generating electricity or as a fuel supplement in the place of coal or other materials. But the total quantity of scrap tires and crumb rubber these products and processes can utilize is also not enough to stem the flow of scrap tire build-up. Therefore, the current situation is that the rate of automobile tire disposition is much greater than the rate of those disposed tires being reprocessed or reused. Consequently, many tire reprocessing plants have huge stockpiles. From time to time, news reports of scrap tire piles burning can be heard/seen, and such burning causes a severe damage to the environment with respect to air pollution and ground water contamination.

Faced with the increasing pressure of this problem, many efforts have been made to find solutions. In fact, as part of these efforts, this article reflects the progress made in a project which is sponsored by Arizona Department of Environmental Quality beginning in March, 1998. The objective of this project is to research and develop new products that can use large quantities of crumb rubber. The theme is that new crumb rubber based products must be developed by using new and advanced technologies. Such technologies will make these products competitive and promising in finding markets with high demand. The crumb rubber application in highway noise barriers is one area of the progress resulting from this project.