The use of asphalt-rubber as an additive to paving material has the potential for becoming the most dominant means of recycling old tires and for solving a difficult tire disposal problem. Currently, about 61% of the 200 million tires produced annually are disposed of at landfills. Of the remaining, about half or 20% are retreaded, 6% are converted into reclaimed rubber, which is used to make new tires, asphalt-rubber and other molded rubber products and 13% are used for slitting, grinding and miscellaneous uses. The potential for increasing the number of retreads and for using a higher amount of reclaim in producing new tires is limited. It is claimed that most of the good old tires are already retreaded and sold as used tires. During the last two decades, the demand for better quality and longer lasting tires has increased steadily. Because reclaim adversely affects the quality of a tire, its use in making new tires has decreased substantially. Industry sources indicate that some reclaimed rubber is used in radial tires, but no tire manufacturer has admitted to its use.
Alternative ways of using old tires have been tried unsuccessfully in the past. For example, a pyrolysis plant was built by Goodyear and Tosco to recover oil and carbon black. The carbon black was not of high enough quality for use in tire manufacture and the process was uneconomic. Even the rise in oil prices over the last few years has not renewed the interest in the process. Tires can be used as fuel to generate energy but there are two problems. Chopped up tires compete against coal and are often more costly on a Btu lb.-', basis. Whole tires were burned by Goodyear during the 1975 -1977 period in special furnaces but the practice ran into a variety of environmental problems. Microwave devulcanization is being practised by Goodyear to reclaim the relatively homogeneous butyl rubber found in inner tire tubes and to displace up to 26% of the virgin rubber used in hose compounds. The process, however, has not been adopted to devulcanize tires which contain non-homogeneous compounds. Another process called the Gould Process is used to obtain a very fine powder from scrap rubber. The powder in used as a substitute for virgin rubber in industrial products. Its use in making tires is in experimental stages.
The use of asphalt-rubber as a road paving material has now been practised for about 15 years. The results show that its use could increase substantially in the near future. This paper investigates the technical, cost and administrative aspects of asphalt-rubber use in roads.
FAILURE OF ASPHALT ROADS -DESCRIPTION OF THE PROBLEM
A pavement, during its life time, is subjected to tensile, shear, compression and flexural stresses due to (1) traffic loads, (2) pavement temperature changes and (3) subgrade and subbase volume changes due to fluctuations in moisture content and temperature. The ability of a pavement to withstand these stresses depends on various strength characteristics including the tensile, shear