Many investigators have found that the strength of the paving mixes can be enhanced by use of a binder formed by modifying available bitumen with certain additives like sulphur and organic polymers. The modified polymers also improve temperature susceptibility and viscosity characteristics and help alleviating some common problems like bleeding of binder during peak summer temperature and stripping of aggregate in moisture prone areas. The polymers used for modification of bitumen for paving purposes are generally styrene-butadiene-styrene copolymer, styrene-butadiene rubber, latex, ethylene vinyl acetate copolymer, polyethylene, polypropylene, etc. Most of these polymers, besides being costly, are not available very commonly. Due to these reasons reclaimed rubbers mainly in the form of used tires have been found to perform satisfactorily for years. The cause of rubber as an additive in asphalt has been discussed and researched for the last 30 years the study carried out in the U. S. can be delineated as follows.
Each year approximately 285,000,000 scrap tires are added to stockpiles, landfills, or illegal dumps across the U. S (Heitzman 1992). It is estimated that the present size of the scrap tire problem is 2-3 billion tires. The introduction of scrap rubber into asphalt pavement has the potential for reducing this waste problem. It has been estimated that if only 10% of all asphalt pavements laid each year in the U. S. contained 3% rubber, all scrap tires produced for that year in this country would be utilized. The potential benefits that this cost-effective product would bring have kept the interest in asphalt-rubber (AR) high throughout the U.S.
The variable studies were indirect tensile strength, peak centre carried load, load point displacement, and unit fracture energy. The results show that higher mean values of indirect tensile strength and fracture energy were obtained corresponding to the higher AR binder content (9%), irrespective of rubber binder contents, particularly at lower temperatures. A statistical analysis of the test results reveals that there are no significant differences among the means of most of the variables studied corresponding to different rubber contents. However, the differences are significant for different binder content.
Asphalt-rubber production can be broken down into "wet" and "dry" processes. The wet process uses the rubber as an additive to the asphalt binder. In this process, anywhere from 10 to 30% rubber, by weight, can be introduced into the binder at a high temperature, and the rubber is allowed react with the binder. The reaction time is usually recommended by the rubber supplier. The resulting AR is typically used in hot mixture, hot laid asphalt concrete, but it can also be used in stress absorbing membrane interlayer, where spray-type applications are common.
The dry process uses rubber as an aggregate. Typically 2-3% rubber is pre-blended with the aggregates prior to charging the mixture with a pure asphalt binder. The most common mixture design for this product has been patented under the trade name "Plus Ride". A generic system, called the TAK system, was developed recently and used on a few construction projects. In Kansas it was added dry, ultra fine rubber to an asphalt concrete mixture in a generic form and has constructed several projects.
The above things as a guideline for further renovation of the strength of bitumen, it was felt that an effort can be made to use polyethylene and kerosene for modification of bitumen in the present investigation.
