An evaluation of permanent deformation properties of crumb rubber modified (CRM) asphalt concrete mixtures

Abstract

The disposal of waste car and truck tires in the U.S. is becoming increasingly problematic, as is true for all forms of non-biodegradable solid waste. Over 200 million tires are discarded annually, of which only 34 percent are recycled in various applications, while the remainder is dumped in stockpiles as scrap. Tire stockpiles pose an environmental threat as fire and health hazards. As a result, government agencies have taken action to mandate the recycling of waste tire rubber by state transportation agencies in highway construction. One recycling alternative is the incorporation of crumb rubber modifier (CRM) in hot mix asphalt concrete (HMAC) mixtures to form crumb rubber-modified asphalt concrete (CRMAC). CRM is introduced to HMAC mixtures as an aggregate through the "dry process", or else as a binder modifier using the "wet process". Experience with experimental CRMAC pavements have shown that under high temperatures, permanent deformation is severe. It has been suggested that the behavior of these mixtures is affected not only by the amount of rubber present in the CRMAC, but also by age, i.e. whether the CRMAC is being used as virgin or reclaimed asphalt pavement (RAP). The scope of this thesis characterizes physical properties of CRMAC mixtures that are associated with permanent deformation. These include Hveem stability, air voids, permeability, creep response, and compressive strength. Three CRMAC mixtures were used, each containing different amounts of RAP. The study aimed at defining how these properties are affected by the RAP content of the mixtures.