This document is a concise overview of the terminology, processes, products, and applications of crumb rubber modifier (CRM) technology. This technology includes any use of scrap tire rubber in asphalt paving materials. In general, CRM technology can be divided into two categories, the wet process and dry process. When CRM is incorporated into an asphalt paving material, it will modify the properties of the binder (asphalt rubber) and act as a rubber aggregate (rubber-modified hot-mix asphalt). The use of asphalt rubber crack and joint sealant is common across the country and is routinely used by manystate maintenance crews. A surface treatment using an asphalt rubber spray application is called a stress-absorbing membrane (SAM). Theuse of CRM in hot-mix asphalt paving materials has broad variability and potential. Composite designs with CRM paving materials are a two-layer system and a three-layer system. The growing nationwide interest in alternative uses for scrap tires has provided CRM technologists with the catalyst to develop new concepts for applying CRM. Themajor interest has been to develop generic dry process mixes and a continuous blending wet process. There are two principal unresolved issues related to the use of CRM in asphalt paving materials. On thenational level, the ability to recycle asphalt paving mixes containing CRM has not been demonstrated. At the state and local level, these modified asphalt mixes must be field evaluated to establish expected levels of performance.
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