Rubber Modified Asphalt Binders

Crumb Rubber Modifier Technology

Highway agencies have been evaluating crumb rubber modifier (CRM) technology applicatioons at different levels of development since the 1970's. Reports have been written to document their findings and observations, but the diversity of terminology makes it difficult to determine the true benefit of a given product. In 1991, FHWA introduced standard terminology to improve the ability to communicate the experience of highway agencies who were evaluating different CRM technologies. The standard terminology has been expanded to include promising new innovations. This report defines the standard terminology and summarizes it in figure 1 below.

Crumb rubber is recylced rubber that has been reduced in size by mechanical shearing or grinding. Crumb rubber modifier is crumb rubber derived from scrap tire rubber that has been reduced to particle sizes less than 6.3mm (¼") and is used in asphalt paving. The methods of producing crumb rubber impart different shape and texture characteristics to each particle. The size, shape, and texture of the CRM have a significant effect on the performance of the asphalt pavement.

CRM is incorporated with asphalt paving materials by one of two construction processes: a wet process or a dry process. The wet process blends the CRM into the asphalt cement to modify the properties of the binder. The method of blending can generally be divided into three categories: batch blending, continuous blending, and terminal blending. Batch Blending defines those wet process technologies that mix batches of CRM and asphalt in production. Continuous blending describes those wet process technologies that have a continuous production system. Terminal blending is associated with wet process technologies that have products with extended storage (shelf life) characteristics and are prodiced at an asphalt cement supply terminal. The terminal blending technologies may use either a batch blending or continuous blending system to actually produce the product at the terminal.

An asphalt cement binder that has been modified with CRM is called asphalt rubber (AR) and can be used in a number of asphalt paving products. The binder modification is achieved through an interaction of the asphalt cement and the CRM, which is commonly referred to as a reaction. The degree of binder modification depends on many factors, including size and texture of the CRM, the proportion of asphalt cement, time and temperature of reaction, degree of mechanical energy during blending and reaction, and the use of other additives. Experienced companies, such as Blacklidge Emulsions, Inc., can assist design engineers and owners with the correct selection of mixture and processes that will achieve desired results.