A Good RAP: Reclaimed Asphalt Pavement

Posted by Mike Cook on Mar 18, 2015

Reclaimed Asphalt Pavement (RAP) can be used as an aggregate and partial oil replacement in the hot recycling of asphalt paving mixtures. The most common method (conventional recycled hot mix) involves a process in which RAP is combined with virgin aggregate and new asphalt cement in a central mixing plant to produce new hot mix paving mixtures. 

Although some form of pavement recycling had been practiced as early as 1915, the first sustained efforts to recover and reuse old asphalt paving materials were conducted during 1974 in Nevada and Texas. Bolstered by the sponsorship of the Federal Highway Administration, (FHWA), more than 40 states performed and documented RAP demonstration projects between 1976 and 1982.

RAP is now routinely accepted in Hot Mix Asphalt (HMA) as an aggregate substitute and as a portion of the binder in nearly all 50 states. Substitution rates of 10 to 50 percent or more, depending on state specifications, are normally introduced in pavements, and recently developed technology has even made it possible to recycle 90 to 100 percent RAP in hot mix.

The use of processed RAP to produce conventional recycled hot mix (RHM) is the most common type of asphalt recycling and is now considered standard asphalt paving practice. There are abundant technical data available indicating that properly specified and produced recycled hot mix asphalt is equivalent in quality and structural performance to conventional HMA in terms of rutting, raveling, weathering, and fatigue cracking. Recycled HMA mixtures also generally age more slowly and are more resistant to the action of water than conventional HMA. 

The maximum limit for RAP content in RHM produced in conventional HMA batch plants is widely considered to be 50 percent, limited by both the heat capacity of the plants and gaseous hydrocarbon emissions. As much as 60 to 70 percent RAP may be processed in drum mix plants. Special plants  based on microwave technology have been developed to limit gaseous emissions from hot mix asphalt production using very high RAP contents (up to 100 percent RAP), but the cost of heating is much higher than that of conventional systems. This process was developed in California and has seen only limited use.

New Caltrans’ specifications that will begin to be implemented in the next few months will encourage additional use of RAP. The new standards will allow approximately 25% replacement in surface courses and up to nearly 40% in base courses. Graniterock currently supplies RAP from facilities in South San Francisco and Redwood City, serving customers with RAP mix needs in the San Jose area and  as far south as Gilroy.

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