A comprehensive review of the literature covering more than 100 published journal articles, conference proceedings, and reports found that although considerable research has been undertaken to understand the advantages and disadvantages of using recycled tire rubber to modify asphalt binders, no published information on PG+X-type initiatives (i.e., focused more on using additional waste tires in asphalt mixes rather than on improving performance of the binder and mix) was found. A number of states have specifications that allow tire rubber as a substitution for styrene-butadiene-styrene (SBS) modification (e.g., California [PG-M], Florida [PG-ARB], and Louisiana [PG-CRM]). The quantities of rubber added and the properties of the rubber particles used are similar to one of the four approaches discussed in this Technical Memorandum: PG+X Approach-1. No published research on adding very small quantities (i.e., less than 0.5 percent by total weight of the mix) in a dry process was located. Preliminary indications from this literature review and from early laboratory testing as part of the study include the following:
∙ The properties of binders modified according to Approach-1, Approach-2, and Approach-4 properties are likely to be influenced by both rubber particle content and rubber particle size. The properties of mixes prepared using Approach-3 are also likely to be influenced by these parameters.
∙ It is unlikely that the PG grading of the PG+X binders prepared according to Approach-1, Approach-2 and Approach-4 will be the same as the base binder. A one grade bump can be expected if five percent rubber by weight of the binder is added and two grade bumps are possible if ten percent rubber is added.
∙ In Approach-2 and Approach-3, the use of smaller rubber particles (i.e., less than 1.0 mm) will probably have less effect on the binder and mix properties than the use of larger particles (i.e., up to 2.36 mm).
∙ Although the objective of the PG+X initiative is to use more recycled tire rubber in asphalt pavements, some benefits in terms of improved rutting, cracking, and moisture resistance performance are still likely despite the small quantities of rubber used.