Crumb rubber modifier is a general type of asphalt modifier that contains scrap tire rubber. Crumb rubber modified asphalt binder pavement products are produced from crumb rubber modifier by several techniques including a wet process and dry process. These crumb rubber modified asphalt binders may contain additional additives or modifiers (i.e., rubber polymers, diluents, and aromatic oils) besides scrap tire rubber.

The primary use of crumb rubber modified asphalt binders in pavement applications include crack and joint sealants; binders for chip seals, interlayers, and hot-mix asphalts; and membranes. The life cycle cost analyses presented in this paper is limited to wet processed crumb rubber asphalt binder as binders used for chip seals, interlayers, and hot-mix asphalt including dense-, gap, and open-graded gradations.

Chip seals
Charles H. McDonald pioneered the United States' development of the wet process (or reacted) crumb rubber modified asphalt binders in the 1960s (1). McDonald first used the asphalt rubber binder for a patching material and identified the operation as a "band-aid" repair technique in Phoenix, Arizona, in 1963. The binder system used for the "band-aid" patch was spray applied and the patch was a "localized chip seal" placed by hand over a limited pavement area. The first "large area" spray application in 1967 produced poor results because of the asphalt rubber's high viscosity relative to the asphalt distributor's capability to spray high viscosity materials. By reducing the crumb rubber modifier concentration, using diluents, and altering the asphalt distribution equipment, successful "large area" spray applications were placed in Arizona in the 1970s (1).

These chip seal coat applications became known as stress-absorbing membranes (SAM). Arizona DOT used CRM asphalt binders or asphalt rubber for chip seals through the 1970s and early 1980s (2) and continue their use on a limited basis today. Other public agencies, including Caltrans, the Texas DOT, and many local governments, continue to use asphalt rubber chip seals.

Asphalt rubber chip seals overlaid with hot-mix asphalt are known as stress-absorbing membrane interlayers (SAMIs). The Arizona DOT placed its first SAMI in 1972 as part of a project to evaluate techniques to reduce reflection cracking (2). Historically, SAMI development followed SAM development. Arizona placed a relatively large number of SAMIs in mid- to late-1980s and a reduced number in the 1990s. Arizona, and other public agencies, continue to use SAMIs today.

Hot-mix asphalt
Crumb rubber modifiers have been used in asphalt binders for hot-mixes since the 1960s (1). They have contained binders prepared from both the wet process (asphalt rubber) and the dry process (rubber modified). Sahuaro, ARCO, Crafco, International Surfacing, and others have supplied asphalt rubber binder for hot-mix applications. The dry process or rubber modified hot-mixes have been supplied by PlusRide or manufactured under the control of public agencies. Dense-, open-, and gap-graded aggregates have been used with crumb rubber modifiers.

Use of CRM in hot-mix asphalt increased substantially in the early 1990s due in large part to the mandate imposed in ISTEA. A survey of state highway administrations conducted by AASHTO in January 1993 indicated that 21 states used CRM in hot-mixes in 1992. However, since the mandate was repealed, the use of asphalt rubber had dropped or ceased in many parts of the United States.

Currently, the majority of crumb rubber binder used in hot-mix asphalt is placed in the states of Arizona, California, Florida, and Texas. Arizona DOT and local governments in Arizona primarily use asphalt rubber binder in open-graded and gap-graded hot-mixes. The use of asphalt rubber binder in open-graded friction courses is now the most popular use of this type of binder by the Arizona DOT. Arizona first placed hot-mix asphalt containing asphalt rubber in 1975. California DOT uses asphalt rubber in dense-, gap-, and open-graded hot mix asphalt. California DOT and local governments in southern California utilize asphalt rubber binders in gap- and open-graded mixtures. Texas DOT uses asphalt rubber primarily in gap-graded mixture identified as coarse matrix, high binder (CMHB) (3).

Florida DOT uses a fine ground rubber at typically 6-12% by weight of asphalt binder in dense- and open-graded hot mixtures. These binders are not asphalt rubber as defined by ASTM (3).

Cost and performance information
The National Cooperative Highway Research Program (NCHRP) completed a Synthesis of Practice on recycled rubber tires in highways in 1994 (1). The Synthesis is based on a review of nearly 500 references and on information from state highway agencies'' responses to a 1991 survey of practice with updates through 1993. A portion of this Synthesis is devoted to the use of crumb rubber modifier (CRM) in paving applications. Specific sections of this report summarize information on performance and life cycle costs associated with chip seals, interlayers, and hot mix asphalt. However, the cost and performance information included is based on performance of sections constructed in the 1970s and 1980s only, since it was derived using interviews with agencies and the review of literature through 1991. A more thorough development of cost and performance was accomplished as part of this study.