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.
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.
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
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
Cost and performance
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.