Glossary of Rubberized Asphalt Terms


The American Association of State and Highway Transportation Officials.
Ambient grinding
method of processing where scrap tire rubber is ground or
processed at or above ordinary room temperature. Ambient processing is typically
required to provide irregularly shaped, torn particles with relatively large surface areas
to promote interaction with the paving asphalt.
Ambient Ground Rubber
processing where scrap tire rubber is ground or
processed at or above ordinary room temperature.
any various natural or synthetic, dark colored, bituminous
substances composed mainly of hydrocarbon mixtures. For purposes of this guide
asphalt is the binder or asphalt cement used in various manners in the paving industry.
It is a product derived from the petroleum refining industry which may contain various
additives and modifiers before its final use in the paving industry. It is an adhesive,
glue, or black and sticky material that is used in seal coating and hot mix paving.
is used as a binder in various types of flexible pavement
construction including surface treatments and hot mixes. According to the ASTM
definition (ASTM D 8, Vol. 4.03, "Road and Paving Materials" of the Annual Book of
ASTM Standards 2001) asphalt-rubber is "a blend of asphalt cement, reclaimed tire
rubber, and certain additives in which the rubber component is at least 15 percent by
weight of the total blend and has reacted in the hot asphalt cement sufficiently to
cause swelling of the rubber particles". In addition asphalt-rubber physical properties
fall within the ranges listed in ASTM D 6114, "Standard Specification for Asphalt-rubber Binder," also located in Vol. 4.03. Recycled tire rubber or scrap tire crumb
rubber is used for the reclaimed tire rubber portion of asphalt-rubber binder. The
asphalt-rubber is formulated and reacted at elevated temperatures and under high
agitation to promote the physical interaction of the asphalt cement and scrap tire
crumb rubber constituents, and to keep the scrap tire crumb rubber particles
suspended in the blend. Asphalt-rubber contains visible particles of scrap tire rubber.
Asphalt-rubber is typically used as either a Type 1 or Type 2 formulation. For purposes
of this guide both Type 1 and 2 are considered as equal.

For additional information about asphalt-rubber go to the Rubber Pavements
Association (RPA) website,, and click on Asphalt Rubber
Standard Practice Guide First Edition

Asphalt-rubber Type 1
consists of asphalt and crumb rubber from scrap tires
only and no other additives, modifiers or extenders. Type 1 is primarily used in Arizona
as well as Texas and Florida.
Asphalt-rubber Type 2
consists of a maximum of 85 % asphalt combined
with a minimum of 15 % rubber. The rubber portion consists of 75% crumb rubber from
scrap tires and 25% from a high natural rubber source. An extender oil is added to the
combination of asphalt and rubber. The amount of extender oil is generally about 2%
of the total mixture. Type 2 is primarily used in California.
is asphalt-rubber asphalt concrete which consists of asphalt-rubber
binder hot mixed with a gap graded aggregate. The resultant mixture is placed as a
final wearing surface or base support layer. It is generally placed from 1-2 inches (25
to 50 mm) in thickness.
is asphalt-rubber friction course which consists of asphalt-rubber hot
mixed with an open graded aggregate. The resultant mixture is placed as a final
wearing course. It is generally placed 0.5 to 1.0 inch (12.5 to 25 mm) in thickness.
is American Society for Testing and Materials.
Automobile tires
tires with an outside diameter less than 26 in. (660 mm)
used on automobiles, pickups, and light trucks.


is any of various natural substances, as asphalt, consisting mainly of
hydrocarbons. For purposes of this guide asphalt, asphalt cement and bitumen refer to
the binder used in various manners in the paving industry and is derived initially from
the petroleum refining process.
Buffing waste
is a high quality scrap tire rubber that is a byproduct from the
conditioning of tire carcasses in preparation for re-treading. Buffing waste contains
essentially no metal or fiber.


California Department of Transportation.
apparatus typically used for ambient grinding and a process that
tears apart scrap tire rubber by passing the material between rotating corrugated steel
drums, reducing the size of the rubber to a crumb particle (generally 4.75-millimeter to
425-micron (No.4 to No.40) sieve).
Crumb Rubber Modifier
a general term for scrap tire rubber that is reduced
in size and is used as a modifier in asphalt paving materials.
Cryogenic/Cryogenic grinding
process that freezes the scrap tire rubber
and crushes the rubber to the desired particle size. The process uses liquid nitrogen to
freeze the scrap tire rubber until it becomes brittle and then uses a hammer mill to
shatter the frozen rubber into smooth particles with relatively small surface area. This
method is used to reduce particle size prior to grinding at ambient temperatures.


refers to a continuously graded aggregate blend typically used
to make hot-mix asphalt concrete with conventional or modified binders.
Devulcanized rubber
rubber that has been subjected to treatment by heat,
pressure, or the addition of softening agents after grinding to alter physical and
chemical properties of the recycled material.
a lighter petroleum product (typically kerosene or similar product with
solvent-like characteristics) added to asphalt-rubber binder just before the binder is
sprayed on the pavement surface for chip seal applications. The diluent thins the
binder to promote fanning and uniform spray application, and then evaporates over
time without causing major changes to the asphalt-rubber properties. Diluent is not
used in asphalt-rubber binders that are used to make asphalt concrete, and is not
recommended for use in interlayers that will be overlaid with asphalt concrete in less
than 90 days due to on-going evaporation of volatile components.
Dry Process
any method that mixes the crumb rubber modifier with the
aggregate before the mixture is charged with asphalt binder. This process only applies
to hot mix asphalt production.


Environmental Protection Agency responsible for administering all
federal environmental rules and regulations nationwide.
Extender oil
aromatic oil used to promote the reaction of the asphalt binder
and the crumb rubber modifier.


Federal Highway Administration that administers all federal funded road
building projects in accordance with federal laws and regulations.
Flush coat
application of diluted emulsified asphalt onto a pavement surface
to extend pavement life that may also be used to prevent rock loss in chip seals or
raveling in asphalt concrete.


aggregate that is not continuously graded for all size fractions,
typically missing or low on one or two of the finer sizes. Gap grading is used to
promote stone-to-stone contact in hot-mix asphalt concrete. This type of gradation is
most frequently used to make rubberized asphalt concrete-gap graded in Arizona it is
referred to as ARAC and in California RAC-G paving mixtures.
produces cubical, uniformly shaped, cut crumb rubber particles
with a low surface area.
apparatus that shears apart the scrap tire rubber, cutting the
rubber with revolving steel plates that pass at close tolerance, reducing the rubber to
cubicle particles generally 3/8 in. to No. 10 (9.5 mm to 2.0 mm) sieve in size.
Granulated rubber
cubical, uniformly shaped, cut crumb rubber particle with
a low surface area which are generally produced by a granulator.
Ground crumb rubber modifier
irregularly shaped, torn scrap rubber
particles with a large surface area, generally produced by a crackermill.
Ground tire rubber
crumb rubber derived from the recycling of scrap tires,
also commonly referred to as GTR.


is hot mix asphalt. It is derived by mixing asphalt and aggregate and an
elevated in a hot plant and placing the resultant mix as the asphalt pavement. It may
be referred to as asphalt paving or blacktop.
High natural rubber
scrap rubber product that includes 40 to 48 percent
natural rubber or isoprene and a minimum of 50 percent rubber hydrocarbon according
to Caltrans requirements. Sources of high natural rubber include scrap tire rubber from
some types of heavy truck tires, but are not limited to scrap tires. Other sources of high
natural rubber include scrap from tennis balls and mat rubber.


commonly used term for the interaction between asphalt binder
and crumb rubber modifier when blended together at elevated temperatures. The
reaction is more appropriately defined as polymer swell. It is not a chemical reaction. It
is a physical interaction in which the crumb rubber absorbs aromatic oils and light
fractions (small volatile or active molecules) from the asphalt binder, and releases
some of the similar oils used in rubber production into the asphalt binder.


Lightweight aggregate
porous aggregate with very low density such as
expanded shale, which is typically manufactured. It has been used in chip seals to
reduce windshield damage.


process that further reduces a crumb rubber to a very fine ground
particle, reducing the size of the crumb rubber below a 425-micron (No. 40) sieve.


aggregate gradation that is intended to be free draining and
consists mostly of 2 or 3 nominal sizes of aggregate particles with few fines and 0 to 4
percent by mass passing the 200 (0.075 mm) sieve. Open grading is used in hot-mix
applications to provide relatively thin surface or wearing courses with good frictional
characteristics that quickly drain surface water to reduce hydroplaning, splash and
Open graded friction course, see open-graded.


Portland Cement Concrete pavement.
is a form of AASHTO and ASTM grading of asphalt by the climatic zone
such that the asphalt meets the required properties for the climatic zone.


Rubber Asphalt Foundation located in Scottsdale, Arizona.
the interaction between asphalt cement and crumb rubber modifier
(CRM) when blended together at a certain temperature for a certain period of time.
The reaction, more appropriately defined as polymer swell, is not a chemical reaction.
It is the absorption of aromatic oils from the asphalt cement into the polymer chains of
the crumb rubber.
Recycled tire rubber
rubber obtained by processing used automobile, truck,
or bus tires (essentially highway or “over the road” tires). The Caltrans chemical
requirements for scrap tire rubber are intended to eliminate unsuitable sources such
as solid tires; tires from forklifts, aircraft, and earthmoving equipment; and other nonautomotive tires that do not provide the appropriate components for asphalt-rubber
interaction. Non-tire rubber sources may be used only to provide High Natural Rubber
to supplement the recycled tire rubber.
Rubber Pavements Association located in Tempe, Arizona.
Rubber Aggregate
Crumb rubber modifier added to hot mix asphalt mixture
using the dry process which retains its physical shape and rigidity.
Rubber-Modified Asphalt Concrete
hot mix asphalt concrete mixture with
dense graded aggregates using a rubberized asphalt type of binder. (Note: The CRM
percentage is generally low (5 to 10%) and is generally the finer mesh (30 mesh size
or smaller).
Rubberized asphalt
is a general term that refers to a broad family of asphalt
binder products that contain scrap tire rubber that may or may not comply with the
ASTM definition of asphalt-rubber (ASTM D 8, Vol. 4.03). This terminology is often
quoted in the press for virtually any asphalt binder that contains some amount of
ground tire rubber derived from scrap tires.
Rubberized asphalt concrete (RAC)
Caltrans terminology for a material
produced for hot mix applications by mixing asphalt-rubber or rubberized asphalt
binder with graded aggregate. RAC may be dense-, gap-, or open-graded.


Sieve Sizes
Sieve sizes various nomenclature;

Sieve Size MeshMetric US Customary
25 mm1 inchNA
19 mm 3/4 inch NA
12.5 mm 1/2 inch NA
9.5 mm 3/8 inch NA
6.4 mm 1/4 inch 3
4.75 mm #4 4
2.36 mm #8 8
2 mm #10 9
1.68 mm #12 10
1.18 mm #16 14
0.8 mm #20 20
0.6 mm #30 30
0.4 mm #40 35
0.3 mm #50 48
0.15 mm #100 100
75 micron #200 200

Strategic Highway Research Program which is a long term research
effort supported by the AASHTO and FHWA to develop new tools, tests and
procedures to design and build longer lasting pavements.
Stress-absorbing membrane (SAM)
the abbreviation for a Stress Absorbing
Membrane. A SAM is used primarily to mitigate reflective cracking of an existing
distressed asphalt or rigid pavement. It is usually associated with an asphalt-rubber
binder sprayed on an existing pavement surface at .60 gallons per square yard (2.9
liters per square meter) and immediately followed by an application of a uniform pre-coated aggregate, which is then rolled and the aggregate is embedded into the binder
layer. The nominal thickness normally ranges between 1/4 and 3/8inch (6 and 9 mm).
Stress Absorbing Membrane Interlayer (SAMI)
for purposes of this guide a
SAMI is an asphalt-rubber membrane beneath an overlay designed to resist the stress
strain of reflective cracks and delay the propagation of the crack through the new
overlay. It is noted, however, that the term SAMI has also been used to include an
interlayer of asphalt-rubber chip seal (SAMI-R), fabric (SAMI-F), or fine unbound aggregate, thus care must be taken in reading reports to determine the type of SAMI
being described.
process that reduces scrap tires to pieces 6 inches (0.15 meter)
square and smaller prior to granulation or ambient grinding.


Terminal blend
is a form of the wet process where scrap tire rubber is
blended with hot asphalt binder at the refinery or at an asphalt binder storage and
distribution terminal and transported to the asphalt concrete mixing plant or job site for
use. This type of rubberized binder reportedly does not require subsequent agitation to
keep the scrap tire rubber particles evenly dispersed in the modified binder. In the
past, such blends normally contained 10 percent or less finely ground scrap tire rubber
by mass (which does not satisfy the ASTM D 8 definition of asphalt-rubber) and other
additives to eliminate the need for agitation. However, new formulations have
reportedly been developed that contain 15 percent scrap tire rubber by total binder
mass but may not meet the asphalt-rubber physical properties contained in ASTM D
6114. In addition the crumb rubber particles are reduced to such a small size by
various means that they are not visible.
Tread Rubber
rubber that consists primarily of tread rubber with less than 5
percent sidewall material (approximately).
Tread peel
pieces of scrap tire tread rubber that are also a by-product of tire
re-treading operations that contain little if any tire cord.
Tread rubber
scrap tire rubber that consists primarily of tread rubber with
less than approximately 5 percent sidewall rubber.
Truck tires
tires with an outside diameter greater than 660 mm (26 inches)
and less than 60 inches (1520 mm) used on commercial trucks and buses.


is the property of resistance to flow (shearing force) in a fluid or
semi-fluid. Thick stiff fluids such as asphalt-rubber have high viscosity; water has low
viscosity. Viscosity is specified as a measure of field quality control for asphalt-rubber
binder production and its use in rubberized asphalt concrete mixtures.
Vulcanized rubber
is a crude or synthetic rubber that has been subjected to
treatment by chemicals, heat and/or pressure to improve strength, stability, durability,
etc. Tire rubber is vulcanized primarily through the use of sulfur. The odor associated
with asphalt-rubber is associated with the use of sulfur in the vulcanized rubber.


Warm Mix Asphalt
Warm mix asphalt (WMA) is the name given to certain
technologies that reduce the production and placement temperatures of asphalt mixes.
Generally, the placement temperatures is between 185°F and 275°F for an asphalt to
be considered warm mix asphalt.
Wet process
any method that blends scrap tire rubber with the asphalt
cement before incorporating the binder into the asphalt paving materials. Although
most wet process asphalt-rubber binders require agitation to keep the scrap tire rubber
evenly distributed throughout the binder, terminal blends may be formulated in a wet
process manner so as not to require agitation.
Whole tire rubber
scrap tire rubber that includes tread and sidewalls in
proportions that approximate the respective weights in an average tire.

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