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 spray.
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 Mesh Metric 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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