Crumb Rubber Modifier in Asphalt Pavement

Appendix A

Glossary

Terms associated with waste tires and crumb rubber modifier have been defined by The Federal Highway Administration (FHWA),(14) ASTM, Lagrone,(15) and Witczak.(16) These terms as defined by these authors are given below in a consolidated glossary.

Ambient ground rubber- processing where scrap tire rubber is ground or processed at or above ordinary room temperature.

ARCO-ARM-R-SHIELD (Arizona refining process) - an asphalt-rubber blend process that was developed in 1975. The blend is composed of approximately 20 percent rubber (of which 40 percent is devulcanized and 60 percent ground ambient vulcanized) and 80 percent AR-4000/ 8000 with 2 to 4 percent Witco extender oil. The granulated rubber has gradings in which 98 percent pass the No.16 mesh and 8 percent pass the No.100 mesh. Diluents are not used routinely.

Asphalt rubber- asphalt cement modified with crumb rubber modifier.

Asphalt-rubber blend- a blend of ground tire rubber (generally finely ground No.16 to No.25 crumb rubber) and asphalt cement, which is used as the "binder" in various types of pavement construction. It generally consists of 18 to 26 percent ground tire rubber by total weight of the blend. The blend is formulated at elevated temperatures to promote the chemical and physical bonding of the two constituents. Various petroleum distillates or extender oils may be added to the blend to reduce viscosity, increase sprayability, and promote workability. The "blend" can be used as the binder in chip seals, seal-slurry coats, and dense- or open-graded asphalt hot-mix construction. When used in this manner, the aggregate gradation can generally conform to typical gradings used with conventional asphaltic concrete mixes. Asphalt-rubber blends can be produced directly at the plant site by adding ground rubber (18 to 26 percent) to the appropriate asphalt cement, and applying heat (1900C to 2180C [3750F to 4250F]) for 1 to 2 hours. Special equipment in the form of mixing chambers, reactor and blending tanks, and oversized pumps are needed. Two types of commercially available asphalt-rubber blends are used frequently: McDonald-Sah uaro (Crafco) process, and ARCO-ARM-R-SHIELD (Arizona refining process).

Asphalt-rubber concrete- implies the use of an asphalt-rubber blend (binder) with dense-graded aggregates in a hot-mix application.

Asphalt-rubber friction course- implies the use of an asphalt-rubber blend (binder) with open-graded aggregates in a hot-mix application.

Automobile tires- tires with an outside diameter less than 66 cm (26 in) used on automobiles, pickups, and light trucks.

Buffing waste- high quality scrap tire rubber which is a byproduct from the conditioning of tire carcasses in preparation for retreading.

Crackermill- 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 mm 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 modifier in asphalt paving materials.

Cryogenically ground rubber- process that freezes the scrap tire rubber and crushes the rubber to the particle size desired.

Devulcanized rubber- rubber that has been subjected to treatment by heat, pressure, or the addition of softening agents after grinding to alter properties of the recycled material.

Diluent- a lighter petroleum product (typically kerosene) added to asphalt-rubber binder just before the binder is sprayed on the pavement surface.

Dry process- any method that mixes the crumb rubber modifier with the aggregate before the mixture is charged with asphalt binder. This method applies only to hot-mix asphalt production.

Extender oil- an aromatic oil used to supplement the reaction of the asphalt and the crumb rubber modifier.

Granulated crumb rubber modifier- cubical, uniformly shaped, cut crumb rubber particle with a low surface area, which is generally produced by a granulator.

Granulator- process that shears apart the scrap tire rubber, cutting the rubber with revolving steel plates that pass at close tolerance, reducing the rubber to particles generally 9.5 mm to 2.0 mm (3/8 in to No.10 sieve) in size.

Ground crumb rubber modifier- irregularly shaped, torn crumb rubber particles with a large surface area, generally produced by a crackermill.

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

PlusRide- a patented form of a rubber-modified asphaltic mix. The product was developed in 1960 in Sweden and patented under the name PlusRide in the United States and Rubit in Sweden. It uses coarse rubber particles (6 mm to 0.6 mm [1/4 in to 1/16 in]) as rubber-filled aggregates, generally about 3 percent weight of mix. The rubber is added directly to a gap-graded aggregate so that a relatively dense-grading between the aggregate and rubber is obtained.

Reaction- the interaction between asphalt cement and crumb rubber modifier when blended together. 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. (Note: Solid tires; tires from fork lifts, aircraft, and earthmoving equipment; other nonautomotive tires; and nontire rubber sources are excluded.)

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- a hot-mix asphalt-concrete mixture with dense-graded aggregates using a rubber-modified asphalt.

Rubber-modified friction course- a hot-mix asphalt mixture with open-graded aggregates using a rubber-modified asphalt.

Rubber-modified hot- mix asphalt-hot-mix asphalt mixture that incorporates crumb rubber modifier primarily as rubber aggregate.

Rubberized asphalt- same meaning as wet process.

SAM- the abbreviation for a stress-absorbing membrane. A SAM is used primarily to mitigate reflective cracking of an existing distressed asphaltic or rigid pavement. It comprises an asphalt-rubber blend sprayed on the existing pavement surface followed immediately by an application of a uniform aggregate which is then rolled and embedded into the binder layer. Its nominal thickness generally ranges between 6 and 9 mm (1/4 and 3/8 in).

SAMI- the abbreviation for a stress-absorbing membrane interlayer. The interlayer may be an asphalt-rubber chip seal, fabric, fine unbound aggregate, or an open-graded asphalt layer. A SAMI is a SAM that is applied beneath an asphalt overlay (which may or may not contain rubber in the mix).

Shredding- process that reduces scrap tires to pieces 0.15 m2 (6 in2) and smaller.

Stress-absorbing membrane (SAM)- a surface treatment using an asphalt-rubber spray application and cover aggregate.

Stress-absorbing membrane interlayer (SAMI)- a membrane beneath an overlay designed to resist the stress and strain of reflective cracks and delay the propagation of the cracks through the new overlay. The membrane is often a spray application of asphalt-rubber binder and cover aggregate.

TLS- the abbreviation for three-layer system. It was developed by Arizona as a means of restoring the rideability of a badly cracked, warped, or faulted PCC pavement. The principle is equally valid for asphalt-concrete pavements. As currently used, the TLS consists of two thin (12.5 mm to 19 mm [1/2 in to 3/4 in]) conventional open-graded friction course layers placed between a low-modulus SAMI (approximately 9 mm [3/8 in] thick). The bottom open-graded friction course layer is placed directly on the existing pavement and functions, in part, as a leveling course. Early in the development of this system, other asphaltic mixes (e.g., dense-graded asphaltic concrete) were used in lieu of the open-graded course.

Tread rubber- rubber that consists primarily of tread rubber with less than approximately 5 percent sidewall rubber.

Truck tires- tires with an outside diameter greater than 66 cm (26 in) and less than 152 cm (60 in); used on commercial trucks and buses.

Vulcanized rubber- rubber that has been subjected to treatment by heat, pressure, or the addition of softening agents after grinding to alter properties of the recycled material.

Wet process- any method that blends crumb rubber modifier with the asphalt cement before incorporating the binder in the asphalt paving project.

Whole tire rubber- rubber that includes tread and sidewalls in proportions that approximate the respective weights in an average tire.

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