ORIGIN
Approximately 280 million tires are discarded
each year by American motorists, approximately one tire for every person in
the United States. Around 30 million of these tires are retreaded or reused,
leaving roughly 250 million scrap tires to be managed annually. About 85
percent of these scrap tires are automobile tires, the remainder being truck
tires. Besides the need to manage these scrap tires, it has been estimated
that there may be as many as 2 to 3 billion tires that have accumulated over
the years and are contained in numerous stockpiles.
Scrap tires can be managed as a whole tire, a
slit tire, a shredded or chipped tire, as ground rubber, or as a crumb
rubber product.
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Whole Tires
A typical scrapped automobile tire weighs 9.1
kg (20 lb). Roughly 5.4 kg (12 lb) to 5.9 kg (13 lb) consists of recoverable
rubber, composed of 35 percent natural rubber and 65 percent synthetic
rubber. Steel-belted radial tires are the predominant type of tire currently
produced in the United States. A typical truck tire weighs
18.2 kg (40 lb) and also contains from 60 to 70 percent recoverable rubber.
Truck tires typically contain 65 percent natural rubber and 35 percent
synthetic rubber. Although a majority of truck tires are
steel-belted radials, there are still a number of bias ply truck tires,
which contain either nylon or polyester belt material.
Slit Tires
Slit tires are produced in tire cutting
machines. These cutting machines can slit the tire into two halves or can
separate the sidewalls from the tread of the tire.
Shredded or Chipped Tires
In most cases the production of tire shreds or
tire chips involves primary and secondary shredding. A tire shredder is a
machine with a series of oscillating or reciprocating cutting edges, moving
back and forth in opposite directions to create a shearing motion, that
effectively cuts or shreds tires as they are fed into the machine. The size
of the tire shreds produced in the primary shredding process can vary from
as large as 300 to 460 mm (12 to 18 in) long by 100 to 230 mm (4 to 9 in) as
wide, down to as small as 100 to 150 mm (4 to 6 in) in length, depending on
the manufacturer, model, and condition of the cutting edges. The shredding
process results in exposure of steel belt fragments along the edges of the
tire shreds.
Production of tire chips, which are normally
sized from 76 mm (3 in) down to 13 mm (1/2 in), requires two-stage
processing of the tire shreds (i.e., primary and secondary shredding) to
achieve adequate size reduction. Secondary shredding results in the
production of chips that are more equidimensional than the larger size
shreds that are generated by the primary shredder, but exposed steel
fragments will still occur along the edges of the chips.
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Ground Rubber
Ground rubber may be sized from particles as
large as 19 mm (3/4 in) to as fine as 0.15 mm (No. 100 sieve) depending on
the type of size reduction equipment and the intended application.
The production of ground rubber is achieved by
granulators, hammermills, or fine grinding machines. Granulators typically
produce particles that are regularly shaped and cubical with a comparatively
low-surface area. The steel belt fragments are removed by a magnetic
separator. Fiberglass belts or fibers are separated from the finer rubber
particles, usually by an air separator. Ground rubber particles are
subjected to a dual cycle of magnetic separation, then screened and
recovered in various size fractions.
Crumb Rubber
Crumb rubber usually consists of particles
ranging in size from 4.75 mm (No. 4 sieve) to less than 0.075 mm (No. 200
sieve). Most processes that incorporate crumb rubber as an asphalt modifier
use particles ranging in size from 0.6 mm to 0.15 mm (No. 30 to No. 100
sieve).
Three methods are currently used to convert
scrap tires to crumb rubber. The crackermill process is the most commonly
used method. The crackermill process tears apart or reduces the size of tire
rubber by passing the material between rotating corrugated steel drums. This
process creates an irregularly shaped torn particle with a large surface
area. These particles range in size from approximately 5 mm to 0.5 mm (No. 4
to No. 40 sieve) and are commonly referred to as ground crumb rubber. The
second method is the granulator process, which shears apart the rubber with
revolving steel plates that pass at close tolerance, producing granulated
crumb rubber particles, ranging in size from 9.5 mm (3/8 in) to 0.5 mm (No.
40 sieve). The third process is the micro-mill process, which produces a
very fine ground crumb rubber in the size range from 0.5 mm (No. 40 sieve)
to as small as 0.075 mm (No. 200 sieve).
In some cases, cryogenic techniques are also
used for size reduction. Essentially, this involves using liquid nitrogen to
reduce the temperature of the rubber particles to minus 87oC
(-125oF), making the particles quite brittle and easy to shatter
into small particles. This technique is sometimes used before final
grinding.
CURRENT MANAGEMENT OPTIONS
Recycling
About 7 percent of the 250 million scrap tires
generated annually are exported to foreign countries, 8 percent are recycled
into new products, and roughly 40 percent are used as tire-derived fuel,
either in whole or chipped form.
Currently, the largest single use for scrap
tires is as a fuel in power plants, cement plants, pulp and paper mill
boilers, utility boilers, and other industrial boilers. At least 100 million
scrap tires were used in 1994 as an alternative fuel either in whole or
chipped form.
At least 9 million scrap tires are processed
into ground rubber annually. Ground tire rubber is used in rubber products
(such as floor mats, carpet padding, and vehicle mud guards), plastic
products, and as a fine aggregate addition (dry process) in asphalt friction
courses. Crumb rubber has been used as an asphalt binder modifier (wet
process) in hot mix asphalt pavements.
As previously noted, of the roughly 30 million
tires that are not discarded each year, most go to retreaders, who retread
about one-third of the tires received. Automobile and truck tires that are
retreaded are sold and returned to the marketplace. Currently there are
roughly 1,500 retreaders operating in the United States, but the number is
shrinking because there is a decline in the market for passenger car
retreads. The truck tire retread business is increasing and truck tires can
be retreaded three to seven times before they have to be discarded.
Disposal
Approximately 45 percent of the 250 million
tires generated annually are disposed of in landfills, stockpiles, or
illegal dumps.
As of 1994, at least 48 states have some type
of legislation related to landfilling of tires, including 9 states that ban
all tires from landfills. There are 16 states in which whole tires are
banned from landfills. Thirteen other states require that tires be cut in
order to be accepted at landfills.
MARKET SOURCES
About 80 percent of all scrap tires are
handled by retail tire vendors. The remaining 20 percent are handled by auto
dismantlers. These two industrial groups, while not the generators of scrap
tires, collect and store tires until they are picked up by transporters,
sometimes referred to as "tire jockeys". These transporters take the tires
to retreaders, reclaimers, and grinders or slitters or to tire disposal
sites (landfills, tire stockpiles, or illegal dumps).
Figure 16-1 presents a graphical overview of
the scrap tire industry.
Figure 16-1.
Overview of the scrap tire industry.
Since tires are combustible, tire storage
sites can be potential fire hazards. Care must be taken to safeguard against
carelessness or accidental ignition, which can occur at tire storage
facilities.
Tire shreds or chips would normally be
available from tire shredder operators. Ground rubber or crumb rubber would
normally be available from scrap tire processors There are probably 100 or
more tire shredders in the United States, but there are only about 15 to 20
scrap tire processors.
HIGHWAY USES AND PROCESSING REQUIREMENTS
Embankment Construction - Shredded or Chipped
Tires
Shredded or chipped tires have been used as a
lightweight fill material for construction of embankments. However, recent
combustion problems at three locations have prompted a reevaluation of
design techniques when shredded or chipped tires are used in embankment
construction.
Aggregate Substitute - Ground Rubber
Ground rubber has been used as a fine
aggregate substitute in asphalt pavements. In this process, ground rubber
particles are added into the hot mix as a fine aggregate in a gap-graded
friction course type of mixture. This process, commonly referred to as the
dry process, typically uses ground rubber particles ranging from
approximately 6.4 mm (1/4 in) down to 0.85 mm (No. 20 sieve).(4)
Asphalt mixes in which ground rubber particles are added as a portion of the
fine aggregate are referred to as rubberized asphalt.
Asphalt Modifier - Crumb Rubber
Crumb rubber can be used to modify the asphalt
binder (e.g., increase its viscosity) in a process in which the rubber is
blended with asphalt binder (usually in the range of 18 to 25 percent
rubber). This process, commonly referred to as the wet process, blends and
partially reacts crumb rubber with asphalt cement at high temperatures to
produce a rubberized asphalt binder. Most of the wet processes require crumb
rubber particles between 0.6 mm (No. 30 sieve) and 0.15 mm (No. 100 sieve)
in size. The modified binder is commonly referred to as asphalt-rubber.
Asphalt-rubber binders are used primarily in
hot mix asphalt paving, but are also used in seal coat applications as a
stress absorbing membrane (SAM), a stress absorbing membrane interlayer
(SAMI), or as a membrane sealant without any aggregate.
Retaining Walls - Whole and Slit Tires
Although not a direct highway application,
whole tires have been used to construct retaining walls. They have also been
used to stabilize roadside shoulder areas and provide channel slope
protection. For each application, whole tires are stacked vertically on top
of each other. Adjacent tires are then clipped together horizontally and
metal posts are driven vertically through the tire openings and anchored
into the underlying earth as necessary to provide lateral support and
prevent later displacement. Each layer of tires is filled with compacted
earth backfill. This type of retaining wall construction was
initially performed in California.
Slit scrap tires can be used as reinforcement
in embankments and tied-back anchor retaining walls. By placing tire
sidewalls in interconnected strips or mats and taking advantage of the
extremely high tensile strength of the sidewalls, embankments can be
stabilized in accordance with the reinforced earth principles. Sidewalls are
held together by means of metal clips when reinforcing embankments, or by a
cross-arm anchor bar assembly when used to anchor retaining walls.
MATERIAL PROPERTIES
Physical Properties
Shredded Tires
Tire shreds are basically flat, irregularly
shaped tire chunks with jagged edges that may or may not contain protruding,
sharp pieces of metal, which are parts of steel belts or beads. As
previously noted, the size of tire shreds may range from as large as 460 mm
(18 in) to as small as 25 mm (1 in), with most particles within the 100 mm
(4 in) to 200 mm (8 in) range. The average loose density of tire shreds
varies according to the size of the shreds, but can be expected to be
between 390 kg/m3 (24 lb/ft3) to 535 kg/m3
(33 lb/ft3). The average compacted density ranges from 650 kg/m3
(40 lb/ft3) to 840 kg/m3 (52 lb/ft3).
Tire Chips
Tire chips are more finely and uniformly sized
than tire shreds, ranging from 76 mm (3 in) down to approximately 13 mm (1/2
in) in size. Although the size of tire chips, like tire shreds, varies with
the make and condition of the processing equipment, nearly all tire chip
particles can be gravel sized. The loose density of tire chips can be
expected to range from 320 kg/m3 (20 lb/ft3) to 490
kg/m3 (30 lb/ft3). The compacted density of tire chips
probably ranges from 570 kg/m3 (35 lb/ft3) to 730 kg/m3
(45 lb/ft3).(9) Tire chips have absorption values that
range from 2.0 to 3.8 percent.
Ground Rubber
Ground rubber particles are intermediate in
size between tire chips and crumb rubber. The particle sizing of ground
rubber ranges from 9.5 mm (3/8 in) to 0.85 mm (No. 20 sieve).
Crumb Rubber
Crumb rubber used in hot mix asphalt normally
has 100 percent of the particles finer than 4.75 mm (No. 4 sieve). Although
the majority of the particles used in the wet process are sized within the
1.2 mm (No. 16 sieve) to 0.42 mm (No. 40 sieve) range, some crumb rubber
particles may be as fine as 0.075 mm (No. 200 sieve). The specific gravity
of crumb rubber is approximately 1.15, and the product must be free of
fabric, wire, or other contaminants.
Chemical Properties
Tire chips and tire shreds are nonreactive
under normal environmental conditions. The principal chemical component of
tires is a blend of natural and synthetic rubber, but additional components
include carbon black, sulfur, polymers, oil, paraffins, pigments, fabrics,
and bead or belt materials.
Mechanical Properties
Limited data are available on the shear
strength of tire chips, while little or no such data are available on the
shear strength of tire shreds. The wide variation in shred size makes it
difficult, if not virtually impossible, to find a large enough apparatus to
perform a meaningful shear test. Although the shear strength characteristics
of tire chips vary according to the size and shape of the chips, internal
friction angles were found to range from 19o to 26o,
while cohesion values ranged from 4.3 kPa (90 lb/ft2) to 11.5 kPa
(90 to 240 lb/ft2). Tire chips have a permeability coefficient
ranging from 1.5 to 15 cm/sec.
Other Properties
Scrap tires have a heating value ranging from
28,000 kJ/kg (12,000 Btu/lb) to 35,000 kJ/kg (15,000 Btu/lb).(2)
As a result, given appropriate conditions, scrap tire combustion is possible
and must be considered in any application.
Tire chips can also be expected to exhibit
high insulating properties. If tire chips are used as a fill material in
subgrade applications, reduced depth of frost penetration compared with that
of granular soil can be expected.
