table of contents - Research and Innovative Technology ...

158 � Transportation Statistics Annual Report 1996
� Tires
Used tires comprise a large part of the solid
waste generated by the transportation sector. No
industry group or governmental agency monitors
scrap tire disposal in the United States.
Therefore, estimates of used tire generation are
based on tire production. (USEPA 1991)
In 1990, an estimated 188 million tires were
placed in landfills or stockpiles, or dumped illegally
(see figure 7-14).Tires are a significant
problem in landfills. The tires often rise to the
landfill surface as other materials around them
settle. The uncovered tires can be breeding
grounds for mosquitoes and other insects. Piles
of tires sometimes ignite and burn, releasing
toxic smoke and fumes. Burning tires emit criteria
air pollutants, metals, and unburned organics.
(USEPA 1991) Burning tires are difficult to
extinguish, and the related residue can cause
groundwater contamination. Several large stockpile
fires in the mid-1980s prompted interest in
alternative uses for scrap tires.
It has been estimated that only about 22 percent
of the tires that were scrapped in 1990 were
recycled. The recycled tires were processed in
two primary ways: fuel combustion (26 million)
and processed-tire products (16 million).
Another 300,000 tires were reused in whole-tire
applications. (While there are many uses for
scrap tires—crash barriers, retaining walls, artificial
reefs, playground equipment, landscaping
material, and other applications—they have not
developed into widely marketed products.) An
additional 12 million tires were exported to
other countries (see figure 7-14).
Burned as fuel, tires have an energy content
of about 15,000 Btu per pound, somewhat higher
than coal (6,000 to 13,000 Btu). Tires are also
compact, consistent in composition, and low in
moisture content.
Processed-tire applications include pyrolysis
products, shredded embankment material, various
molded rubber products, and crumb rubber
for asphalt paving. Of these, pyrolysis and
crumb rubber are the most common.
Pyrolysis thermally breaks scrap tires down
into three marketable products—pyrolytic gas,
oil, and char. The gas has a heat value similar to
natural gas. The oils can be used for gasoline
additives and fuel oil, and char can be substituted
for carbon black in some applications. EPA
maintains that pyrolysis units will have minimal
air pollution impacts, because most of the pyrogas
generated in the pyrolysis process is burned
as fuel, and organic compounds are destroyed
during burning. (USEPA 1991)
Crumb rubber modifier (CRM) is primarily
used to produce asphalt rubber and rubber modified
hot mix asphalt (RUMAC). Asphalt rubber
is a combination of asphalt cement binder and
CRM and is used in sealants, thin surface treatments,
and hot mix asphalt (HMA). RUMAC is
used in hot mix asphalt only (see next section).
(USDOT FHWA and USEPA 1993, 4)
Use of asphalt pavement containing recycled
rubber has been encouraged through provisions in
the Intermodal Surface Transportation Efficiency
Act of 1991 (ISTEA). ISTEA prohibits disapproval
of highway projects on grounds of use of
asphalt pavement with recycled rubber. It requires
studies on the performance and recyclability of
asphalt pavement containing recycled rubber and
evaluation of the environmental impacts of its use.
ISTEA provisions setting minimum utilization
requirements for recycled rubber in asphalt pavement
were repealed by Congress in the 1995
National Highway System Designation Act.
� Asphalt and Concrete Pavement
Asphalt and concrete pavement removed
from roadways constitute a significant portion
of the solid waste produced from transportation
infrastructure. Fortunately, this material can be
reclaimed and used in a number of transportation-related
applications. Estimates vary about