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Desirable Properties
Tire chips (scrap tires cut into 1- to 12-inch pieces) have a
number of qualities that make them well-suited for use in road
and bridge construction. Tire chips are:
- lightweight
- low-pressure
- free-draining
- good thermal insulators
- durable
- low-price
Tire chips can help reduce fill weight and address slope
stability, landslide, and embankment settlement problems. Tire
chip unit weights (compacted in place) range from 40 pcf for a
thin fill with no soil cover to 60 pcf for a thick fill covered
with a thick soil cover. Gravel compares at 125 pcf. For
retaining wall and bridge abutments, tire chips reduce wall
pressure, which can save money. For example, using tire chips as
backfill can lower pressure at the base of a 5-foot wall by 50%.
Tire chips are more permeable than most granular aggregates
and can be used in roadside French drains or drainage layers.
Tire chip permeability is greater than 10 cm/s. Tire chips have
also been shown to be a great thermal insulator, eight times
better than gravel, in fact. Moreover, tire chips are durable
and cheap.
Tires chips are generally uniformly graded with specific
gravities ranging from 1.02 to 1.27 depending on whether steel
belted, glass belted, or a mixture were measured. Specific
gravities for soils are typically 2.60 to 2.80, more than double
that of tire chips. Water absorption capacities generally range
from 2 to 4.3%. Unlike most soils, water content does not affect
tire chip compaction. Compacted dry unit weights of tires range
from 38 - 43 pcf, approximately 1/3 the unit weight of soils.
However, the unit weights of tire chips does increase under the
weight of overlying soils and tire chips.
Large volumes of tires can be used in civil engineering
construction applications. As a guideline, 75 tires yield about
1 cubic yard of compacted tire chip fill, and 1,000 tires will
fill a 14-cubic-yard dump truck.
Tire Chip Compressibility
Tire chips used in road and bridge applications settle during
construction and within a month or two after construction due to
the weight of the overburden. Settlement varies, but a 14-foot
thick tire chip layer (3-inch pieces) covered with about 6 feet
of soil settled more than 3 inches after the soil was in place.
Temporary post-construction deflection also occurs each time a
vehicle drives over the pavement. Deflections decrease as the
thickness of the overlying soil increases and tend to be less
for 3-inch chips than for 12-inch chips. Initial tests show that
soil cover for maximum pavement life using 3-inch tire chips
should be 2.5 to 5 feet to minimize deflection. It is important
not to underestimate the compressibility of tire chips.
Design Considerations - Paved Roads
Tire chips should be wrapped in an appropriate geotextile,
with 18-inch overlaps at the seams, to prevent surrounding soil
from being washed between the tire chips. The 3-inch nominal
chips are easier to shape to the desired grade than 12-inch
chips. To compensate for post construction compression, it is
necessary to overbuild the tire chip layer so that the
compressed elevation of the tire chips is at the desired level.
Moist soils compact much more easily over tire chips. Final
grading and paving should be delayed to allow for tire chip
settlement.
Mixing soil with tire chips to minimize compression is not
recommended. It's difficult to mix the soil and chips, which
increases construction costs. Improper mixing may lead to
long-term settlement problems. Also, soil decreases the benefits
tire chips offer.
Design Considerations - Unpaved Roads
Soil cover on unpaved roads should be thick enough to prevent
rutting and will depend on the thickness of the tire chip layer
and on traffic loads. Use of geotextile may be unnecessary with
3-inch chips.
Design Considerations - Retaining Walls and Bridge
Abutment Backfills
Because tire chips exert less than half the pressure of
gravel, retaining walls built with chips can be thinner and,
therefore, cheaper. When using 3-inch chips, a reasonable
coefficient of lateral earth pressure at rest is 0.40 for
design. Geotextiles should be used to separate the tire chips
from the surrounding soil using a "belt and suspenders" design
at the contact between the geotextile and the back of the wall.
Design Considerations - To Limit Frost Penetration
Tire chips provide thermal insulation to reduce frost
penetration depths. The chips have been shown to reduce
penetration by up to 25%. The thermal conductivity of tire chips
(0.1 to 0.2 Btu/hr-ft-E F) is eight times lower than that of
typical soil.
Design Considerations - Drainage Layers
Tire chips have very high permeability and are an attractive
substitute for granular soils in highway edge drains, French
drains, and drainage layers at the bottom of subgrades. Tire
chips need to be completely enclosed in geotextile to prevent
fines from reducing permeability.
Environmental Considerations
Experiments indicate that tire chip leaching tests are below
the limits for metals with primary drinking water standards, and
for metals with secondary standards, except for manganese and
iron. Tests for volatile and semi-volatile organics were at the
A non-detect level for all measured compounds above ground
water. Tire chips are not recommended for use below groundwater
tables at this time.
Constructions Specifications
- Pay quantity: Be specific as to how
quantity is measured, i.e. loose-in-truck, compacted
in-place, or compacted with overlying soil cover. The
general contractor assumes liability for the quantity of
tire chips loose-in-truck. Tire chip suppliers or general
contractors assume liability for chips compacted in-place.
Buying tire chips by the ton may reduce the contractor's bid
price, but requires the project manager to keep track of
weight tickets.
- Stockpiling: Tire chips may need to be
stockpiled on-site to keep construction projects on track.
- Spreading: Tire chips can be spread
most easily with a track-mounted dozer or track-mounted
loader. A smaller dozer is easier to use than a large one.
- Compacting: Twelve-inch compacted lift
thickness work best for 3-inch chips, using a heavy
vibratory smooth drum or vibratory sheeps foot roller, or
heavy, standard-width track dozer in 6 to 8 passes. Granular
soils for cover are easier to compact over tire chips if
they are slightly wet of optimum.
Exothermic Reactions in Tire Chip Fills
Of 70 installations of tire chip fill applications in the US,
there have been 3 that have experienced exothermic, or
heat-producing, reactions. These were all very large
installations with a number of common features which should be
avoided in the future, including: free access to oxygen, thin
soil cover, topsoil placed directly on tire chips, tire chips
contaminated with liquid petroleum, lots of exposed steel,
contact of tire chips with fertilizer, and concentrations of
crumb rubber.
In general, recommended preliminary construction procedures
are to: provide at least 4 feet of soil cover to reduce oxygen
and water infiltration (soil should contain a minimum of 25%
fines); prevent topsoil or fertilizer from coming in direct
contact with tire shreds; use large tire shreds (8-inch nominal
for fills of 10 feet or more); limit exposed steel belts; and
limit the amount of crumb rubber included with the shreds (no
more than 1-2% passing #4 sieve).
ASTM Approval
ASTM "Standard Practice for Use of Scrap Tires in Civil
Engineering Applications" has been adopted. This Standard
Practice will includes information on material characterization,
construction practices, and leachate. The ASTM Subcommittee
D-34.15 Construction and Other Secondary Applications of
Recovered Materials has approved the Standard Practice, as has
the ASTM Committee D-34 on Waste Management.
Original report written by Dana Humphrey, Ph.D., P.E.,
University of Maine, Summary Prepared by Recycling and Recycled
Products Program, General Services Division, Texas Department of
Transportation.
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