M Issue: For engineers specifying natural aggregate, geotechnical parameters and
requirements for specific types of construction applications are generally
available in published information and design manuals, as well as
from the engineer’s personal knowledge of material behaviors and properties. With new materials, however, parameters are
often derived from laboratory test results.
Even when lab tests are performed, it is always desirable to
have a range of typical values for comparison.
Glass aggregate is a relatively new construction
aggregate material. In general,
glass aggregate is durable, strong, and easy to place and compact.
This best practice presents typical geotechnical parameters
for aggregate consisting of 100% glass as well as mixtures of glass
and natural aggregate. Best Practice: The geotechnical parameters of cullet aggregate are
largely dependent upon the percentage of glass content, gradation
and compaction level, and to a lesser degree on the type or source
of glass. The experience gained from construction and
lab results indicates that moisture and debris content (within reasonable
ranges) have a relatively minor effect on the geotechnical performance. The insensitivity to moisture content is likely
due to the fact that glass aggregate is more free-draining than most
natural aggregate. This is
for two reasons: first, glass has a smooth, non-porous surface that
does not retain water; second, glass aggregate contains no clay materials
to hinder drainage. Hence,
only a limited amount of moisture can be retained by the cullet.
In the case of debris content, the insensitivity is due to
the fact that the debris content by weight is much lower than a two
dimensional visual judgment would suggest.
This is because the typical contaminants; paper labels, plastic
rings, and metal caps, have much lower densities than glass and tend
to lie flat on a stockpile, presenting their largest face.
Because the debris content appears to be much greater than
it actually is, load rejections are likely to occur before the contamination
reaches a point of actually affecting the engineering performance. One hundred percent cullet
fill is typically used in lightly loaded or non-loading applications
in which it should be compacted to about 90% to 95% of the maximum
dry density as determined using the ASTM D698 (Standard Proctor) test
method. As indicated previously, the parameters for
natural aggregate are typically available in published information. For mixtures of cullet and natural aggregate,
some parameters such as unit weight can be linearly interpreted based
on the mix percentage. The
ranges of typical values presented here take into account the effects
of gradation and compaction level.
For non-critical structures or applications, high range values
can be safely assumed. For critical structures or applications, the
parameters should be confirmed by laboratory testing. Geotechnical parameters should always be selected or interpreted
by qualified engineering personnel. Unit Weight The unit weight of a compacted,
100% cullet fill can range from 95 to 115 pounds per cubic foot (pcf).
The unit weight generally increases with decreasing cullet
size. Also, well-graded cullet
has a higher unit weight than poorly-graded cullet.
Every piece of glass processing equipment generates a somewhat
different shape and gradation of glass aggregate.
Glass aggregate processors should test the unit weight of their
products and keep it on file for potential users.
Bearing Capacity The bearing capacity is a function
of loading geometry, embedment depth, unit weight of fill, and the
strength of the fill. Cullet
aggregate with 100% cullet can be used for support of light loads. For small load areas such as footings or piers,
typical bearing capacities of 1,000 to 1,500 pounds per square foot
(psf) can be considered for glass.
For large loading areas such as mat or rigid pavement, a subgrade
reaction modulus in the range of 100 to 200 pounds per cubic inch
(pci) can be considered. Lateral Pressures Lateral pressures are required for the design
of retaining structures. These
pressures are functions of unit weight, material strength, and the
deflection of the retaining structure.
For 100% cullet fill, the following parameters can be used
for design purposes: Lateral Pressure Equivalent Fluid Density (pcf) Active
25 to 30 Passive 250 to 300 At-Rest 35
to 45 Note that these parameters
do not include hydrostatic pressure that must be taken into account
if drainage is not provided. Also,
the passive values include a safety factor of 1.5. Angle of Repose A high angle of repose is desirable
so that temporary excavation can be sloped without shoring. The angle of repose will largely depend on
the gradation, compaction level, and the weather condition. Typically, a 100% cullet can be stable at a
slope angle of 2H: 1V. (2 horizontal to 1 vertical). Temporary slopes (less than a few weeks in general) can be excavated
at 1H:1V. The slope must
be flattened if the material is under groundwater. Also, the excavation must be in conformance with local safety regulations.
Permeability Cullet fill has a medium to high
permeability depending mainly on the gradation. The permeability values typically range from 0.05 to 0.25 cm/sec.
These parameters compare favorably with those of natural sand
and gravel. Implementation: The parameters presented in
this Best Practice reflect some of the research performed on glass
aggregate to date. This information
can be a starting point for qualified geotechnical engineers to make
their own judgments based on local conditions and availability of
processed material. Benefits: Dissemination of the information presented herein should
help geotechnical engineers, contractors and permitting authorities
become familiar with glass aggregate fill materials and ultimately
to increase their use in construction. Application
Sites: Design
offices, construction sites, and testing laboratories. Contact: For more information about
this Best Practice, contact CWC, mailto:info@cwc.org. References: Case Studies for the Use of Post Consumer
Glass as a Construction Aggregate, report number GL-97-5rpt, CWC,
1997 (available from CWC web site). Issue Date /
Update: November 1996
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