Material: Recycled Glass Issue: Glass is a relatively new construction aggregate material. The term “glass aggregate” includes, for this
Best Practice, 100% glass, glass-soil, and glass-aggregate mixtures. In general, glass aggregate is durable, strong,
easy to place, and easy to compact.
The material can be used for construction applications including
general backfill, roadways, utility backfill, drainage medium, and
in miscellaneous uses such as landfill cover and underground storage
tank backfill. For each application, the material should be
specified based on the cullet content, gradation, debris level, and
compaction level. Criteria
for developing the specifications for any aggregate rely on a combination
of technical data and practical historical experience.
Currently, the availability of such criteria for glass aggregate
is limited. This lack of information
is a barrier to the increased use of the material. Best Practice: This best practice presents quality control measures
applicable for all glass aggregate.
The intent is to define the general parameters that must be
considered when developing specifications.
A more detailed development of specifications for glass aggregate
in load-supporting (see the
Behavior of Glass Aggregate under Structural Loads). Processing and Mixing: Specifications may require that the processed glass
be blended with natural aggregate to a specific percentage. Because blending adds extra costs and can be
difficult in the field, the specifying engineer should give serious
consideration to the need for uniform blending. In many drainage or non-structural applications, it may be permissible
to switch between 100% glass and 100% natural aggregate during the
job without sacrificing quality.
In structural applications, it may be more important to attain
uniform blending. The blending
process should prevent segregation of particles and debris.
Gradation. From an engineering standpoint,
it has been shown that 1, ¾, ½, or ¼-inch minus cullet all perform
well in appropriate applications.
Glass of ¼-inch minus has a grain size close to that of a fine
to coarse sand, whereas glass of ¼-inch plus is similar to a fine
to coarse gravel. In general, cullet particles over one inch
in size become platy in shape and are susceptible to breaking and
chipping. A cullet fill containing
greater than 10% of such coarse particles can experience gradation
change during transportation and compaction, and possibly volume reduction
upon loading. On the other hand, glass particles smaller
than US No. 200 sieve will have a large surface area and can retain
a relatively large quantity of moisture. A cullet fill containing
greater than 10% of such fine particles can become sensitive to moisture
content during compaction, and may be difficult to use during wet
weather. The specifying engineer should begin with the
same gradation as is required for natural aggregates in the application,
then consider whether the function of the glass is to replace the
natural aggregate with the same gradation or complement it with a
gradation that improves the density of the fill. Debris Level: Debris may be defined as any materials that may impact the performance
of the engineered fill if present in sufficient quantities. Organic materials may decay and result in volume
reduction. Metals, ceramics, and plastic, if present
in large enough quantities, can affect the engineering properties. A visual classification method has been developed
for field determination of debris level. See the Visual Inspection for Glass Construction Aggregate
Best Practice. The debris level obtained using this visual procedure is much higher
than the debris content measured by weight or volume. This is because paper residue, which appears
to represent 10% by two-dimensional classification, may actually be
less than 2% by volume or weight.
Finally, a specification should always indicate that no hazardous
materials are allowed. Compaction. In order to achieve the desired
engineering properties in the field, glass aggregate should be compacted
to a specified minimum level in the field. The compaction levels are typically specified using maximum dry
densities determined in the laboratory.
For 100% cullet, the compaction data is found using a Standard
Proctor test (ASTM D698). For
glass-soil or glass-aggregate mixture, a Modified Proctor test (ASTM
D1557) is typically used. The
desired level of compaction is generally 90 to 95% of maximum dry
density. The glass processor
should keep data on the dry density of processed glass from that facility,
and, if possible, a lab confirmation should be performed for the specific
job. The level of compaction should
be field-verified by in-situ testing.
The frequency of testing is typically one per 2,500 square
feet of fill but not less than one per lift of fill.
Nuclear densometers are the most commonly used device for density
testing. However, due to the
porous and heterogeneous nature of the material, modifications to
the common test procedures should be specified when appropriate. Such modifications are presented in the following Best Practices:
1) Compaction of Glass Aggregate, 2) Density Test of Glass Aggregate Using a Nuclear Densometer,
and 3) Moisture Content Test of Glass Aggregate Using a
Nuclear Densometer. Design Considerations. Considerations
should be made regarding the exposure of the general public to cullet
fills. Depending on the application,
landscaping soil and vegetation, asphalt pavement, or concrete could
be used to cover the cullet fills.
Also, considerations should be made regarding cullet fills
which are placed in contact with synthetic liners, geotextile, PVC
pipes or pipes with protective coatings.
Implementation: This Best Practice presents
a starting point for specifying engineers to begin to consider the
kinds of construction applications in which they will use recycled
glass aggregate. Given the considerations above, engineers should
judge their own potential uses based on the properties of glass aggregate,
the availability of properly processed glass, and local economics. Benefits:
The
material behaviors of cullet fill and thus the criteria for developing
specifications are similar to those of natural sand and gravel. Dissemination of the best practice information presented here will
help engineers, contractors and permitting authorities to be familiar
with cullet fill materials and ultimately increase their potential
use in construction. Application
Sites: Glass
processing facilities, construction sites, and testing laboratories. Contact: For more information about this Best Practice, contact
CWC, mailto:info@cwc.org. References: Numerous studies and reports have been generated for
the use of glass in construction applications.
See the Best Practice Studies of Glass in Construction Applications for descriptions
of the primary references. Shin, C. J., S&EE, Inc.,
Bellevue, WA Issue Date /
Update: November 1996
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