Material: Recycled Glass Issue: Recycled glass aggregate is a relatively new construction aggregate material.
For this Best Practice, glass aggregate includes 100% glass and
glass-aggregate mixtures. In general, glass aggregate is durable, strong,
and easy to place and compact. For
each application, specifications regarding the cullet content, cullet
gradation, debris level, and compaction level are required. Specifications should be generated based on
criteria that are related to the engineering behavior of the in-place
material. When the material
is used in structural load applications, the behavior and properties
must be especially well understood. Best Practice: This best practice presents the material behaviors
that can be expected for glass aggregate in load-supporting applications.
General specifications, which relate mainly to quality control measures,
are presented in the Developing Specifications for Glass Aggregate Best
Practice. Load-supporting backfill includes
fills that support heavy stationary loads such as fill beneath footings
and slabs, fluctuating loads such as those beneath reciprocating pumps,
compressors or other machinery, and light-loaded conditions such as
fill placed beneath pedestrian sidewalks. Load-supporting fills must
be strong, with minimal settlement potential under material self-weight
and applied loads. The strength
requirement can be achieved by compacting the material to a pre-determined
density. The settlement potential can be minimized by
controlling the gradation and deleterious debris content. Glass aggregate is a granular material that
will deform elastically under load, but will return to the original
volume when the load is removed. However,
both organic and inorganic debris in the glass can effect the elasticity
of the aggregate. No long-term
deformation is expected if the debris is limited to less than 5% to
10% as determined by visual inspection. (See the Visual Inspection for Recycled Glass as Construction Aggregate
Best Practice). Cullet fill will apply lateral
loads including active, at-rest, and passive pressures to a retaining
structure. The magnitude of
these loads is a function of the strength and density of the fill. Since glass aggregate is non-cohesive, its
strength can be represented by its internal friction angle, which is
typically 38 to 42 degrees. Glass
aggregate is generally lighter than natural aggregate because the specific
gravity values of glass cullet (about 2.0 to 2.5) are less than those
of natural aggregate. Therefore, the density of glass aggregate is
a function of both the percentage of cullet content and the gradation
of the material. Generally,
the internal friction angle of a granular material is proportional to
its density, which is in turn proportional to the level of compaction
achieved in the fill. Frictional resistance develops
at the interface of fill particles and at the structure surface. In construction applications, the load-applying
surfaces may include concrete, wood, steel, or plastic. Typically, the frictional resistance can be
estimated using about 2/3 to 3/4 of the internal friction angle of the
fill material. For critical
structures, a laboratory direction shear test is recommended for the
determination of the interface frictional resistance.
For fill under cyclic loading,
both the strength and durability of the material are critical. The latter depends on gradation and material
characteristics. The suitability
of such fill can be evaluated using laboratory tests such as CBR (California
Bearing Ratio), Resistance R-Value, or Resilient Modulus tests. The resilient modulus can be determined by
cyclic triaxial tests. However,
this test requires special equipment and is not commonly conducted. In engineering practice, the resilient modulus
is often obtained from other test values such as CBR. For data on several gradations and mixtures of glass aggregate see
(1). The workability of a glass
aggregate is generally good. The
material is typically free-draining, therefore, its compaction characteristic
is insensitive to moisture content.
In dry weather, wetting the material is necessary for dust control. Implementation: Enough laboratory research
and field data has been accumulated to have a good understanding of
the physical characteristics of glass as a construction aggregate (see
the references below). However,
in every region research data and anecdotal experience must be combined
with the characteristics of local materials to determine the niche glass
aggregate can fill in local aggregate applications. Benefits: The material behaviors of cullet fill are similar to
those of natural sand and gravel, and thus the criteria for specifications
are relatively similar to those of natural materials. However, because
glass aggregate is a relatively new construction material, the performance
of glass aggregate in construction applications may draw more scrutiny
than expected for customary materials.
This makes it all the more important to be familiar with and
understand the behavior of cullet fill materials. Application Sites: Glass processing facilities, materials recovery
facilities, construction sites, and testing laboratories. Contact: For more information about
this Best Practice, contact CWC, mailto:info@cwc.org. References: Developing Specifications for Waste Glass and Waste-to-Energy Bottom Ash
as Highway Fill Materials, Volume 2 of 2 (Waste Glass), Paul J. Cosentino Ph.D.,
P.E., et al., Florida Institute of Technology, 1995. Case Studies for the Use of Post Consumer
Glass as a Construction Aggregate, CWC report GL97-5rpt, 1997. Issue Date /
Update: November 1996
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