Material: Recycled
Glass
Issue: The engineering properties of granular fill materials such as 100% glass
cullet, or cullet-soil or cullet-aggregate mixtures, are related in
large part to the density of the fill and the gradation of the mixture. The gradation requirement is usually confirmed
by laboratory testing prior to the fill operation, whereas the density
requirement is typically checked by in-place or field density testing
during the operation. Field density testing is performed to confirm
that the fill has been compacted to a density that meets or exceeds
a specified level. If this level
has not been reached, further compaction or other adjustments will be
required in the field. If the
compaction criterion has been reached or exceeded, the fill is said
to be acceptable and engineering performance characteristics such as
strength and compressibility are ensured.
Field density tests are typically performed using a nuclear densometer. For granular materials such as cullet and gravel, the test accuracy
may suffer from the presence of voids inside of the materials. In addition, the presence of hydrocarbon-containing
organic content such as labels in cullet fill may be erroneously read
as moisture by the instrument. Best Practice: A study sponsored by the Clean Washington Center investigated
the suitability of nuclear densometer testing on glass cullet aggregate.
The study compared density measurements obtained using a nuclear
densometer with those obtained using a sand cone.
The latter is a physical test that determines the density of
the compacted material by measuring its volume and weight.
The nuclear densometer tests included the backscatter mode (ASTM
D2922-96e1) which measures the density near the surface, and
direct transmission mode (ASTM D5195-91(1996)) with the source probe extending
to depths of 6 to 12 inches. The
study concluded that nuclear densometers could be used for the testing
of cullet aggregate. No correction
to the density measurements is required and the test procedures can
be the same as those used for natural materials. The test frequency is recommended to be the same as for natural
material at one test per lift per 2,500 square feet of fill, but not
less than one per lift. Cullet aggregate has been used
since 1994 in the Seattle area. Experience
has been gained in the quality control of this fill material using a
nuclear densometer. Some observations
and recommended adjustments to the test procedure are provided in the
following paragraphs.
(1)
Cullet aggregate is typically compacted by vibratory compaction equipment. The vibration can cause the finer particles
to migrate toward the bottom of each lift.
As a result, the void space reduces and density increases in
the bottom portion of the lift. Such
uneven distributions of particle sizes and non-uniform density profiles
can wrongly indicate a poorly graded material.
Hence, the backscatter mode of the nuclear density test should
be avoided as this test mode measures the density in the upper portion
of the lift. It is recommended
that the test be performed using the direct transmission mode with the
test probe extending the full depth of the lift.
(2)
To get the most accurate overall reading, it is recommended that four
measurements be obtained at each test location with the nuclear densometer
rotated 90 degrees between measurements.
The average of the measurements should be used for record purposes. This procedure reduces the effect of non-homogeneity on the density
measurement.
(3)
The surface of cullet aggregate is typically uneven and highly permeable. Such surface conditions will normally reduce
the density measurement of a nuclear densometer because the instrument
will be supported on the highest peak.
To avoid this effect, a thin layer of sand should be used to
fill the voids and even the surface prior to measurement.
(4)
A parallel check on the accuracy of the density measurements
by a nuclear densometer can be performed using physical tests such as
the sand cone method (ASTM D1556)
or rubber balloon method (ASTM D2167).
(5)
The moisture measurement may be affected by the non-homogeneity
of the compacted fill and the organic content in the cullet debris.
If necessary, a moisture compensation should be included in the
densometer operation. Details of such compensations are presented
in the Moisture Content Measurement
of Glass Aggregate Using a Nuclear Densometer Best Practice. Implementation: In regions where the use of
glass in construction is not common, seminars or discussion sessions
should be held with permitting authorities at the city, county, and
state levels so that quality control procedures are standardized and
acceptable. In the future, it may be possible to create
a modified practice to be incorporated into ASTM test standards. Owners,
developers, equipment and material suppliers, architects, engineers
and contractors should also be informed of these issues so that the
best procedures are followed in the field and the results are agreeable
among project team members. Benefits: Nuclear densometers are the most popular tool to test
the density of fill materials. The
procedure is quick and easy to perform, and the test results are available
at the completion of the test. Hence,
the quality of the fill can be evaluated immediately and adjustment
to the placement or compaction procedures can be made without delay
to the fill operation. Ultimately,
this simple test method allows the quality of fill to be controlled
effectively and efficiently. The
validation of methods for testing of cullet aggregate is a necessary
step for glass aggregate to be considered for construction applications.
Application Sites: All construction sites that use cullet aggregate
as a fill material for the purposes of general fill or fill for special
applications. Contact: for more information about
this Best Practice, contact CWC, mailto:info@cwc.org. References: Annual Book of American
Society for Testing and Materials, Volume 4.08 for Soil and Rock Shin, C. J., S&EE, Inc., Bellevue, WA Suitability of
Nuclear Densometer Testing on Glass Cullet Aggregate, Clean Washington Center Report
GL-94-2, 1994. This report is
available only in hardcopy. Contact
the CWC. Issue Date /
Update: November 1996
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