Material: Recycled Glass Issue: The moisture content of a fill material will affect its compaction characteristics
and needs to be evaluated during quality control of the fill operation.
When the fill is drier than its optimum moisture content, any
moisture in the fill serves as a lubricant and helps the relative movement
of the particles when densifying under vibratory compaction.
When the fill is wetter than its optimum moisture content, the
moisture absorbs part of the compaction energy and prevents the formation
of a denser material. Hence, moisture content determination constitutes
an important part of quality control because the information is used
in evaluating the compaction efficiency and material behaviors. Accurate assessment of the moisture content
of a fill material is important also because the density requirement
of fill is based on the dry density of the material, and the dry density
is calculated based on the measurements of wet density corrected for
moisture content. When using a nuclear densometer, the most popular test
device, these measurements are conducted simultaneously using separate
mechanisms. The dry density
is then calculated by an integrated circuit in the densometer. Therefore, an accurate measurement of the moisture
content is necessary for an accurate assessment of dry density and meaningful
quality control. For a free-draining
material such as cullet aggregate, the accuracy of moisture content
measurement by a nuclear densometer may suffer from non-uniform distribution
of the moisture in the material. Because
the nuclear gauge measures moisture content through the level of hydrogen
it detects, organic content in the cullet fill may cause erroneous moisture
content measurements. For a
complete discussion of using the nuclear densometer to test compaction,
see the Density Test of Glass Aggregate Using a Nuclear Densometer
Best Practice. Best Practice: A study was conducted by the Clean Washington Center
to investigate the suitability of nuclear densometer testing on glass
cullet aggregate. The study
compared the results obtained using a nuclear densometer with those
obtained using a sand cone. The
latter is a physical test which determines the density of the compacted
material by measuring its volume and weight, and the moisture content
by measuring the loss of moisture using an oven.
The study concluded that nuclear densometers could be used to
test cullet aggregate. No correction to the test results is required and the test procedures
are 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 1993 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.
Due to the
free draining nature of the cullet aggregate, it has been found that
moisture content increases toward the bottom of the lift. Since a nuclear densometer can only measure
moisture content near the fill surface, the measurement may not be representative
of the entire lift. It is recommended
that confirmation tests be conducted with the upper portion of the fill
excavated and moisture content measurements taken for the lower portion
of the lift. The excavation
must be large enough to avoid side effects while using the densometer. The operation manual of the densometer should
be consulted to obtain the required excavation size. A moisture offset based on an average moisture
content should be required to be input into the densometer so that an
average value can be used in the dry density calculation.
2.
The nuclear densometer measures the moisture content of
a fill material by proportioning its hydrogen content. Since water is the only source of hydrogen in most construction
materials, the nuclear densometer will measure the moisture content
correctly in most cases. However,
for measurements of hydrogen-rich materials such as cullet containing
organic debris, the nuclear densometer will mistakenly yield moisture
content measurements that are higher than the actual amount. If not corrected, this elevated moisture content will result in
a lower dry density, and erroneously report a lower compaction level. It is recommended that for cullet with debris
content of 10 percent or higher by the visual classification method
described in the Visual Inspection for Recycled Glass as Construction Aggregate Best Practice, moisture
content be measured using the laboratory test method ASTM D2216. Test results should be compared to
those obtained using a nuclear densometer.
Based on this comparison, an offset of moisture content measurement
by the nuclear densometer can be implemented in the field. Implementation: Distribute this best practice
among engineers and contractors and invite input to the practice. In the future, a modified practice may be incorporated
into ASTM test standards. Seminars
or discussion sessions should be held with the permitting authorities
at the city, county, and state levels so that quality control procedures
are standardized and acceptable. Modified
practices should also be disseminated among owners, developers, equipment
and material suppliers, architects, engineers and contractors so that
valid procedures are followed in the field and results are agreeable
among project team members. Benefits: Nuclear densometers are the most popular tool in density
testing of fill materials. The
test is easy to perform and requires only a few minutes to complete. 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 of the fill operation. 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 crucial
step for the material to be considered for construction applications. Application Sites: All construction sites that use cullet aggregate
as a fill material. 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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