Material: Recycled Glass Issue: Engineers and contractors often
require examination or characterization of fill materials prior to import
to a project site. This characterization or testing is typically performed
on one or a set of samples which must be representative of the gross
material in storage. This best practice describes procedures that pertain
to sampling of glass for construction applications. These descriptions
are not comprehensive. Rather, they have been developed as a general
introduction to typical procedures. Best Practice: The physical properties of
glass cullet as a granular material have been well enough established
to give specifying engineers confidence in its efficacy in properly
designed construction applications. However, the engineering characteristics
and workability of any specific lot of material depend to a large extent
on the gradation and debris content of that particular lot. Glass aggregate
in storage or stockpiles may become segregated by size or may contain
debris such as food residues, metal caps, plastic, paper, and other
non-aggregate materials. Sampling procedures must be conducted to insure
that changes in gradation and debris content are within tolerances.
A separate report, Methods for Sampling and Testing Recycled
Glass, is available for information on sampling procedures for
other glass applications. Sampling of recycled glass
can be achieved directly from storage locations such as bins, bunkers
and stockpiles, or from conveying units such as conveyor belts and flowing
streams. In each case, a set of sub-samples is collected to form a composite
field sample. The composite sample is then sub-divided into smaller
sample sizes using a quartering procedure. The quartering procedure
is repeated until a desired sample size is achieved. The following paragraphs
detail procedures for sampling from storage units and conveying units. Determine statistical sample size There are standard analytical procedures for
the determination of statistical sample size. ASTM
D75-97, Standard Practice for
Sampling Aggregates, contains a table correlating maximum nominal
aggregate particle size with minimum recommended field sample size. Sub-sample collection
1.
Sampling from
storage units: Collect samples
at three to five locations from the exposed faces of the material in
the storage units. The material in the top six inches should be avoided,
and attempts should be made to retrieve material at different depths.
Glass is cohesionless, and excavation to depth may require shoring.
Power equipment is recommended for sample collection. Particles should
be collected from below the disturbed surface where gradation change
is minimal. For large bunkers and stockpiles, the sample locations should
include the top third, mid third, and bottom third of the stored material.
For storage bins and small bunkers, the sample locations should be scattered
on the horizontal plane.
2.
Sampling from
conveying units: Estimate the
length of time for the stored material to be sampled to pass the sampling
point. Determine the time interval for the collection of three to five
sub-samples. The conveying unit may need to be stopped to allow sample
collection. At each interval of sample collection, all particles from
coarse to fine and debris must be collected. Quartering Combine the collected sub-samples into a composite sample. Repeat the
quartering procedure until a desired sample size is achieved. Quartering
can be conducted on a smooth and clean concrete surface or on a plastic
tarp where collected material will not be lost and foreign material
will not be introduced. Perform the following steps. (a) Place the sub-samples
on the surface and mix thoroughly by turning the entire composite sample
over at least three times. Compaction by static or vibratory forces
should be avoided. (b) Shovel the entire sample into a conical pile
by depositing each shovelful on top of the preceding one. (c) Flatten
the conical pile to a uniform thickness and diameter by spreading with
a shovel. The material should have a diameter about four to eight times
the thickness. (d) Divide the flattened mass into four equal quarters
with a shovel or trowel. (e) Remove two diagonally opposite quarters
for further quartering. Testing Sampling is only the first part of the Sampling and Testing sequence.
After a statistically valid sample has been obtained, tests must be
run on the sample to determine whether the lot being examined meets
the specifications. The testing may be for any characteristics agreed
upon between the buyer and the seller. Implementation: Sampling procedures are used
to confirm the quality of delivered materials and in dispute resolution
when two parties disagree on the delivered quality. Procedures should
be established in advance of delivery and used as needed to satisfy
both buyer and seller. Benefits: Acceptance limits or specifications
of glass aggregate include material characteristics such as grain size
and debris level. These characteristics are usually obtained through
visual inspection performed in the field or testing of small samples
in the laboratory. Procedures
that can produce representative samples are required to link the characteristics
of the samples to those of the bulk material in storage, so that engineers
and contractors can predict the performance of the material. As such,
implementation of sampling procedures for glass aggregate is a necessary
step for the material to be considered for construction applications.
Once the procedures become widely accepted, the efficiency in
material characterization will increase, and this will ultimately increase
the ability of the material to be considered for construction use. Application Sites: Glass processing facilities;
materials recovery facilities; construction sites; testing laboratories;
all manufacturers using glass feedstocks. Contact: For more information about
this Best Practice, contact the CWC info@cwc.org. References: Annual Book of American Society for Testing and Materials,
Volume 4.08 for Soil and Rock; Methods for Sampling and Testing
Glass Cullet, Clean Washington Center, 1996; Shin, C. J., S&EE, Inc., Bellevue, WA Issue Date /
Update: November 1996
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