Density Testing of Glass Aggregate
Using a Nuclear Densometer

 
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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