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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Waste glass that has been crushed and screened has the potential for use as a granular base material. Glass that has been reduced to a fine aggregate size fraction (less than 4.75 mm (No. 4 sieve) in size) exhibits properties similar to that of a fine aggregate or sandy material, with relative high stability, due to the angular nature of crushed glass particles. Blending with other coarse conventional materials will typically be required to meet required granular base gradation specifications.
No documented demonstrations or commercial applications of waste glass in granular base applications have been identified. Nonetheless, recent evaluations of glass aggregate properties suggest that properly processed waste glass blended with appropriately sized aggregates is well suited for use as a granular base material.(1)
MATERIAL PROCESSING REQUIREMENTS
Crushing and Screening
Waste glass should be crushed and screened prior to use to produce a material that will be free of sharp edges and glass slivers, and that will meet the requirements of a fine aggregate material, as defined by AASHTO M29.(2)
Waste glass should be free of ferrous and nonferrous metal, and the level of inorganic and organic debris should be reduced as much as practical. It has been recommended that levels of debris in the waste glass should be limited to 5 percent as determined by the American Geophysical Institute (AGI) test method.(3) The AGI test method is a visual test in which small samples are placed on a grid and extraneous debris counted and measured by weight.
Some of the properties of waste glass that are of particular interest when glass is used in granular base applications include gradation, density, friction angle, bearing capacity, durability, and drainage characteristics.
Gradation: Crushed glass collected from Material Recovery Facilities can be expected to exhibit a relatively wide variation in top sizes. Differences in gradation are dependent, in great part, on the type of glass crushing equipment used. In general, however, crushed glass can be expected to be a well-graded material, and properly sized cullet or cullet-aggregate mixtures can yield engineering properties that compare very well with natural aggregates used in granular base applications. Waste glass should be crushed and screened to produce a material that satisfied the grading requirements of granular base specifications, such as AASHTO M147.(4)
Unit Weight and Compacted Density: Crushed glass has a unit weight of approximately 1120 kg/m3 (70 lb/ft3), which is lower than that of conventional aggregate. The compacted density of crushed glass will vary with the size and grading of the glass as well as the degree of contamination (extraneous debris, such as paper, plastic caps, and soil). A maximum dry density of approximately 1800 to 1900 kg/m3 (111 to 118 lb/ft3) has been reported, which is also somewhat lower than that of conventional granular material. Crushed glass exhibits a relatively flat moisture-density curve, which indicates that the compacted density is insensitive to moisture content.
Stability: Relatively high angles of internal friction (compared with conventional aggregates) of greater than 50 degrees have been reported for crushed glass with top sizes of 19 mm (3/4 in) and 6.4 mm (1/4 in). California Bearing Ratio (CBR) test results of crushed glass blended with conventional aggregate were found to exhibit values ranging from 42 to 125 percent for blends of 50 percent glass with crushed rock. Lower glass additions of 15 percent were found to exhibit values almost identical to that of the crushed rock used in the tests (approximately 133 percent).(1)
Durability: Larger size glass particles have marginal durability, as measured by the Los Angeles Abrasion test, with values of approximately 40 to 45 percent. This suggests that additional processing (crushing) of the waste glass would be desirable to eliminate the larger, less durable glass fraction.
Drainage: Crushed glass is a free-draining material that exhibits permeabilities ranging from 10-1 to 10-2 cm/sec, depending on the glass gradation.
Crushed waste glass (cullet) used in granular base applications should be limited to the replacement of fine aggregate sizes. Fine crushed glass contains durable sand-like particles and exhibits consistent properties. Recommended gradations are presented in Table 20-6. Crushed glass in this size range will perform as a highly stable (angular) fine aggregate material. It has been recommended that maximum cullet content should be limited to 15 percent in granular base applications and 30 percent in subbase applications.(1)
Table 20-6. Recommended cullet gradation for use as a structural fill or granular base material.(1)
Maximum dry density of cullet-aggregate mixes should be determined by the Modified Proctor Test, ASTM D1557. Debris levels should be limited to 5 percent as determined by the American Geophysical Institute visual method(3) to ensure the use of a clean material.
Conventional AASHTO pavement structural design procedures can be employed for granular base containing waste glass.
Storage and Material Handling
The same general methods and equipment used to handle conventional aggregates are applicable for waste glass. When combined with natural aggregates, crushed glass should be uniformly mixed.
Placing and Compacting
The same methods and equipment used to place and compact conventional aggregate can be used to place and compact waste glass.
The same field test procedures used for conventional aggregate are recommended for granular base applications when using waste glass. Standard laboratory and field test methods for compacted density are given by AASHTO T191(5), T205(6), T238(7), and T239(8).
Monitored field demonstration programs should be undertaken to better document the performance of granular glass bases in actual applications. Field density test methods, using the nuclear gage density test, require verification.