Waste Glass - Material Description

ORIGIN

Glass is a product of the supercooling of a melted liquid mixture consisting primarily of sand (silicon dioxide) and soda ash (sodium carbonate) to a rigid condition, in which the supercooled material does not crystallize and retains the organization and internal structure of the melted liquid. When waste glass is crushed to sandlike particle sizes, similar to those of natural sand, it exhibits properties of an aggregate material. In 1994 approximately 9.2 million metric tons (10.2 million tons) of postconsumer glass was discarded in the municipal solid waste stream in the United States. Approximately 8.1 million metric tons (8.9 million tons) or 80 percent of this waste glass was container glass.(1)

CURRENT MANAGEMENT OPTIONS

Recycling Over the past decade, there have been widespread efforts to recover postconsumer glass. Bottle bill legislation, which provides for deposits during purchase of containers and deposit bottle returns at retail outlets, has been introduced in some states, but more often glass recovery and recycling have been attempted through the collection of waste glass at recycling centers or Material Recovery Facilities (MRFs). MRFs are facilities that are designed to sort, store, and market municipal solid waste recyclables that are collected at the curb (source-separated materials). Curbside separation of glass involves homeowner or apartment dweller separation of container glass (or glass commingled with other recyclables such as ferrous and aluminum cans and plastic containers) in preparation for the transport of these materials to the MRF. Some communities that do not have curbside collection offer waste glass drop-off locations for collection. At most MRFs, waste glass is hand-sorted by color (white, amber, and green), and crushed for size reduction (generally to less than 50 mm (2 in) in size). Crushed (color-sorted) glass, which is commonly referred to as cullet, is marketed in most locales as a raw material for use in the manufacture of new glass containers. Traditionally, glass recycling has involved the collection and sorting of glass by color for use in the manufacture of new glass containers. Recycling postconsumer glass from the municipal solid waste stream for use as a raw material in new glass products is limited, however, by the high cost of collection and processing (hand sorting) of waste glass,(2,3) and specifications that limit impurities (e.g., ceramics, ferrous metal, paper, plastics and mixed-colored cullet) in the glass production process. In addition, during collection and handling of glass, high percentages of glass breakage (30 to 60 percent)(4) limit the quantity of glass that can actually be recovered using hand-sorting practices. Given these limitations, traditional glass recycling rates have been relatively low. Disposal The EPA has estimated that in 1994 approximately 9.1 million metric tons (10.1 million tons) or 77 percent of the waste glass generated in the United States was landfilled.(1)

MARKET SOURCES

Figure 1 provides a schematic representation of the flow of container glass in the United States along with potential recycling options. In most cases, mixed-colored waste glass can be obtained by contacting the municipality that is responsible for the collection of recyclables or the MRF operator. The quality of waste glass obtained from MRFs, which collect and process glass from municipal recycling collection districts, can vary widely and can contain dirt, paper, and plastics. Glass gradation can also vary widely. It can range in top size from 25 mm (1 in) to 100 mm (4 in). The exact physical characteristics of the glass from any given source will depend on the processing equipment (e.g., crushing and screening equipment) at the MRF and the degree of processing afforded the mixed colored glass.

HIGHWAY USES AND PROCESSING REQUIREMENTS

Asphalt Concrete Aggregate Waste glass has been used in highway construction as an aggregate substitute in asphalt paving. Many communities have recently incorporated glass into their roadway specifications, which could help to encourage greater use of the material. Granular Base or Fill Crushed glass or cullet, if properly sized and processed, can exhibit characteristics similar to that of a gravel or sand. As a result, it should also be suitable for use as a road base or fill material.
Figure 1. General overview of glass cycle in the United States
When used in construction applications, glass must be crushed and screened to produce an appropriate design gradation. Glass crushing equipment normally used to produce a cullet is similar to rock crushing equipment (e.g., hammermills, rotating breaker bars, rotating drum and breaker plate, impact crushers).(4) Because MRF glass crushing equipment has been primarily designed to reduce the size or densify the cullet for transportation purposes and for use as a glass production feedstock material, the crushing equipment used in MRFs is typically smaller and uses less energy than conventional aggregate or rock crushing equipment. Successful production of glass aggregate using recycled asphalt pavement (RAP) processing equipment (crushers and screens) has been reported.(3) Magnetic separation and air classification may also be required to remove any residual ferrous materials or paper still mixed in with the cullet. Due to the relatively low glass-generation rates from small communities, stockpiles of sufficient size need to be accumulated to provide a consistent supply of material in order for glass use to be practical in pavement construction applications.

MATERIAL PROPERTIES

Physical Properties Crushed glass (cullet) particles are generally angular in shape and can contain some flat and elongated particles. The degree of angularity and the quantity of flat and elongated particles depend on the degree of processing (i.e., crushing). Smaller particles, resulting from extra crushing, will exhibit somewhat less angularity and reduced quantities of flat and elongated particles. Proper crushing can virtually eliminate sharp edges and the corresponding safety hazards associated with manual handling of the product. Uncontaminated or clean glass itself exhibits consistent properties; however, the properties of waste glass from MRFs are much more variable due to the presence of nonglass debris present in the waste stream. Table 1 lists some typical physical properties of waste glass collected from a number of MRF facilities.(2)
Table 1. Selected physical properties of waste glass.
Test Glass Samplesa ASTM Test Method
Coarse Fine
Particle Shape Angularity Flat (%) Flat/Elongated (%) Angular 20-30 1-2 Angular 1 1 ASTM D2488
Specific Gravityb 1.96 - 2.41 2.49 - 2.52 ASTM D854c ASTM C127c
Permeability (cm/sec) ~2 x 10-1 ~ 6 x 10-2 ASTM D2434
a. Coarse and Fine glass samples represent minus 19 mm (3/4-in) and minus 6.4 mm (1/4-in) samples, respectively, collected from several MRF facilities unless otherwise indicated.b. Coarse specific gravity samples represent glass fraction greater than 4.75 mm (No. 4 sieve); fine samples represent glass fraction less than 4.75 mm (No. 4 sieve). c. ASTM D854 and C127 procedures were used for the coarse and fine fractions, respectively.
Glass collected from MRF facilities can be expected to exhibit a specific gravity of approximately 2.5. The degree of variability in this value depends on the degree of sample contamination and is reflected in the range of specific gravity data shown in Table 1. Crushed glass, which exhibits coefficients of permeability ranging from 10-1 to 10-2 cm/sec, is a highly permeable material, similar to a coarse sand. The actual coefficient of permeability depends on the gradation of the glass, which, in turn, depends on the degree of processing (crushing and screening) to which the glass is subjected. The particle size distribution of glass received from MRF facilities can vary greatly. Table 2 presents typical glass gradation values for crushed glass received from a New York City MRF.(5)
Table 2. Waste glass gradation results.a
Standard Sieve Size Average % Finer
25.4 mm (1 in) 100.0
12.7 mm (1/2 in) 98.7
6.35 mm (1/4 in) 86.0
3.18 mm (1/8 in) 32.6
0.84 mm (No. 20) 6.4
0.42 mm (No. 40) 3.2
0.21 mm (No. 80) 1.5
0.075 mm (No. 200) 0.6
a. Represents waste cullet collected from a City of New York MRF in 1993.
Chemical Properties Glass-formers are those elements that can be converted into glass when combined with oxygen. Silicon dioxide (SiO2), used in the form of sand, is by far the most common glass-former. Common glass contains about 70 percent SiO2. Soda ash (anhydrous sodium carbonate, Na2CO3) acts as a fluxing agent in the melt. It lowers the melting point and the viscosity of the formed glass, releases carbon dioxide, and helps stir the melt. Other additives are also introduced into glass to achieve specific properties. For example, either limestone or dolomite are sometimes used in lieu of soda ash. Alumina, lead, and cadmium are used to increase the strength of the glass and increase resistance to chemical attack. Various iron compounds, chromium compounds, carbon, and sulfur are used as coloring agents. Most glass bottles and window glass are made from soda-lime glass, which accounts for approximately 90 percent of the glass produced in the United States. Lead-alkali-silicate glasses are used in the manufacture of lightbulbs, neon signs, and crystal and optical glassware. Borosilicate glasses, which have extraordinary chemical resistance and high temperature softening points are used in the manufacture of cooking and laboratory ware.(6) Table 3 lists the typical chemical compositions of these glasses.
Table 3. Typical chemical composition of glass types.
Constituent Soda-Lime(7) Lead(7) Borosilicate(8)
SiO2 70 - 73 60 - 70 60 - 80
Al2O3a 1.7 - 2.0 -- 1 - 4
Fe2O3 0.06 - 0.24 -- --
Cr2O3b 0.1 -- --
CaO 9.1 - 9.8 1 --
MgO 1.1 - 1.7 -- --
BaO 0.14 - 0.18 -- --
Na2O 13.8 - 14.4 7 - 10 45
K2O 0.55 - 0.68 7 --
PbO -- 15 - 25 --
B2O3 -- -- 10 - 25
a. Higher levels for amber-colored glass. b. Only present in green glass.
Glass is generally considered an inert material; however, it is not chemically resistant to hydrofluoric acid and alkali. Expansive reactions between amorphous silica (glass) and alkalis (such as sodium and potassium found in high concentrations in high alkali Portland cement) could have deleterious effects if glass is used in Portland cement concrete structures.(8,9) Mechanical Properties Typical mechanical properties for glass are listed in Table 4.(2) Glass is a brittle material that fractures from tensile stress. Gravel-sized glass particles that are greater than 4.75 mm (No. 4 sieve) in size exhibit relatively poor durability when compared with conventional aggregate materials. The internal friction angle or shear strength and the bearing capacity of crushed glass blended with conventional aggregates is relatively high, and its compactibility is relatively insensitive to moisture content. Due to its vitreous, inert nature, crushed glass can be expected to exhibit good soundness properties, but poor frictional properties. Typical mechanical properties if waste glass are shown in Table 4.
Table 4. Typical mechanical properties of waste glass.
Test Results Test Method
Los Angeles Abrasion (%) 30 - 42 ASTM C131
Maximum Dry Density, kg/m3 (lb/ft3 Optimum Moisture (%) 1800 - 1900 (111 - 118) 5.7 - 7.5 ASTM D1557
Angle of Internal Friction (deg) 51 - 53 ASTM D3080
California Bearing Ratio (%) 15% glass 50% glass 132 42 - 125 ASTM D1883
Hardness (measured by Moh's Scale of Mineral Hardness) 5.5(10)  
Other Properties (Thermal, Reflection, and Glare) Glass is known for its insulating or heat-retention properties (low thermal conductivity). Aggregates and aggregate mixtures with low thermal conductivity can help to decrease the depth of frost penetration. Studies conducted at the Colorado School of Mines in the early 1970's reported that glass aggregate pavements take a longer time to cool down due in part to the lower thermal conductivity of glass, when compared to natural aggregates.(11) Recent thermal conductivity test results (ASTM C518) for mixed-colored crushed glass are presented in Table 5.(2) Comparison of the results for crushed glass with those for a natural gravelly sand aggregate mix show that glass can be expected to exhibit higher heat retention than natural aggregate materials.
Table 5. Thermal conductivity test results.
Material Apparent Thermal Conductivity(1) Watts/Meter - °K
Sample 1 Sample 2
Crushed Glass 0.315 0.260
Gravelly Sand 0.463 0.638
1. Results are presented for two separate samples.
The high reflective properties of glass can be a desirable property in highway construction if they assist in delineating the roadway surface from the surrounding environs. Excessive reflection could, however, result in glare that could adversely affect roadway visibility. There are no documented studies on the quantities of size fractions of glass in pavements that are likely to produce excessive glare. There is, however, a noticeable glass reflection in pavements with glass fractions exceeding 15 percent by weight.

REFERENCES

  1. EPA. Characterization of Municipal Solid Waste in the United States: 1995 Update, EPA 530-R-96-001, March 1996.
  2. Washington State Department of Trade and Economic Development, Glass Feedstock Evaluation Project, 1993.
  3. Chesner, W. H. and Petrarca, R. W. "The Ecosphere Recycling System and the Use of Glass as a Construction Aggregate Material,"Proceedings of the 1988 Conference on Solid Waste Management and Materials Policy. Legislative Commission on Solid Waste, New York State, January 1988.
  4. Egosi, N. G. "Mixed Broken Glass Processing Solutions," Utilization of Waste Materials in Civil Engineering Construction. Editors H. Inyang and K. Bergeson, American Society of Civil Engineers, 1992.
  5. Chesner, W. H. "Waste Glass and Sludge for Use in Asphalt Pavement," Utilization of Waste Materials in Civil Engineering Construction. Editors: H. Inyang and K. Bergeson, American Society of Civil Engineers, 1992.
  6. Samtur, H. Glass Recycling and Reuse, University of Wisconsin, National Science Foundation, NTIS PB239674, March 1974.
  7. Ahmed, I. Use of Waste Materials in Highway Construction, Purdue University, FHWA/INJHRP-91/3, January 1991.
  8. Phillips, J. C., D. S. Cahn, and G. W. Keller. "Refuse Glass Aggregate in Portland Cement Concrete," Proceedings of the Third Mineral Waste Utilization Symposium, U.S. Bureau of Mines, Chicago, Illinois, March 1972.
  9. Johnston, C. D. "Waste Glass as Coarse Aggregate for Concrete." American Society for Testing Materials, Journal of Testing and Evaluation, Vol. 2, No. 5, 1974.
  10. Duckett, E. J. Cullet from Municipal Waste, Ceramic Industry, April 1979.
  11. Dickson, P. "Cold Weather Paving with Glassphalt," Albuquerque Symposium on Utilization of Waste Glass in Secondary Products, 1973.

Waste Glass - Asphalt Concrete

INTRODUCTION

Waste glass that is crushed and screened can be used as a portion of fine aggregate in asphalt paving mixes. Satisfactory performance has been obtained from hot mix asphalt pavements incorporating 10 to 15 percent crushed glass in wearing surface mixes. The term "glasphalt" has at times been used to describe these pavements. Higher blends, incorporating perhaps up to 25 percent, could potentially be used in base or binder course mixes. Hot mix asphalt surface course pavements with more than 15 percent waste glass may experience deterioration due to stripping of the asphalt cement binder from the waste glass.

PERFORMANCE RECORD

At the present time, the commercial use of waste glass in asphalt paving applications has been limited to communities such as the City of New York, where the quantity of waste glass produced and collected provides sufficient incentive to recycle it in pavement applications. Most of the earlier applications of glass use have been limited to test pavements or specialty applications. In the late 1960's and early 1970's a number of studies and field demonstrations were undertaken in the United States. to examine the potential for using waste glass as an aggregate substitute material in hot mix asphalt. (See references 1,2,3,4,5.) During this period test paving strips were placed at approximately 33 locations throughout the United States and Canada.(6) From the mid-1970's through the mid-1980's, the City of Baltimore made use of glass in its street pavement program. At least 17 streets were paved with glass to produce a "sparkle" effect, resulting from the reflection of sunlight or street lamp light off the glass pavement. In the mid-1980's research activities were undertaken on Long Island and a glass processing plant was designed and began operations, processing over 12,600 metric tons (14,000 tons) of mixed waste glass for use as an aggregate substitute in paving applications.(7) More recently, numerous paving projects using waste glass have been undertaken around the country. However, by far the most aggressive program has been undertaken by the City of New York's Department of Transportation, where from 1990 through 1995 approximately 225,000 metric tons (250,000 tons) of glass has been used in resurfacing applications.(8) Flat and elongated particles that could contribute to pavement raveling, stripping, poor skid resistance, abnormally high tire wear, and excessive glare were all identified by early researchers as potential problems. Since glass does not absorb any of the asphalt cement binder, and since glass is also "hydrophilic," moisture damage (stripping) is a particular concern that has been identified, especially when high percentages and large gradations are introduced into a surface course mix.(9) Many of the early investigators recommended the addition of lime as an antistripping agent to reduce potential stripping problems. Early glasphalt projects used high percentages of glass (greater than 25 percent by weight of the mix) with coarse glass gradations (greater than 12.7 mm (1/2 in)). Current data suggest that the use of high glass percentages and large particles of glass probably contributed to most of the stripping and raveling problems that were reported during the early test pavement demonstrations of the 1960's and 1970's. The high angularity of cullet, compared with rounded sand, can enhance the stability of asphalt mixes where properly sized cullet is used. Stabilities comparable and, in many cases, better than conventional mixes have been reported.(5,7,10) Other beneficial characteristics include low absorption and specific gravity and low thermal conductivity, which reportedly offers enhanced heat retention in mixes with glass.(10)

MATERIAL PROCESSING REQUIREMENTS

Cleaning When used in asphalt concrete, glass processing must include the removal of ferrous and nonferrous metal, plastic, and paper. In most waste glass processing plants this requires screening, magnetic and eddy metal current (non-ferrous metal) separation, air classification, and/or handpicking operations. Although 100-percent removal of all paper, plastic, and debris from postconsumer glass streams is unlikely, an acceptable glass product can be achieved in most instances, particularly if mix designs limit glass to 10 to 15 percent of the mix. Crushing and Screening Crushing and screening are required to achieve proper sizing and to eliminate flat and/or elongated and sharp-edged glass particles.

ENGINEERING PROPERTIES

Some of the glass properties that are of particular interest when glass is used as fine aggregate in asphalt paving include gradation, specific gravity, and durability. Gradation: Waste glass used in asphalt surface pavements should be processed to a fine aggregate size (less than 4.75 mm (No. 4 sieve) and blended with conventional aggregates to conform to gradation requirements in accordance with AASHTO T27.(11) Larger top sizes ranging from 9.5 mm to 15.3 mm (3/8 to 5/8 in) should be suitable for use in base course mixes. Specific Gravity: Due to a specific gravity approximately 10 to 15 percent below conventional aggregates, waste glass can be expected to provide a greater yield (more volume of asphalt concrete per ton). Durability: Glass is a brittle material and coarse particles greater than 4.75 mm (3/8 in) in size can be expected to break down during handling. Consequently, it is preferable to process (crush and screen) waste glass into a fine aggregate size, which is minus 4.75 mm (No. 4 sieve), prior to its use in surface course asphalt paving mixes. Some of the properties of an asphalt mix containing glass that are of particular interest include frictional properties, mix stability, stripping resistance, and reflectivity. Frictional Properties: Skid resistance tests results that have been reported have shown waste glass pavements to fall within recommended skid resistance testing limits. Nonetheless, large glass particles (greater than 19 mm (3/4 in) in size) that have at times been incorporated into poorly processed surface course pavements could become slick when wet and should be avoided in all surface mixes containing glass. Mix Stability: The angular shape and high friction angle (approximately 50°) of well-crushed glass contributes to good lateral stability. This is a positive feature, particularly where vehicular braking and acceleration are considerations. Stripping Resistance: Glass is not absorptive and bonds poorly to asphalt binder. Antistripping agents such as hydrated lime introduced as 2 percent of the aggregate mix by weight have been used in previous demonstrations to reduce potential stripping problems. Poor immersion-compression test results (retained stability), a measure of the potential for stripping problems, can also be expected where a high percentage of oversized glass particles are introduced into a mix.(10) Reflectivity: Large percentages of glass in a surface pavement (greater than 15 percent by weight) produce a noticeable increase in the reflectivity of the pavement. Depending on the size of the glass particles, this could produce a noticeable glare, particularly on wet pavements. Smaller glass particles and lower percentages of glass can help to reduce reflective glare problems.

DESIGN CONSIDERATIONS

Mix Design Asphalt mixes containing crushed glass can be designed using standard laboratory procedures. Conventional fine hot mix aggregate gradations, as specified in AASHTO M29,(12) may be used. It is recommended that mix design testing include stripping potential evaluations as outlined in AASHTO T283.(13) Currently most highway departments allow the use of 5 to 10 percent glass in their asphalt mixes. Although some areas use 6.4 mm to 12.7 mm (1/4 in to 1/2 in) gradations and larger, many users are taking a more conservative approach to gradation size. The City of New York has lowered its specified gradation top size in its mix design to minus 9.5 mm (3/8 in) from 15.3 mm (5/8 in). Los Angeles has specified the use of minus 9.5 mm (3/8 in) glass. Studies in Virginia and Florida also have recommended that minus 9.5 mm (3/8 in) gradation be used.(9) Most data at the present time indicate that larger gravel-sized glass particles will reduce pavement performance, and that optimum performance can best be achieved by using crushed glass as a sand or fine aggregate substitute (less than 4.75 mm, or No. 4 size sieve). When waste glass is used as a fine aggregate substitute material, glass performance in hot mix asphalt should be comparable to conventional mixes.(14) Where larger, gravel-sized glass particles are used, raveling and stripping in particular could be a problem. The introduction of an antistripping agent such as hydrated lime (approximately 2 percent by weight of aggregate) could be beneficial, but performance should be satisfactory if only fine-grained (minus 4.75 mm (No. 4 sieve) glass is used and substitution rates do not exceed 15 percent. Base course applications, being less susceptible to stripping, rutting and skid resistance, could tolerate the introduction of glass particles up to 15.3 mm (5/8 in) in size, resulting in substitution of both coarse and fine aggregates in the mix. Although higher percentages could probably be introduced, a 15 percent weight limitation would provide for a conservative design. Extraneous debris (e.g., paper, dirt, etc.) sometimes associated with waste glass can be expected to adversely impact mix quality. The introduction of excessive debris from Material Recovery Facilities can be expected to increase the void content of most mixes, if the asphalt content or the mix gradation is not adjusted. Recent recommendations suggest that extraneous debris should be limited to 5 percent of the glass by weight to avoid significant impacts on glass quality.(15) Structural Design Conventional AASHTO pavement structural design methods are appropriate for asphalt pavements incorporating waste glass in the mix.

CONSTRUCTION PROCEDURES

Material Handling and Storage The same methods and equipment used to store or stockpile conventional aggregates are applicable for waste glass, particularly where glass is properly precrushed to a fine, sand-size fraction, where additional breakdown is not a concern. Mixing, Placing, and Compacting The same methods and equipment used for conventional pavements are applicable to asphalt pavements containing waste glass. Quality Control The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing waste glass. Mixes should be sampled in accordance with AASHTO T168,(16) and tested for specific gravity in accordance with ASTM D2726(17) and in-place density in accordance with ASTM D2950.(18)

UNRESOLVED ISSUES

The development of uniform specifications concerning sizing, levels of debris and mix limitations are needed to facilitate glass use. There is some uncertainty regarding the need for antistripping agents such as lime if glass is reduced to a very fine aggregate size (less than 6.35 mm (1/4 in)). The most limiting constraint to glass use is the lack of an adequate and consistent supply of the product. In only a few instances, such as in the City of New York, have provisions been made to establish a continuous market supply of glass. The elimination of hand sorting and crushing of all glass to produce a market-ready aggregate product is probably required to achieve more widespread glass use.

REFERENCES

  1. Leite, B. J. and D. D. Young. Use of Waste Glass as Aggregate for Pavement Material, Department of Engineering and Construction, City of Toledo, Ohio, 1971.
  2. Abrahams, John H., Jr. "Recycling Container Glass -- An Overview," Proceedings of Third Mineral Waste Utilization Symposium, Chicago, Illinois, 1972.
  3. Molisch, W. R., T. E. Keith, D. E. Day, and B. G. Wixson. "Effects of Contaminants in Recycled Glass Utilized in Glasphalt," University of Missouri - Rolla, Proceedings of the Third Mineral Waste Utilization Symposium, Chicago, Illinois, 1972.
  4. Abrahams, J. "Road Surfacing with Glass Aggregate," Albuquerque Symposium on Utilization of Waste Glass in Secondary Products, 1973.
  5. Molisch, W. R., et al. Use of Domestic Waste Glass for Urban Paving,, University of Missouri -- Rolla, 1975.
  6. Samtur, H. Glass Recycling and Reuse, IES Report No. 17, University of Wisconsin-Madison, 1974.
  7. Chesner, W. and R. Petrarca. Report on Glass Aggregate Pavement for the Browning Ferris Industries' Merrick Transfer Station Located in Hempstead, New York, August 1987.
  8. Slater, William, New York City Hamilton Avenue Asphalt Plant Manager, Telephone Communication, November 1995.
  9. Flynn, Larry. "Glasphalt Utilization Dependent on Availability." Roads and Bridges, February 1993.
  10. Petrarca, R. "Use of Glasphalt," Paper Presented to the Long Island Society of Asphalt Technologists, 1988.
  11. American Association of State Highway and Transportation Officials. Standard Method of Test, "Sieve Analysis of Fine and Coarse Aggregates," AASHTO Designation: T27-84, Part II Tests, 14th Edition, 1986.
  12. American Association of State Highway and Transportation Officials. Standard Specification for Materials, "Fine Aggregate for Bituminous Paving Mixtures," AASHTO Designation: M29-83, Part I Specifications, 14th Edition, 1986.
  13. American Association of State Highway and Transportation Officials. Standard Method of Test, "Resistance of Compacted Bituminous Mixtures to Moisture Induced Damage," AASHTO Designation: T283-85, Part II Testing, 14th Edition, 1986.
  14. Chesner, W. "Waste Glass and Sewage Sludge Ash Use in Asphalt Pavement," Utilization of Waste Materials in Civil Engineering Construction. American Society of Civil Engineering. 1992.
  15. Washington State Department of Trade and Economic Development, Glass Feedstock Evaluation Project, 1993.
  16. American Association of State Highway and Transportation Officials. Standard Method of Test, "Sampling Bituminous Paving Mixtures," AASHTO Designation: T168-82, Part II Tests, 14th Edition, 1986.
  17. American Society for Testing and Materials, Standard Specification D2726-96, "Bulk Specific Gravity and Density of non-Absorptive Compacted Bituminous Mixtures," Annual Book of ASTM Standards, Volume 04.03, West Conshohocken, Pennsylvania, 1996.
  18. American Society for Testing and Materials, Standard Specification D2950-96, "Density of Bituminous Concrete in Place by Nuclear Methods,"Annual Book of ASTM Standards, Volume 04.03, West Conshohocken, Pennsylvania, 1996.

Waste Glass - Granular Base

INTRODUCTION

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.

PERFORMANCE RECORD

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) Cleaning 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.

ENGINEERING PROPERTIES

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.

DESIGN CONSIDERATIONS

Mix Design 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 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 6. Recommended cullet gradation for use as a structural fill or granular base material.(1)
Size % Finer
1/4-inch 10 - 100
No. 10 0 - 50
No. 40 0 - 25
No. 200 0 - 5
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. Structural Design Conventional AASHTO pavement structural design procedures can be employed for granular base containing waste glass.

CONSTRUCTION PROCEDURES

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. Quality Control 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).

UNRESOLVED ISSUES

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.

REFERENCES

  1. Washington State Department of Trade and Economic Development, Glass Feedstock Evaluation Project, 1993.
  2. American Association of State Highway and Transportation Officials. Standard Specification for Materials, "Fine Aggregate for Bituminous Paving Mixtures," AASHTO Designation: M29-83, Part I Specifications, 14th Edition, 1986.
  3. American Geophysical Institute. AGI Data Sheet 15.1 and 15.2, Comparison Chart for Estimating Percent Composition, 1982.
  4. American Association of State Highway and Transportation Officials. Standard Specification for Materials, "Aggregate and Soil-Aggregate Subbase, Base and Surface Courses," AASHTO Designation: M147-70 (1980), Part I Specifications, 14th Edition, 1986.
  5. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Sand Cone Method," AASHTO Designation: T191-86, Part II Tests, 14th Edition, 1986.
  6. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Rubber-Balloon Method," AASHTO Designation: T205-86, Part II Tests, 14th Edition, 1986.
  7. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil and Soil-Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T238-86, Part II Tests, 14th Edition, 1986.
  8. American Association of State Highway and Transportation Officials. Standard Method of Test, "Moisture Content of Soil and Soil Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T239-86, Part II Tests, 14th Edition, 1986.