Crushed glass is being recycled on many construction projects as a 100% substitute for natural aggregates or in an aggregate blend. Glass aggregate and blends are strong, clean, safe, and economical. However, glass aggregates do not look the same as conventional aggregates and behave somewhat differently. To facilitate site inspection of glass aggregates, this technology brief compares their characteristics to those of conventional aggregates.

Appearance

As a post-consumer recycled product, crushed glass often contains paper and plastic not normally found in natural aggregates. In a 100% glass aggregate product, this debris can appear to have a large volume because of its plate-like nature. Caps and labels, the most common contaminants, present a large surface plane to the eye as they rest in the pile, but are proportionally thin. Visual estimates of the percent of debris based on exposed surface area are 2.5 to 5 times greater than the measured debris content by weight (1). Glass can also pick up dirt and become scratched and pitted during processing, giving the glass a gritty surface. As a result of the nature of the debris and the processing, crushed glass often appears "dirty" compared to conventional aggregates.

The Washington State Department of Transportation (WSDOT) specifications for glass in construction aggregate applications limit the amount of debris to 10% by visual inspection, depending on the application (2). This corresponds to 2% to 4% debris by weight. The visual inspection method is an easily performed field test derived from methods developed by the American Geological Institute. More detailed information on this procedure can be found in the Glass Feedstock Evaluation Project report (1). It is recommended that field inspectors use this procedure to form a sense of what is"typical" appearance for glass aggregates.

Safety

Bottle glass, the major source of recycled glass, is "soft" compared to plate glasses and has a different breaking pattern. Broken bottle glass contains very few shards. Also, as glass is crushed, it becomes more rounded and less sharp. As a result, crushed glass aggregate is no more dangerous than crushed rock aggregate. Safety clothing worn for working with crushed rock aggregates is adequate for working with crushed glass.

Some contractors have expressed concern about the health effects of the dust generated from dumping, leveling, and compacting glass aggregate. Bottle glass consists primarily of amorphous, or non-crystalline, silica (2). It is crystalline silica that has been associated with silicosis. Accordingly, OSHA regulations governing the permissible levels of dust classify glass dust as a"nuisance" dust and not a hazardous dust.

Permeability

WSDOT has approved glass aggregate as a 100% replacement for natural aggregates in six applications: backfill for walls, backfill for pipe bedding, backfill for sand drains, sand blanket, and bedding material for flexible pipe. The table at right shows that a much higher percentage of fines is allowed to be used with glass aggregate than with the traditional materials (2,3).

Compaction

Glass aggregate and glass aggregate blends have favorable compaction characteristics which provide good workability of the material.

The blending of crushed glass with other aggregates in low percentages (<15%) will not noticeably affect the compaction characteristics of the major constituent of the blend. Since glass aggregate has a lower specific gravity than conventional aggregates, compacted blends may have a lower density. This lower density should be accounted for in the project specifications.

The compacted density of 100% glass aggregate and higher percentage blends are relatively insensitive to moisture content. This means that the material can be placed and effectively compacted during wet weather, keeping construction downtime to a minimum.

Full-scale testing shows that a nuclear densometer can be used to measure the density and moisture content of 100% glass aggregate fills without the need for correction factors. No change is required in standard nuclear densometer test procedures (5). The older sand cone and water balloon test methods are also reliable with 100% glass aggregates.

References

(1) Glass Feedstock Evaluation Project. Clean Washington Center, 1993.

(2) Amendment to 1994 Standard Specifications Section 9-03.21. Washington State Department of Transportation, June 3, 1994.

(3) Standard Specifications for Road, Bridge, and Municipal Construction. Washington State Department of Transportation, 1994.

(4) Respiratory Health Aspects of Ground Class vs. Ground Silica Fact Sheet. Clean Washington Center, 1994.

(5) Suitability of Nuclear Densometer Testing on Glass Cullet Aggregate. Clean Washington Center, 1994.

This technology brief was prepared by the Recycling Technology Assistance Partnership (ReTAP). ReTAP's mission is to advance industry's use of recycled materials through technology extension services. ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology.

Reports Dated: 1993 and 1994
Fact Sheet Update: September 1995