City Finds Cutting: Edge To Recycling Glass

In efforts to provide a market for glass cullet - broken or colored glass that typically is landfilled - a number of states have investigated its use in roadway construction. For example, in early 1995, the Texas Department of Transportation (DOT) asked the College of Engineering at Texas Tech University, Lubbock, Texas, to develop specifications for using glass cullet in roadway construction.

The study's results, plus the success of glass cullet use in other states, prompted the DOT and the Texas Natural Resource Conservation Com-mission (TNRCC) to seek places to test the material's longevity. Meanwhile, the city of Devine, Texas, was searching for funding and alternative methods to reconstruct many of their roadways since the city budget did not provide for major street construction.

Encouraged by the Alamo Area Council of Governments' solid wastes coordinator, the city applied for and received a TNRCC grant in May 1996 to use glass cullet in the reconstruction of approximately 4,000 linear feet of its streets.

Two 20-foot-wide streets were chosen for the project in the city's northern portion. Prior to their reconstruction, the roadways consisted of a single coarse surface, with numerous potholes filled with approximately four inches of asphaltic concrete, on six inches of compacted base material and a sand subgrade.

To save landfill space, the old asphaltic concrete was set aside to be mixed with the road base to form the subgrade for the new streets. Also, limestone rock asphalt, native to Texas, was slated to be used for the roadway surface.

Originally, the glass cullet was to be shipped weekly from Vista Fibers, San Antonio, Texas, to the road site to be used in the compacted base material; however, the size and grading of the cullet was not consistent with DOT gradation standards. So, an additional contractor, Vulcan Materials, San Antonio, Texas, was used to further reduce the cullet's size.

The mixture of two loads of limestone followed by one load of glass cullet followed by two more loads of limestone were placed in a crusher. The material then was crushed to a 31/44 inch maximum size. This gradation size versus the more typical one- and one-half inch grade, reduced the size of plastic bottle caps and other debris in the cullet.

Samples taken from Vulcan's stockpile showed that the combined mix was uniform in glass distribution and gradation. Laboratory tests confirmed that the amount of glass did not exceed 20 percent by weight of the total base material, which met DOT specs. More than 2,400 tons of the material then was produced, diverting more than 435 tons of glass cullet from the landfill.

The project, designed by Garcia & Wright Consulting Engineers Inc., San Antonio, Texas, included safety issues, such as the potential hazard of exposed glass on the roadways' shoulders. In response, the shoulder width beyond the pavement's edge was minimized and treated with asphalt emulsion prime coat material. In addition, existing gravel driveways also were paved to reduce concerns about exposed glass in Vulcan's combined base material.

The project's contractor, Evans & Evans Inc., New Braunfels, Texas, found that working with the combined base material did not impose any significant restraints to construction. However, plastic bottle caps were observed, particularly at the roadway's edge. Apparently plastic "floats" to the top and edges during processing, causing small "pockets" during construction. Pull tabs from drink cans also were noted, but were evenly dispersed within the material.

Although the construction costs of the combination material are comparable to those with standard materials, they still are somewhat higher. Some of this extra cost can be attributed to the additional handling of the glass cullet required during crushing. Notably, removing glass cullet debris either at the recycling center or at the aggregate plant can raise costs.

Vista Fibers saved money since it didn't have to pay disposal fees for the cullet used in the project. This savings will allow the center to buy the necessary equipment to process the glass prior to shipping it to the base material supplier. Cleaner glass cullet could process easier, cost less and ultimately be more competitive with standard materials.