Waste material can be categorized as construction wastes, industrial wastes, mining or mineral wastes, agricultural wastes, or domestic wastes (of which scrap tires are a significant subset). Many advanced recycling programs have been established to make use of these wastes, such as requiring identifying codes for the base resin in plastic products to enable more refined recycling of plastics. Some of these wastes are not suitable for or do not make a significant recycling contribution to highway use. For example, only a small amount of the total crop waste (estimated to be about 9 percent of all the total nonhazardous solid waste generated each year in the United States) has a beneficial highway use. Potential uses are as an asphalt extender or portland cement additive.
In another example, it has been shown that wastes can be rendered essentially benign when used in asphaltic concrete installations. In a demonstration to the Minnesota Department of Transportation, the toxicity of bottom ash from a municipal sewage sludge incinerator was shown to be less than or equal to the toxicity of the asphaltic concrete matrix to which it was attached (Request for Approval of WIA in MnDOT Asphaltic Concrete Non-Wear Course Projects, Final report, S. David, Jan 16, 2002).
The following articles contain brief descriptions of the types of wastes that research has indicated have the potential for use in highway projects (NCHRP Synthesis 199).
