Recycled, Co-Product, Waste Material - What's The Difference?
Recycled Materials
Recycled materials are obtained from an old pavement and are included in materials to be used in the new pavement. Common recycled materials include reclaimed asphalt pavement or recycled concrete pavement. Depending on the regional market, these materials would be "waste" if not recycled, ending up in a landfill. Allocation of environmental impact between the manufacture of the original material and its reuse in the new material is based on the processing needed to make this material suitable for use in the new pavement. The demolition of the existing pavement and its transportation to a processing plant is allocated to the old pavement.
Co-Products
Co-products are derived as part of another process-often industrial but possibly agricultural-that brings value to the overall process. For pavement applications, some of the most common co-products result from the production of pig iron for steel making, including slag cement and air-cooled iron blast furnace slag aggregate. Allocation for co-products is based on some agreed upon approach, but most often is based on economic worth of the various co-products.
Waste Materials
Wastes are materials that normally would be sent to a landfill, for which the cost of transport and processing is the only source of economic value. If the material has value beyond this, it is no longer considered a waste, but instead a co-product. Recycled asphalt shingles is an example of one such waste material as long as the economics stay consistent with the above definition. The classification of fly ash is more complex, as in some regional markets it would fit the definition of waste whereas in other markets it is clearly a co-product because it has economic value beyond the cost of transport and disposal.
For more information, see Chapter 3 (.pdf) of the Reference Document.