Chapter 2: Construction and Demolition Wastes

2.3. COMPOSITION OF WASTE (CDW)

   Construction waste does not typically have the same characteristics as demolition waste. Building material inputs are principally needed during construction and operation. A builder will typically order excess sizes or quantities of materials to ensure that there will be enough materials on hand to construct the building.

            One economic objective in the construction of a building is to produce a facility in the most cost-effective manner possible. For a contractor, this may involve procuring standardized building products to take advantage of the economies of scale. A contractor purchases rolls of insulation, sheets of gypsum wall board, etc., rather than single pieces. To accommodate the specific situation of a particular system or configuration, these standard units must be trimmed to fit. By weight or by volume, wood, drywall and cardboard combined make up between 60 and 80 percent of jobsite waste in construction projects. Even prefabricated assemblies, such as doors and windows, which are packaged in large quantities of cardboard, metal or plastic strapping, and wood tend to produce a significant amount of waste at construction sites. Concrete is also one of the largest and most visible components of construction waste. 

          In new construction, partial truckloads of concrete have long been a disposal problem. To avoid creating waste, ready mix plants have come up with many innovative solutions. Unused portions are returned to produce concrete retaining wall blocks or highway dividers, or the unset concrete is washed to recover the coarse aggregate for reuse. Construction waste will also vary according to the type of project. For example, the construction of an interior partition will generally involve discarding lumber cutoffs, scrap pieces of gypsum wall board, and excess insulation. The framing of wood buildings typically produces, by percent to square foot of area, the most waste. However, an efficient design and framing plan cannot only reduce the required amount of material but can also decrease disposal costs.

           Demolition produces a significantly different waste than construction or renovation. Demolition debris is more likely to contribute materials contaminated by undesirable components and/or potentially toxic substances such as lead paints, stains, and adhesives. In some cases, the physical and chemical makeup of the individual constituent material has been changed by the use of finishes or maintenance techniques. Wood flooring, for instance, when finished with a sealer, such as varnish, has an altered chemical composition. In terms of solid waste management, the biggest problem with demolition debris is that the various types of debris are mixed together during the course of the demolition. 

          Even with careful manual disassembly of the structure (deconstruction), different types of materials mixed together are still possible. For example, the removal of exterior walls in a load-bearing masonry system will result in a combination of masonry units, such as concrete block or brick, mild steel reinforcing metal, metal ties, and grout. In this case, further processing beyond typical deconstruction practices would be required to separate all of the dissimilar materials.

            As mentioned above, the composition of C&D waste varies according to the type of project and the method of construction and demolition. In terms of the individual components, there are sixteen categories of materials that make up C&D waste in general. Table given below lists these categories and their sources. Each class of materials consists of several different types of materials. This information is a survey of all individual components that may or may not be found in a building.

               Several of these classes of materials, such as concrete, masonry, and ceramics are considered inert by solid waste authorities, because they will not degrade by bacterial activity once landfilled. There are, however, many components of C&D waste that are not inert in nature and, therefore, are putrescible. Wood is the best example of a material the will putrefy under the right conditions in a landfill. There are also several types of materials that can be considered chemically-reactive, such as paint, paint thinner, etc., and must be handled in a special manner.
(This Chapter is continue.....)