Structural waste: Where is away?


    In the book Cradle to Cradle (2002), M. Braungart and W. McDonough ask the question, "When something is thrown away, where is ’away’?" The book questions our current approach to material use, disposal, and recycling. It also questions the assumption that structures have to be bad for the environment. There is no simple hierarchy to indicate which structural materials are better for our planet. But a thoughtful approach to structural waste and closing the materials loop at the end of a building’s life is an important part of good design.

    Waste from industrial processes can be thought of as either solid waste or molecular waste. Solid waste from building construction and demolition may end up in a landfill, be reused, be recycled, or biodegrade. Molecular wastes are liquid and gas by-products that infiltrate the atmosphere, oceans, rivers, soils, plants, wildlife, and people. Industrial waste flows are largely hidden from view and the amount of waste generated to produce usable goods can be enormous in comparison to the final product (Natural Capitalism, Hawken, Lovins & Lovins, 1999).

    An intelligent perspective on waste is explained in Cradle to Cradle. Waste happens, but waste should "equal food." Waste that could be considered food falls into one of two categories: biological nutrients and technical nutrients. Biological nutrients are those that biodegrade and return to the earth as nourishment when they break down. Building material examples include wood, bamboo, and straw. Technical nutrients consist of industrial products that can be recycled such as steel, aluminum, glass, and some types of chemicals and plastics. In the best scenario, all useful nutrients are non-toxic to human health and the planet, and industrial wastes that are known to be toxic are phased out of use.

    Waste streams are not the only factor in structural material selection, but educated choices add value to our projects and reflect our concern for human and ecological health. Best choices when considering waste reduction and closed-loop material systems include the following:
    Reuse whole structural systems in existing buildings.
    Use structural materials that can be disassembled, salvaged, and reused. Salvaged materials do not go back through the industrial process. This eliminates additional solid and molecular wastes, along with the energy inputs involved with recycling. Material-grade marking, good as-built drawings, and the ability to disassemble and separate structural components will significantly increase the probability of reuse for materials such as reclaimed timbers, steel members, and pre-cast concrete elements. A design handbook by Bill Addis of Buro Happold Consulting Engineers, Building with Reclaimed Components and Materials (2006), includes case studies and design guidance on the topic.
    Where appropriate, use renewable, minimally processed natural materials that easily biodegrade without leaching toxins.
    Use structural elements fabricated from technical nutrients that can be completely recycled and where the resulting material quality is comparable to that of the original. This is only a best choice when the industrial recycling process can eliminate harmful solid and molecular waste by-products and where the re-manufacturing is powered by renewable energy sources.

    The proposed structure’s size, required durability, expected life, regional climate, and multiple other factors will further influence which of these options, and which material, best suits a project.

    Many recycled content materials are not best choices from a waste and toxicity perspective (although some are good choices). Two definitions from Cradle to Cradle explain why. "Monstrous hybrids" are materials that mix biological nutrients and technical nutrients. Such materials are nearly impossible to separate and recycle again. They either end up in the landfill, where biological nutrients are contaminated instead of replenishing the earth, or they are "downcycled." Downcycling occurs when the recycling of dissimilar materials of varying quality produces a material of even lower quality. The cycle continues with the material getting weaker and less useful with each recycling effort. The waste and energy to produce these materials can be significant and some require toxic inputs or long-lasting binders that cannot safely biodegrade in a reasonable amount of time. The use of recycled content is diverting waste from landfills in the short term. But monstrous hybrids and low-quality materials that result from downcycling will eventually also end up in a landfill.

    Looking at materials and systems through this lens provides a starting point to better understand the effects of what we specify and how we put building components together. Doing what we can to close the materials loop is a step in the right direction to eliminate structural waste that cannot later be used as food for another structure or nourishment of ecological systems.

    Carla Dhillon, P.E., LEED AP is a project engineer for Lionakis, an A/E firm with headquarters in Sacramento, Calif. She is a member of SEI’s Sustainability Committee, SEAOCC, the Northern California chapter of the USGBC, and the California Straw Building Association. She can be reached at The SEI sustainability committee website is