Recycled content plays a pivotal role in structural design in structural design

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    Have you been tasked with filling out a "green checklist" for a sustainably design project and had no any idea where to start? You might want to start with recycled content, since it’s likely you’re already specifying materials containing recycled content.

    The major reason sustainable design encourages specification of materials that use recycled content is because this action helps to reduce our reliance on the extraction and processing of virgin materials (which have high monetary and environmental costs). Using recycled content can encourage the reduction of solid waste and the decreased use of water and energy. On the larger-world scale, specifying recycled content can ultimately help reduce greenhouse gas emissions.

    The commonly accepted definition of recycled content is the ratio of how much recycled material is contained, by weight, in a finished product. Recycled material is defined to include both post-consumer and pre-consumer content.

    Post-consumer waste is material produced by the end consumer (households, commercial, industrial, or institutional facilities) that can no longer be used for its originally intended purpose.

    Pre-consumer reclaimed content (also called post industrial) is material diverted from the waste flow during a manufacturing or fabrication process; this material is then reintroduced into another production process, but not necessarily reworked from or reclaimed within the same process.

    Recycled building materials

    Steel is the most well-known recycled material. Each year, millions of tons of pre- and post-consumer steel products, including cars, appliances, cans, and construction materials, are recycled by steel mills into new steel. For example, nearly 100 percent of the feedstock for the production of reinforcing steel is recycled scrap.

    The recycled content of concrete can be obtained through the use of fly ash, ground granulated blast slag, and silica fume. These post-industrial wastes are used as partial replacements for Portland cement. Post-consumer recycled concrete can be broken up and used in the form of concrete aggregate.

    Concrete masonry units, similar to concrete, may contain supplementary cementitious materials such as fly ash, silica fume, and slag cement. Concrete masonry can also incorporate glass or other recycled materials. Mortar may contain recycled materials such as fly ash.

    Both recycled plastic lumber and wood-plastic composite lumber products are extruded into standard lumber forms or custom shapes. Plastic lumber is made from recovered plastics such as HDPE, LDPE, PET (many of the plastics disposed in municipal recycling programs). Wood/plastic composite lumber is made from plastic resins (such as trash bags) and reclaimed wood (sawdust); however, this material cannot be recycled again.

    Certification of recycled content for a particular product comes in the form of a written affidavit from the manufacturer or supplier stating the material has the specified amount of recycled material within the product. Additional information provided to justify the recycled content is product cost, manufacturer data, and pre-consumer/post-consumer recycled content percentage. Most certification systems require third-party certification to receive credit, and reputable claims are usually backed up by such certification.

    Early in the design phase, recycled content goals are established. Research is required to determine the achievable recycled content goals for the project materials. Once the project materials are selected and a preliminary design has been completed, a preliminary calculation can be performed to set project objectives for recycled content. As the project continues, the structural engineer may be asked to obtain documentation from suppliers, manufacturers, and vendors to confirm the actual recycled content for each material or product.

    Although recycled content is an important consideration for a sustainable design strategy, remember that recycling is the last of the 3Rs of waste hierarchy—reduce, reuse, and recycle. It is best to first reduce (material efficiency), and to then reuse (salvaged materials). But when you do turn to recycling, consider the following:

    * Choose materials with the highest recycled content available. Specifically, search for the highest post-consumer content available. Reducing manufacturing waste is beneficial, but recycling post-consumer material has a more positive impact.

    * Consider materials that are not just recycled prior to the project use, but can be recycled after their useful life.

    * Learn how the material is produced. If an inordinately high amount of energy or other natural resources are used in production of the material; should the material be considered for use in a "green" building?

    * Understand how the material can be recycled from its current form; or will it be difficult or impossible to ever recycle again?

    Sustainably minded structural engineers who understand and appreciate the importance of specifying recycled content in building materials can be great assets and resources to project teams. Understanding the life cycle of the materials we use is a key component to success in working with recycled content. So the next time that "green checklist" comes around, you can be sure that as a structural engineer, you have something to add.

    Gary D. Garbacik, P.E., is a structural engineer with Gannett Fleming, Inc., in Harrisburg, Pa. He can be reached at ggarbacik@gfnet.com. Garbacik is a member of the ASCE Structural Engineering Institute’s Sustainability Committee. The committee website is www.seinstitute.org/committees/sustainable.cfm.