by sadia_badhon | February 26, 2020 11:22 am
by Alana Fossa
From commercial buildings and rehabilitation projects to bridges and highways, the recycling and reuse of construction materials is a great way to increase economic and environmental value. Although steel is considered to be a leader in sustainable building materials, its reuse is sometimes overlooked as it is known to rust. In reality, steel and galvanized steel can be reclaimed with minimal reprocessing and be reinstated to provide the same strength, durability, sustainability, and protection again and again. Integrated with both environmental and economic advantages, there are a wide variety of methods to reuse steel and galvanized steel to achieve sustainable designs that will last for years.
Steel and zinc: Sustainable building materials
As more architects, engineers, and owners subscribe to the green building movement, an effort to minimize the impact of construction on future generations has become a primary focus for many projects. In line with these goals, the specification of structural steel offers specifiers a high strength-to-weight ratio and low carbon footprint (1.16 tons of carbon dioxide [CO2] per ton of fabricated hot-rolled steel), thereby establishing steel as a popular building material for sustainable development. Further, all steel waste from production, fabrication, and erection is captured and recycled (read More than Recycled Content: The Sustainable Characteristics of Structural Steel by the American Institute of Steel Construction [AISC]).
There is an opportunity for additional environmental benefits when steel is hot-dip galvanized for corrosion protection. After fabrication, hot-dip galvanizing (HDG) is a coating process where steel is cleaned and then immersed in a bath of minimum 98 percent pure molten zinc. The result is the formation of a metallurgically bonded zinc coating comprising intermetallic alloy layers covered by a layer of pure zinc (Figure 1). In atmospheric environments, this zinc coating provides durable, maintenance- and emission-free protection for 70 years or more in most regions (Figure 2) (consult Performance of Hot-Dip Galvanized Steel Products in the Atmosphere, Soil, Water, Concrete, and More by the American Galvanizers Association [AGA]). Energy consumption and emissions data measured during zinc production, the galvanizing process, and life cycle of steel were used to conduct a life-cycle analysis (LCA) for hot-dip galvanized steel and demonstrate the environmental impact from production to end-of-life (Figure 3) (refer to Hot-dip Galvanizing for Sustainable Design by AGA). Over the life cycle, the use of HDG is unique because all material and energy inputs and emission outputs are isolated to the production phase because there are no emissions or maintenance after installation (refer to Hot-dip Galvanizing for Sustainable Design by AGA).
HDG can not only provide benefits in terms of lower environmental impact during use, but also allows steel to be reused or recycled efficiently for other projects at the end of its life. At end-of-life, both steel and zinc are recycled without waste or the loss of any physical or chemical properties. This means rather than being down-cycled into other or lesser products, zinc and steel can be reused again and again without compromising structural integrity (read Life-Cycle Assessment study for hot-dip galvanized balcony system compared with painted balcony system by S. Vares, K. Tattari, and T. Hakkinen). Also, steel has the highest recycled content of any building framing material at 93 percent, and 98 percent of all steel is currently being recycled (read More than Recycled Content: The Sustainable Characteristics of Structural Steel by AISC). Similarly, zinc has a high reclamation rate of 80 percent in North America and 95 percent in building projects (consult Zinc: A Sustainable Material Essential for Modern Life by the International Zinc Association [IZA]).
Impact for LEED projects
In the building industry, increased interest in using recycled and reused materials is driven by individual environmental awareness in addition to the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) rating system. As proven and sustainable products for commercial building construction, the initial use and reuse of steel can be used to claim credits for LEED projects within the Materials and Resources (MR) category. To assist specifiers document these LEED credits, the American Institute of Steel Construction (AISC) has published separate industry-wide (generic) environmental product declarations (EPDs) for fabricated hot-rolled structural sections, fabricated hollow structural sections, and fabricated plate. Similarly, the American Galvanizers Association (AGA) has EPDs for after-fabrication, hot-dip galvanized, hot-rolled structural sections, hollow structural sections, and plate. Health product declarations (HPDs) are also available based on the type of zinc used by the galvanizer to obtain a building product disclosure optimization (BPDO) credit for selecting products that are extracted and sourced in a responsible manner.
In addition to the environmental advantages, the reuse of steel provides economic savings to free up funding for new construction. The additional specification of HDG achieves many decades of corrosion protection, and thus eliminates maintenance costs in the future. Reductions in maintenance costs achieved through HDG have proven to be significant over time in comparison to other corrosion protection systems. For example, in the early 1970s an evaluation of bridges in Stark County, Ohio, determined 110 of them were structurally deficient and 50 percent of the budget was spent maintaining paint systems used to protect the bridges from corrosion. In an effort to relieve the budget of these maintenance costs, all decks and beams from the deficient bridges were removed, sandblasted, galvanized, and reinstalled using the original steel components (Figure 4). Thanks to HDG, these bridges are still rust-free and maintenance-free today. The resultant savings motivated the officials to convert all painted guardrail in the county to galvanized by reusing the steel. By 1987, the cost of paint maintenance for the department declined to less than 10 percent. With the potential for great economic savings, specifiers can benefit from similar evaluations for reuse.
Reuse of steel and galvanized steel
As long as the steel has not suffered from severe corrosion, there is a variety of ways the material can be reused and recycled without increasing construction waste or impacting future recyclability. The steel can be reused in place, reinstalled at another location, or reclaimed for fabrication on a new project. Options to reuse the steel include but are not limited to:
Steel that was previously bare or painted, but is otherwise fit for purpose, can also be hot-dip galvanized for corrosion protection because the after-fabrication galvanizing process is the same for new and reused steel. To prepare the reused steel for the galvanizing process, existing coating must be removed by abrasive or chemical methods. Next, articles not originally designed for HDG may require minor fabrication steps to ensure proper galvanizing. These steps may include the addition of vent and drainage holes, and cropped corners to allow the flow of pretreatment chemicals and zinc, which ensure full coverage of the parts during immersion in the process tanks and molten zinc bath (refer to “The Design of Products to be Hot-dip Galvanized After Fabrication” by AGA). When galvanizing reused steel, it is important to note the impact on appearance for projects where aesthetics are of primary concern, such as architecturally exposed structural steel (AESS). The galvanized coating forms at a uniform rate across all surfaces, and will not fill in dimples, roughness, damages, or holes present on the reused steel members. While some architects may prefer the imperfect appearance, these visual aspects may not meet the requirements of all projects.
A rehabilitation project for the Indian Mill truss bridge in Wyandot County, Ohio, demonstrates the ability to easily expand and galvanize an existing painted steel structure (Figure 5). Originally constructed in 1913, this single-lane vehicular steel truss bridge was deemed functionally obsolete in 2010 and required significant repairs. To preserve the original bridge aesthetic and to expand it, a similar truss was designed to incorporate pieces of the original steel. These components were disassembled, galvanized, and reused in the new construction along with the new galvanized steel members. The result was a fully galvanized bridge that will provide the county and rural community with maintenance-free longevity for another 100 years.
On the other hand, sometimes the steel to be reused on a project has already been galvanized. For galvanized steel that has previously been in service, the components can be reused as is, regalvanized, or painted over. If inspection reveals suitable coating thickness, no further processing may be required prior to reuse. Regalvanizing requires steel to be disassembled, stripped of any remaining coating at the facility, galvanized, and then reassembled at the desired location. This process effectively restores the galvanized steel to its initial condition in a very short time. Painting over the existing galvanizing can be achieved in the field, and is most suitable for parts that cannot be removed from service or where a shorter extension in coating life is needed.
Regalvanizing was a successful choice for the reuse of guardrail panels, originally galvanized in 1955, for the M-102 Bridge Rail Reconstruction Project (Detroit, Michigan). For 50 years of Michigan weather, traffic mishaps, road grime, and salts, HDG provided maintenance-free protection from corrosion, but highway traffic over time damaged 20 percent of the panels. Under a ‘Keep It Green’ initiative supported by the Michigan Department of Transportation (MDOT), the old but undamaged guardrail was stripped of the remaining galvanizing, regalvanized, and returned to service. Since only 20 percent of the existing steel required replacement, MDOT saved more than half of the budget allocated for this project. In addition to freeing up funds for future projects, the reuse of HDG will provide the M-102 Bridge Rail reconstruction project maintenance-free longevity for over 50 years.
Although the above examples demonstrate the many ways steel can be reused efficiently and cost-effectively with minimal or no processing, additional opportunities may be revealed on further evaluation of existing steel projects. Additionally, different steel articles from a variety of industries are also well-suited for reuse, including balcony railings, security barriers, fencing, highway products, architectural panels, sculptures, and recreation equipment. As sustainability becomes a larger part of specifying and engineering, the use of materials like steel and zinc ensures designs stand the test of time while improving the quality of life for future generations. Reusing steel and specifying HDG for corrosion protection can provide additional opportunities to make any project more sustainable.
Alana Fossa is the senior corrosion engineer for the American Galvanizers Association (AGA). Fossa provides assistance to architects, engineers, fabricators, owners, and other specifiers regarding technical issues and the processing of hot-dip galvanized steel (HDG). She also manages AGA studies and research on performance, application, and processing of HDG steel. Fossa can be reached via e-mail at firstname.lastname@example.org.
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