Embodied carbon is the total impact of all greenhouse gases emitted (measured in CO2-equivalent or CO2-e) into the atmosphere by a material, product, or system. For structural materials, embodied carbon is linked to the extraction of the raw material, manufacturing, transportation, construction, maintenance during service life, demolition, and end of life. CO2-equivalent is a unit of measurement based on the relative impact of a given gas on global warming or its Global Warming Potential (GWP). Therefore, embodied carbon and GWP are often used interchangeably.
Net-zero embodied carbon is when the upfront embodied carbon is reduced to the greatest extent possible. The remaining embodied carbon is then offset so that the emissions over the lifecycle of the building are effectively eliminated. Net-zero embodied carbon is also referred to as neutral embodied carbon.
Carbon sequestration is the process of capturing and storing atmospheric carbon dioxide. Wood, concrete to some extent, and other renewable materials can be used to sequester and “store” carbon within the material. However, once the structure is demolished, the sequestered carbon can be re-released into the atmosphere. Carbon sequestration (a reduction in embodied carbon) may only be credited to a product if the end of life fate of that carbon is considered in the life cycle assessment. Therefore, the end of life assumptions for structural materials is critical.
Embodied carbon is essential for structural engineers to address. As buildings’ and infrastructures’ operational emissions work towards achieving net-zero, embodied carbon will become drastically more critical to address and reduce.
Carbon released during the production of materials and construction of buildings/infrastructure is emitted earlier than carbon released during operation and therefore has a more immediate impact on climate. This is known as the time value of carbon. It is thereby crucial that structural engineers work to reduce the upfront embodied carbon of the structural system to the greatest extent possible.
A life cycle assessment (LCA) is a method of evaluating the environmental impacts, including embodied carbon, associated with all of the stages of a product or building’s life. The stages include raw material extraction, manufacturing, distribution, construction/assembly, maintenance/repair, and disposal/recycling. Guidelines and standards for LCA’s are ASTM E2921, ISO 14040, and ISO 14044.
Environmental product declarations (EPDs) are third party verified reports measuring the environmental impacts, including embodied carbon, of a product or material from a life cycle assessment. The typical life cycle stages measured by EPDs include raw material extraction, transportation to processing/manufacture, and manufacturing of the product or material. Notably, most EPDs do not include the product’s end of life considerations. The standard for EPDs is ISO 14025.
For a holistic view of the embodied carbon of a structural system, it is essential to conduct a whole building LCA (WBLCA). For example, steel and concrete tend to have larger embodied carbon footprints between the stages of raw material extraction to production. However, during design and construction, less of the material can be used due to their inherent strength, and both can be recycled/reused during their end of life. These two factors can greatly reduce the structure’s overall embodied carbon, which can be captured by a whole building LCA.
The location of a material or product’s extraction and manufacturing can significantly influence the magnitude of its embodied carbon. Producers, manufacturers, and fabricators who source local raw material, use a larger percentage of recycled material, or obtain electricity from a renewable energy source can greatly reduce the embodied carbon of their material or product. Therefore, always try to obtain and compare EPDs from local manufacturers that may supply the projects’ material over using industry averages.
Some additional strategies structural engineers can use to reduce a structure’s embodied carbon are: using alternative materials like fly ash and slag in concrete, selecting an efficient structural system for the building type and usage, optimizing structural materials, conducting whole building life cycle assessments to inform decision making, and increasing the service life of a building, to list a few.
