The following article by Anna Peterson, PE, Structures Group, is an excerpt from the report, Embodied Carbon: A Clearer View of Carbon Emissions.
Overview
As we become more aware of the importance and urgency of reducing embodied carbon, we must look past a prescriptive approach based only on a regional radius to better identify products that result in the lowest total embodied carbon. While regional materials boost local economies and minimize the impacts associated with shipping and transportation, teams must assess possible tradeoffs between regional production and distant suppliers that provide higher quality products or more efficient processes.
Over the last 20 years, LEED has encouraged design teams to focus on selecting regional materials and, in turn, design and construction teams are now adept at documenting a project’s regional content. Specifying materials that are extracted, manufactured, or assembled in proximity to a project site can support the local economy and minimize impacts from transporting building materials.
The impact of shipping construction materials is typically proportional to weight, though not all shipping modes produce the same environmental impact. For example, transporting one ton by truck emits nearly four times the amount of CO2 as transporting by barge. The graph below shows the effects of each transportation method available in Tally. This leads to the question, is there a case where materials from greater distances result in a net carbon benefit?
We recently faced this question when performing a whole building life cycle assessment (WBLCA) for City of Hope, a concrete framed medical office building in Duarte, California. The project used WBLCA to minimize embodied carbon and to achieve the LEED WBLCA credit. Our initial WBLCA found that the concrete structure was responsible for the majority of the embodied carbon in the building’s structure and enclosure. Using supplementary cementitious materials (SCMs) is a typical, and effective, strategy to minimize embodied carbon by reducing the cement content of concrete. However, a fly ash shortage and other logistical concerns required the team to explore different strategies to reduce cement content.
While a local quarry for concrete aggregate is just over two miles from the project site, the team examined whether using a coarse aggregate source from Vancouver, British Columbia— nearly 1300 miles away—could result in an improved environmental impact. The aggregate from British Columbia is stronger, stiffer, and shaped to enable high-performance concrete with minimum cement content. Our analysis considered both the additional transportation impacts and savings from reduced cement content. We found that the environmental impact reduction achieved with a lower cement content, even without the use of SCMs, outweighed the increased transportation impacts.
This illustrates the need for project teams to ask suppliers for material-specific product data through producer-specific EPDs (Environmental Product Declarations), and to use impact data when selecting material suppliers. While specifying local materials may provide benefits, teams seeking to minimize embodied carbon must make more robust quantitative comparisons that consider not only transportation impacts, but also any impact reductions a non-regional supplier may achieve through manufacturing or procurement optimization.
