Resilience and embodied carbon are often considered to be separate goals within the built environment and in fact, sometimes competing ones. This doesn't have to be the case.
Concrete construction is well known for providing resilient and durable structures, and this includes reinforced concrete masonry unit (CMU) structures. The primary source of Global Warming Potential (GWP) with all concretes is due to Portland cement as a raw material even though it makes up a relatively small proportion of the concrete mix. During cement manufacturing, roughly 50% of CO2 emissions are due to the chemical reaction of calcination, in which high heat chemically breaks apart limestone, and roughly 40% of CO2 emissions are due to the heat required. Incredible strides have been made within the cement industry to lower embodied carbon. The American Cement Association (ACA), formerly the Portland Cement Association (PCA), has published its roadmap to carbon neutrality that includes replacing cement with low-carbon cement blends, using alternative fuels and renewable energy in manufacturing, and introducing carbon capture technologies into the cement manufacturing process. <u1:p></u1:p>
Carbon sequestration is another area of research in the concrete world. Carbon sequestration results from carbonation, which is part of the chemical reaction that occurs when concrete hydrates. Different types of concrete sequester different amounts of carbon dioxide, and at different speeds. Historically, most or just about all of the research around concrete and carbon sequestration had been performed on wet-cast, or poured-in-place concrete. Massachusetts Institute of Technology (MIT) has a sequestration calculator called the Whole Life Cycle Carbon Uptake Tool which provides sequestration rates for different mixes and exposures of wet-cast concrete, and includes estimates for scenarios at the End-Of-Life (EOL) if the concrete were to be crushed and exposed to the atmosphere. <u1:p></u1:p>
In 2020, CMHA (Concrete Masonry and Hardscapes Association) began new research specific to carbon sequestration rates for concrete masonry. This research discovered something that had not been quantified before, which is that there is a difference in carbon sequestration rates between wet-cast concrete and dry-cast concrete masonry. CMU has accelerated sequestration rates due to its unique dry-cast manufacturing process. This manufacturing process creates a unique structure within the concrete itself that allows air to penetrate beyond just the outer few millimeters, accelerating carbonation throughout. For CMU, the majority of sequestration occurs immediately after manufacture, and within the first few years after construction, and is not dependent on being crushed in EOL (End Of Life) scenarios. More on CMU and sequestration rates can be found in CMHA FAQ – 022-24 Do Manufactured Concrete Products Sequester CO2?
Accelerated carbon sequestration is one of the reasons that concrete masonry construction is low carbon. The manufacturing process not only creates the unique matrix that accelerates carbon sequestration, it also enables less cement to be used due to compaction and vibration. A third factor that makes CMU construction low embodied carbon is the fact that CMU has cores, or a hollow shape. For partially grouted walls, there is simply less concrete in the wall to begin with. For more on the factors affecting the embodied carbon of CMU, SE 2050 has published a resource called Top 10 Things Every Structural Engineer Should Know about Masonry. And for a much deeper dive, The Block Design Collective Learning Hub has several AIA accredited learning modules dedicated to both resiliency and the CMHA embodied carbon research, including quantifying the embodied carbon of CMU assemblies using LCA studies that factor in all 3 top CMU embodied carbon contributors; less cement, more sequestration, and less volume of concrete in the wall. <u1:p></u1:p>
Embodied carbon is best considered across the whole life cycle, and not only at the factory gate or A1-A3. Concrete in general, is its most carbon intensive when it is first made, but as durability, longevity, resilience (and for concrete masonry use-phase (B1) carbon sequestration) play out in Whole Building LCA use phase scenarios, the embodied carbon picture for concrete improves drastically.
Modern, code compliant, reinforced CMU structures perform inherently well, withstanding natural disasters with minimal repairs. This saves embodied carbon. They offer resilient solutions without embodied carbon trade-offs. On projects where resilience and low embodied carbon are both priorities, what stands in the way of choosing concrete masonry?<u1:p></u1:p>
<u1:p></u1:p>Heidi Jandris, Assoc. AIA, is a sustainability consultant to Concrete Masonry & Hardscapes Association (CMHA). She grew up immersed in all things CMU through her family's New England concrete block business, and today researches and implements ways to lower the environmental impacts of concrete masonry. She holds a Bachelor of Architecture from Pratt Institute, a Master of Science in Sustainable Building Systems from Northeastern University, and a WBLCA-P micro-credential from British Columbia Institute of Technology.
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Heidi Jandris Assoc. AIA
Kore Consulting
Worcester MA
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