Embodied carbon: Why it matters to the structural engineer—and how to reduce it
November 01, 2023
November 01, 2023
To decarbonize the building industry, structural engineers need to focus on embodied carbon and look at alternative materials
A version of this blog appeared as “Carbon emissions, concrete, and steel” in Design Quarterly Issue 19.
Why are we talking about structural engineering and carbon?
For years, structural engineering wasn’t at the forefront of the sustainability conversation. But sustainable buildings have grown up. Today, we can design and build high-performance buildings that are efficient to operate. These new buildings use less energy for heating and cooling, trending toward net zero in some cases.
At the same time, demand has been increasing for the design of healthier spaces—in terms of air, light, and nontoxic materials. Thus, when we look at the emissions metrics from high-performance buildings, operational carbon is rapidly declining, leaving us with embodied carbon. The emissions required to build the building—its materials, their manufacture, and the construction methods—are what make up a building’s embodied carbon. Viewed through this lens, buildings still account for significant emissions. And structural engineers are more and more at center stage when it comes to embodied carbon reduction. This is a sea change for many of us.
Designers can improve operational carbon incrementally. In contrast, once the building has been designed and built, the embodied carbon is invested. So, before construction is when the structural engineer can make impact the embodied carbon in buildings. When we’re talking about embodied carbon, we’re generally referring to concrete, masonry, and steel. So, what really are the tools and approaches structural engineers can take to reducing embodied carbon, and what’s standing in their way?
Fly ash as supplementary cementitious material (SCM): Supplementary cementing materials amplify concrete’s strength and durability. One way we can reduce embodied carbon is by using fly ash as an SCM in an alternative to sack Portland cement. Fly ash is a byproduct of coal burning currently used as an SCM.
The problem? As we shutter coal plants, the fly ash supply is diminishing. And it’s also in demand for roadway construction. Without fly ash, we may be stuck using straight sack cement, which has a higher embodied carbon. So, we need to be on the lookout for other options, for example, concretes that sequester carbon to help with building decarbonization.
Concrete with calcined clay limestone cements (LC³): When burned at a high temperature, naturally occurring clay takes on properties that make it a great SCM. It has been used in buildings across Europe for years, but it hasn’t been widely adopted across North America. It’s promising as a replacement for the limestone used in cement. Manufacturers say metakaolin or calcine clay (CC) and LC³ (cement that blends limestone and calcined clay) can reduce from 30 to 40 percent of CO₂ emissions compared to ordinary Portland cement. And it performs just as well.
The challenge? Both CC and LC³ are not yet widely used in North America. That means it will take some early adopters to demonstrate the viability of these materials in the marketplace.
Steel: The good news when it comes to steel in North America is that we’re already recycling it at a high rate. With industrial electric arc furnaces, US and Canadian plants manufacture steel that is roughly 93 percent recycled in content.
The bad news is that the supply of recyclable steel is dwindling. Cars, previously a reliable source of steel, are made of less and less steel. And it is not cost effective to recycle much of the steel out of razed buildings. So, we’re facing a future where we will need to mine and manufacture more native material to meet the steel demand.
Mass timber, glulam, cross-laminated timber (CLT): Wood feels good, it looks good—it has biophilic properties. With mass timber and CLT, we can build using a sustainable, renewable, carbon-sequestering resource. Mass timber buildings are lighter, requiring less concrete (and carbon) in their foundations. We’re seeing more mass timber in regions where the product is readily available—the Pacific Northwest and British Columbia, for instance.
But to be truly sustainable, mass timber must be harvested from well-managed forests. Codes that limit the height of mass timber buildings, the question of fire ratings, its relative expense, and lack of experience in certain regions are limiting its acceptance for now. But all of this is changing.
Before construction is when the structural engineer can make impact the embodied carbon in buildings.
Embodied carbon modeling: One sure way to reduce embodied carbon today is for our teams to design buildings that simply use less concrete and steel. We can rapidly generate multiple conceptual designs and then model their embodied carbon based on the building materials. In this way, we can see in real time which possibilities have a smaller embodied carbon bill.
The more we can incorporate embodied carbon in our modeling in the early design stages, the better decisions we make on the use of low-carbon alternatives. We are working to make embodied carbon analysis an automated part of our design workflow.
Building reuse: As the saying goes, “they don’t build them like they used to.” As structural engineers, we have a key role in identifying existing buildings with good bones that can be reused to support our current programming needs and minimize our impact to landfills and harvesting virgin materials. The “greenest building” may already exist.
Roadblocks: Availability, pricing, and lack of knowledge and experience are holding back uptake of materials that can reduce embodied carbon—even when a client is enthusiastic.
Take a recent project for example. The client wanted to use carbon cure concrete—a product that has been injected with CO₂ to keep it out of the atmosphere—for the slab on a new facility. The supplier for the product told us they didn’t have a plant in the area—that would make delivery prohibitively expensive.
What will it take for clients to invest in building structures with lower embodied carbon? Laws, for one. Updating the codes to promote building design and health that are carbon friendly are underway in North America.
Brand is important. The companies and organizations that walk the talk and focus on building decarbonization have a positive story to tell. They can connect their values and practice with the clients, the staff, and the investors who take climate change and environmental mission seriously.
New and repurposed buildings are key for residential, commercial, or industrial uses. To move toward carbon neutrality while meeting demand for the built environment, structural engineers will need to tackle embodied carbon on every project. Beginning now.