As we’ve noted previously on this blog, climate action in the building industry had been focused, appropriately, on reducing the operational carbon of our buildings through efficiency and renewable energy. However, science and recent number-crunching indicates we must shift more attention to reducing the embodied carbon of our buildings as well. That’s the carbon emissions related to extracting, producing, and transporting building materials, and the biggest contributor is concrete.
That doesn’t mean we can, or even would want to, eliminate concrete, but we can be more strategic to lower the carbon emissions.
Credit: Architecture 2030
Concrete is strong, versatile, fire-proof, and made of some of the most abundant materials on earth – 6% air, 18% water, 25% sand and 41% gravel, plus, of course the trouble maker: 10% cement. Portland cement is a high emitter because of the very high temperatures needed to make it, plus the release of CO2 as part of the chemical reaction in making concrete. For that reason, there are two main approaches to reducing the climate impacts: reduce the amount of cement in concrete, and reduce the amount of concrete itself. Here are some key places to get started.
1. Communicate your intent: Tell your team on day one that you want lower embodied carbon and see what ideas come from your structural engineer, builder and concrete supplier. The National Ready Mixed Concrete Association knows they’re under the microscope and has a target to reducing its carbon emissions by 50% by 2030. Your local supplier likely has lots of resources.
2. Encourage Innovation: Don’t be overly-prescriptive in specifying your mix design. Fly-ash is still a good option, but don’t stop there or set unnecessary caps. There are new cement alternatives coming out all the time, plus admixtures and supplementary cementitious materials (SCM’s) that reduce the need for Portland cement. Aggregate size and type, air content and water ratio can further influence cement content, so it’s best to focus on performance and allow for innovative ways to get there.
3. Fine-tune performance criteria: If you don’t need full strength in 28-days, allow for a longer cure time like 60, 90 or even 120 days, which allows for more SCM’s to be used. Invest in the additional design time to optimize beam size and slab thickness, and employ life cycle analysis (LCA) to compare options. Consider non-concrete alternatives where feasible, such as hempcrete.
4. Allow for sequestration: Concrete has some ability to absorb carbon over its lifetime, but only when it’s exposed to air, so leave concrete exposed when feasible. When demolishing concrete, crush it to be used as road base or spread out in some way to further absorb carbon. And keep an eye out for new technology that actually injects carbon into the concrete mix.
A small carbon reduction in the mix design of a large project, such as this student housing tower in San Jose, could result in a significant reduction in emissions overall.
Concrete is complicated, with a ton of variables that influence workability, strength, finish, cost and, critically, carbon emissions. Operating and constructing buildings is one of the largest contributors to climate change; those of us lucky enough to be in the building industry have the opportunity, and the responsibility, to reduce those impacts as quickly as possible.
For more ideas and resources, check out Architecture 2030’s Carbon Smart Materials Palette, as well as Building with Strength.