Low Carbon Concrete Specification

At LMN, we specify an embodied carbon reduction averaged across all mixes, so some mixes may have higher or lower reductions than others. This includes a spec section that requires EPDs for 90% of all concrete mixes and an excel file that includes calculations. Email me at kanderson@lmnarchitects.com and I can walk you through it. Kjell Anderson FAIA, LEED Fellow Principal, Director of Sustainable Design LMN Architects lmnarchitects.com M 206 812 6546 O 206 682 3460 S Linkedin | X | Instagram

We don't specify the exact mix. Instead, we set performance criteria and allow the contractor to submit mix designs that meet the criteria. Note that the 40% reduction target is based on our experience in the Seattle area with our local supply of gravel and sand and how it behaves in the mixes. That target should be adjusted in other regions based on the local supply and mix designs. Even in nearby Portland OR, it's harder to hit that 40% target because of how the local gravel performs in the mix. I.e., they have to use more cement to achieve the strengths required. In Part 1 we require EPDs for all the mixes In Part 2 of the spec, under Performance Criteria, we include the following language. We drew on example language provided by CLF. I think it was MKA who drafted the initial language and shared it: Carbon Footprint Reduction: Provide concrete mix designs with EPDs showing maximum Global Warming Potential (GWP) as listed below. Total reduction based on weighted average of all mix designs can be used to demonstrate compliance: [you need to coord the list of mix design strengths and cure times with structural] Mix designs with 28 day strengths: 40% GWP reduction from 2019 Carbon Leadership Forum baselines. Strength 3,000 psi: Maximum: 192 kilograms of CO2 equivalent/cubic yard. Strength 4,000 psi: Maximum: 238 kilograms of CO2 equivalent/cubic yard. Strength 4,500 psi: Maximum: 292 kilograms of CO2 equivalent/cubic yard. Mix designs with 56 day strengths: 50% GWP reduction from 2019 Carbon Leadership Forum baselines. Strength 6,000 psi: Maximum 256 kilograms of CO2 equivalent/cubic yard. Submit mix designs for review using Type 1L cement, supplementary cementitious material, carbon-reducing admixtures, and adjusted water-cement ratios as needed to meet the Carbon Footprint Reduction targets. Conform to mix design requirements shown on General Structural Notes. Then elsewhere in Part 2 we include product requirements for the various components like cement, SCMs, and admixtures. For example, we include Carbon Cure as an acceptable carbon-reducing admixture, but we don't require its use. Hope that helps. Kristian Kicinski AIA, LFA, LEED AP BD+C (he, him) Associate Principal / Director of Sustainability direct: 206.536.1370 Send me files

Agreeing here that performance specs are the way to go. The NRMCA has issued guidelines for issueing performance specs, which can then be aligned with NRMCA Regional Concrete Mix Benchmarks. The performance can be stipulated based on the psi, as Kristian has listed above, OR the specs can call for an overall percent reduction for the project as a whole. The latter approach provides the ready mix supplier with the most possible flexibility. 

And I would be remiss if I didn't link to the BSA / CLF Boston NE Hub's recently published Low Carbon Concrete Report, which was illustrated by Building Green. Thanks to BG, it's more fun than it sounds - I promise! https://www.clfboston.com/_files/ugd/271969_351fa3b215f4442990814c94ca268d32.pdf

We use a similar method to Kristian but we take the GWP baselines from EC3 for the region because the values are so varied across the country On Wed, Oct 2, 2024, 3:38 PM Kristian Kicinski wrote:

Our approach is similar to what Kristian and James described, but, because there is so much regional variety, even from one part of the state to another, we define the GWP limits for different strengths in conversation with suppliers.

Terry, if your project is in Washington, I might have a spec specific to the area in question, and would be happy to share.

Agree on performance specs, setting a "whole building" (all concrete) target and giving suppliers some flexibility (we typically provide the targets per mix, for transparency but specify the overall, project, weighted average target). Also agree on regionally based targets. We use NRMCA (latest 2022 report) for baselines, which CLF 2023 cites as a source, but then seems to be about 20% higher than NRMCA. I like CLF, but the ups and downs of their baselines makes it harder for me to understand. As we've gotten into for a variety of projects, we've found that a reasonably achievable target is widely variable, based on the specific region, sophistication and competitiveness of suppliers, willingness of client to potentially pay a premium, and the type and use of concrete being poured. We've had projects (mass timber with exposed polished floors as primary concrete use) that had to really push for a 5% reduction in the reasonably competitive Portland project, we had another tilt-up project in Seattle achieve a 50%+ reduction.