Throughout the construction industry, there is a movement toward sustainability that is affecting what and how we build, and a central part of this movement is the use of low-carbon materials. Although all materials are subject to scrutiny, concrete has received specific attention, and rightfully so, concrete is the most widely used material in the world. On a unit mass basis, concrete has one of the lowest carbon footprints of all manufactured materials.
Still, the enormous volume used acts as a multiplier that makes concrete worldwide one of the largest single sources of emitted carbon dioxide (CO2). Simultaneously, concrete is the cornerstone of civilization. From transportation systems to water and sanitary systems to building construction for both private and public owners, concrete is essential. So, for concrete, our climate challenge is to reduce its carbon footprint, knowing its use will continue.
Stepping up to this challenge, the ClimateWorks Foundation has recently launched a Task Force to support a series of articles, tests, and concrete project case studies that help educate and de-risk the use of lower-carbon concrete. This article is the first of a series that will frame this challenge, identify opportunities for impact, educate on important issues around lower carbon concrete, and then, through shared case studies, show examples of lower carbon concrete in use.
Where to Focus
While Portland cement is often 15% or less of the final concrete mix volume measured by weight, it makes up nearly 90% of its global warming potential (GWP). So, concrete avoidance and substitution are keys to the material’s carbon reduction strategies. Avoidance is achieved by reducing the total cement in a mixture or optimizing the concrete volume required within a design. Substitution is accomplished by using an alternative cementitious material (ACM) to fully replace the Portland cement, a supplementary cementitious material (SCM) to replace a portion of the Portland cement or both.
Historically, the most common SCMs have been coal fly ash or blast furnace slag. Still, with the closure of domestic coal-fired power plants and the limited supply of blast furnace slag, there is a growing list of alternative supplementary cementitious materials (ASCMs) starting to be considered. We should continue to use our proven SCMs to their full availability. But “additionality,” which includes both lower carbon approaches to making cement binders and new ASCMs, is also needed.
For new approaches to be effectively used and scalable, we need testing and industry standards to establish acceptance criteria. This includes testing for consistency, strength development with time, pumpability, placement predictability, and long-term durability. Of course, cost also needs to be considered.
Currently, national building codes such as ACI 318 recognize ASTM C150 Portland cement, ASTM C595 blended cement, and ASTM C1157 performance-based cement as acceptable, with SCMs limited to fly ash, slag cement, and silica fume. Concrete mixtures are also subject to approval by the licensed design professional and local code official based on the use case and specific mixture designs. Trial batch testing and mixture performance data are typically required prior to approving a mixture for use.
Diligence in testing and review should not be compromised. However, which ASTM and ACI standards should be used for testing new material is not clear and often not consistently applied, so the Task Force would help provide guidance.
When a new material has been appropriately validated through third-party validation programs such as that being launched by ACI, there is still another significant barrier to its adoption. Unless prior batch testing data exists, or the ready-mix supplier is brought on early, and they are given a budget for testing, the design/construction/ready-mix supplier team is often reluctant to consider new mixtures or materials. There is typically no time to generate new mixture performance data, and there is a disincentive to take any new material or financial risk.
But if we are to accelerate cleaner and lower carbon concrete use, we need to de-risk this barrier. Fortunately, there is an opportunity we are yet to leverage fully. Large projects often require hundreds of concrete placements over the course of many months to years. All projects, even small ones, have an opportunity to run parallel studies during the concrete’s construction, finding non-critical locations to test a trial batch of next-generation material and collecting data to inform either later placements on the same project or future projects down the road. Advancing an industry culture of continual testing and evolving improvement is another approach the Task Force will be encouraging.
Who’s Working On This?
While far from being the only ones working on this challenge, the material and structural engineering leaders of the ClimateWorks Foundation Task Force will be supporting a series of placed concrete mixture demonstration projects. This will include evaluating mixtures with a 30% and a more ambitious 60% or greater GWP reduction compared to baseline mixtures from each case study project. The Task Force will target multiple strategies that are regionally appropriate across the country. The goal is to evaluate multiple scalable strategies that meet carbon reduction targets while retaining performance and constructability needs. Where trade-offs occur, those will be documented as well. Shared case studies will highlight the lessons learned and advance a collective industry understanding. Another notable and related effort, one this Task Force will collaborate with, is coming from the tech sector Green DC Futures Team (see: https://climateaccord.org/news/greener-concrete-for-data-centers-an-open-letter/).
Where Do We Go From Here?
Breaking down the puzzle of lower carbon concrete into smaller and more manageable topics and then working solutions to those topics is something we collectively know how to do if we lean into the challenge and share our knowledge. The forthcoming articles and case studies from the ClimateWorks Foundation Task Force are a step in that direction. Future articles will include inputs on performance-oriented concrete specifications, standardized test protocols, in-field placing demonstration standards, and other topics developed as this effort evolves. ■