Navigating Sustainable Concrete

To view the figures and tables associated with this article, please refer to the flipbook above.

With increased awareness of the influence of materials on global carbon emissions and initiatives like the SE2050 Commitment, structural engineers are adapting traditional workflows to incorporate embodied carbon reduction goals. In today’s environment of material innovation, practicalities can be lost in translation. This article promotes a nuanced approach to working with low-carbon concrete from the perspective of the supplier and contractor—advocating for increased collaboration, adaptability, and a willingness to embrace innovative solutions in a rapidly evolving industry.

The authors’ collective experience spans the breadth of the industry—from traditional structural design, construction project management, material engineering, field crew operations and logistics, and concrete placement techniques. Many aspects should be considered when implementing novel materials on a project, so authors' recommendations are distilled here into three categories: materials, placement, and design documents to highlight how to start. However, one major theme that will be consistent throughout is the need for open discussion between the Owner, Engineer of Record (EOR), Contractor and Ready-mix Supplier about the project’s sustainability goals.

In any concrete mix, it’s the cement that dominates the Global Warming Potential (GWP) due to the manufacturing process. While aggregates and cement make up about 70% and 10% of the volume respectively, in a traditional mix, the cement accounts for somewhere between 75 to 90% of the concrete’s GWP (Fig. 1, Reference Cemex).

As such, to reduce a concrete’s CO2 from a materials perspective, one needs to reduce the impact of the cement. Two of the primary levers to reduce cement’s CO2 are:

SCMs include industrial waste materials such as Fly Ash (FA), Blast Furnace Slag (GGBFS or slag), silica fume and other pozzolans that have long been used in concrete to reduce the amount of cement. While these materials have historically been used to reduce costs, they also improve the concrete by:

Fly ash, slag, and silica fume are already in use, but are not used to maximum effectiveness in most cases. For example, many specifications limit FA to 15 to 20% but many applications can go higher. Several alternative SCMs are also in development, and in most cases, they perform well. The issue is typically “scalability” and having a consistent, long-term supply of material available to make the effort of dedicating bin space, adding hoppers and developing the new mix designs with the alternative SCM worthwhile.

One downside is that SCMs may impact early age strength gain because they typically react with the byproducts of cement hydration and therefore are slower to gain strength. However, the end concrete is a denser, stronger, and more durable material that has lower embodied carbon. SCMs may also affect other plastic properties, but these can be typically addressed by the engineer, contractor, and supplier working together to evaluate the impacts to the overall project.

Blended cements are cements where part of the clinker is substituted with other materials such as limestone, slag, FA, silica fume, calcined clays, and other manmade or natural pozzolans at the cement plant (vs the Ready-mix plant as above). Blended cements must conform to the requirements of ASTM C595 (or AASHTO M 240) and are used in all aspects of concrete construction in the same manner as portland cements. Generally, there are four main categories:

The advantage of all blended cements is that they reduce the amount of clinker in the final product and that lowers CO2. Because of this, we are in a blended cement revolution. Currently, about 53% of all cements sold in the U.S. are blended cement, with 1L cements making up approximately 97% of those cement sales. Still, the use of the other ASTM C595 blended cements (e.g. Ty 1P, 1S, and IT) is also expanding and they make up 15% or more of blended cements usage in states such as New Jersey, Nebraska, Delaware, and parts of New York, (Source: USGS April 2024 Cement Statistics and Information https://www.usgs.gov/centers/national-minerals-information-center/cement-statistics-and-information).

Compared to portland cements and for the same cementitious quantities, blended cements tend to have lower early strength and lower heat of hydration, but they also tend to have comparable 28-day strength and higher ultimate strengths. However, it is important to understand that in addition to meeting the engineering requirements for an application, cement content is also based on other project specific issues such as environmental conditions and contractor placement requirements. As such, while early strength may be reduced, the mix could still meet the early opening requirements. Early discussions between the engineer, contractor, and producer, and working together can address most project specific requirements and applications.

Long term usage in Europe, Canada, and elsewhere, as well as our own experience has shown that using 1L cement does make good concrete (e.g. it is concrete). However, the material can behave differently and while this is usually not a problem, isolated issues related to specific placements or conditions have occurred.

This re-emphasizes the important point that in transitioning to all types of blended cements, it is NOT always a 1:1 replacement for straight cement mix. While the total amount of cementitious material will most likely be similar to the mix of ordinary portland cement with different amounts of SCMs, performance will vary, and adjustments may need to be made.

To minimize issues, as cement manufacturers move to all types of blended cements into the marketplace, it is critical that they have open dialog, coordination, and communication with the ready-mix suppliers, contractors, engineers, and other customers about their plans. Some specific items that need to be shared include:

While it may be argued that using SCM replacement at the cement plant or the ready-mix plant results in the same overall CO2 reduction; the primary advantage to having the blending occur at the cement plant is the ability optimize the blending so that the final cement has higher consistency, optimum fineness, and improved chemical control so that it behaves better. Using a blended cement product also helps smaller producers who have only limited silos and space to produce low carbon concrete.

However, no matter what cement type is used, specifications should always allow for ready mix suppliers to add additional SCMs at the plants. Because the local producers will know their local materials best and can dial in the mix for most given project specific needs, this will help ensure a low carbon concrete can be obtained. Still, care and appropriate and adequate testing are needed to ensure performance.

From a contractor’s perspective the goal is to provide the owner with the best combination of schedule, quality, safety, and cost that will make the project a success and enhance a relationship based on mutual trust. Going forward, embodied carbon will also be weighted in the balance of project characteristics. Each project type and location will present a unique combination of challenges that can be modified related to sustainability and would be nearly impossible for an engineering firm to know unless and until there is open communication with the owner, design team, contractors, and local ready mixed producers.

The construction team can provide feedback on which elements of the project may have less critical strength needs. The earlier in the construction process the team members are identified and this discussion takes place, the more effective the team will be in addressing the project sustainability goals with minimal or no construction schedule impact. If the closest ready mixed producers do not have enough SCM silos or aggregate capacity to provide low embodied carbon concrete which meets the owners goals, the team will need to be more creative and explore alternate blended cements or different suppliers. Admixture advances have provided the option of longer hauls in remote locations or congested metropolitan areas to use hydration stabilizers to extend set times beyond the traditional 90 minutes while maintaining workability and overall quality. While not an ideal sustainability solution because of the additional hauling, it does open the range of potential producers if material availability, experience, or the timeline to produce Environmental Product Declarations (EPDs) may stop all sustainability efforts.

Finishability and pumpability will need to be tested on novel slab mixes, particularly non-air entrained, troweled mixes with high flatness standards. Many contractors have reported differences or issues with placement of low carbon concrete. Additional lab testing and field mockups large enough for project appropriate equipment testing will reduce the risks. Starting with mixes the team has successful history with and making incremental adjustments is one strategy.

The testing laboratory technicians also will need support for the team to succeed. Many of the blended cements or mixes with higher SCM contents are more sensitive to extreme temperatures, so to maintain the cylinders within the ASTM C31 curing range, adequate preparation and jobsite logistics are needed. High or low initial curing temperatures will yield low breaks, according to multiple studies.

When presented with a specific project opportunity, it is incumbent upon the structural engineer to understand how the client’s overall sustainability goals can be successfully applied to the project, and specifically how the structural AND non-structural items, such as sidewalks and duct-banks, inform the full picture of the project’s embodied carbon footprint.

Ensure you are given access to the client’s Basis of Design (BOD) and Division 001 specifications. These hold the key to understanding big picture goals.

Balance areas where you can be aggressive with areas where you can’t. For example, mixes made to achieve high-early strength have proportionally higher cement content. By knowing this, we can balance these mixes with areas like foundations, which traditionally can accommodate a longer curing duration.

Bring non-structural concrete providers into the discussion, such as those providing concrete for hardscape, equipment pads, or duct-bank concrete in the case of data centers.
Come prepared to meetings with an approximate “Carbon Budget” to outline your path to success. Table 1 is an example demonstrating a quick calculation done via spreadsheet to map out early carbon expectations and reductions. Tools like this provide clear communication to clients, designers, and trade partners.

Boilerplate specifications which don’t directly apply to (or even worse contradict) project goals are a frequent source of confusion for concrete contractors.

The authors recommend the following best practices for adapting contract documents for novel mix use:

Start proactively communicating now to your clients that you, as the EOR, need to be involved early to guide this process. A successful implementation of aggressive goals can only happen with clear communication early and often between all parties. This is your chance to show why your firm is invaluable in early project planning. Reach out to your area ready mix suppliers and concrete contractors for a plant tour or stay after a project meeting to understand their workflow and current supply chain. Attend an area ACI Chapter event, Concrete Contractor Society meeting, Structural Engineers’ Association, Carbon Leadership Forum Meeting, or U.S. Green Building Council local chapter event—these are great places to broaden your network and understanding of local capabilities. Be willing to share your successes and failures—everyone will benefit, and you will be a sought-after resource on future projects. ■

About the Authors

Erika Winters-Downey, SE, LEED AP BD+C, is a structural engineer and Director of Material Innovation & Impact for the Clayco/Lamar Johnson Collaborative enterprise.

Mike Hernandez, PE, FACI, LEED AP BD+C, is Technical Director for the American Society of Concrete Contractors. He has over 25 years of construction operations experience constructing concrete buildings and bridges.

Kyle Kammer, PE, is the Director of Quality for Concrete Strategies. He has over 15 years of experience in concrete construction and leads Concrete Strategies’ sustainability efforts in conjunction with the Clayco Enterprise team.

Jim Mack joined CEMEX in September 2007 and is currently Director, Market Development – Infrastructure and Sustainability. In this position, he works with agencies, contractors, and other professionals to identify and develop sustainable concrete and cement-based solutions for pavement and building applications.