To view the figures and tables associated with this article, please refer to the flipbook above.
If you are a practicing structural engineer, you are likely familiar with the idea of a concrete mix design table and concrete general notes. For many projects, these are supported with additional detail provided in specification section 03 30 00—Cast in Place Concrete. While the level of detail and amount of information included may vary depending on what type of project you find yourself working on, nearly every project involves some amount of concrete. Unlike other materials, concrete can vary significantly in performance among suppliers and is made from multiple constituent materials, each with their own ASTM standards. It can’t be defined by a single ASTM standard as is the case for steel or a species and grade from the NDS supplement as is the case for wood. Specifying concrete often seems to require a deep knowledge of multiple different ACI codes, an understanding of various rules of thumb and navigating a few contradicting opinions from contractors and peers.
Many structural engineering firms have started to update their existing concrete design criteria because of two emerging and important industry trends: performance-based specifications and embodied carbon. The goal of this article is to provide a summary of the different variables that engineers need to consider when specifying concrete and explicitly address how these variables are related to performance-based criteria and embodied carbon. A detailed reference related to these topics is provided at the end of the article which has been developed by the National Council of Structural Engineering Association (NCSEA) Sustainable Design Committee.
Performance-Based Specifications
Performance versus prescriptive—what is the difference when it comes to concrete specifications? Prescriptive design criteria do not guarantee performance, but often have historic precedence or are backed up by research indicating a correlation to performance. Engineers rely on these based on this historic precedence even though the concrete industry is constantly evolving. Procedures for production are different, materials are more advanced, and our ability to collect and report data has significantly increased. A common example of prescriptive design is limiting the water content in a mix to achieve adequate durability of the finished surface, reduce long term shrinkage, and ensure a quality mix. Another is specifying a minimum slump to ensure the concrete is placeable and consolidates well, or a maximum slump as a limitation on excessive water content.
Prescriptive specifications are when an intermediate or proxy value is being specified with the intention that it will correlate or lead to the desired performance. They are one lingering example in many structural drawings where engineers are still specifying the means and methods of the contractor.
Performance-based specifications are when the desired results are specified directly. Using the same examples noted earlier, slump and max w/cm ratios are replaced with shrinkage data. As engineers, we do need data for a performance-based specification to be successful, which is often a drawback. We may need data that the supplier doesn’t immediately have and that will increase the lead time for mix design submittals and concrete placement. The sophistication of a supplier is potentially highly variable within a given market and especially between different size markets and regions. Additionally, the industry does rely on prescriptive based requirements when there are not time-proven and reliable performance-based metrics available, such as the durability requirements outlined in ACI 318, Chapter 18.
Performance-based specifications are when the desired result or performance characteristic of a concrete mix design are specified directly without dictating the method by which the mix supplier must use to achieve the required performance. They inherently allow for optimization by the ready-mix suppliers, allowing the responsibility for performance to shift to the entity that can affect the change.
The National Ready Mixed Concrete Association (NRMCA) has advocated for this idea through their Prescription to Performance Initiative (P2P), which began in 2002. The initiative was developed to educate suppliers and engineers on the benefits of movement towards performance-based specifications. They highlight the incentive it creates for concrete suppliers to innovate. It rewards them for decreasing variability in their test data and increasing the performance of their materials. Along these lines, it inherently supports reducing embodied carbon, even if it wasn’t being tracked or stated as a goal on a project.
Concrete Class Properties
Historically, a variety of different variables have been specified for concrete mixes by structural engineers. A non-exhaustive list is shown in Table 1. Included in the table for reference is an indication whether the variable is primarily performance-based or prescriptive in nature and the variable’s impact on the embodied carbon and cost of a mix, i.e., does specifying a higher value for the variable tend to correlate to an increase in embodied carbon or does it have an inverse relationship (or “it depends”)? For prescriptive properties, a performance-based property that can be substituted is indicated.
Embodied Carbon Background
Concrete producers and specifiers have another variable that they must consider and balance along with structural performance, durability, place-ability and pumpability: embodied carbon. Both globally and domestically, buildings are responsible for 35% of greenhouse gas emissions. While a large portion of these emissions are the result of building operations, embodied carbon has been gaining more and more attention as a smaller but significant portion of the problem (Fig. 1). With the focus on embodied carbon increasing, both the structural engineer and the concrete industry should be strategizing on how to make reductions and learning how to appropriately respond to the demands of architects and building owners. Many are realizing, if it wasn’t clear already, that this is a complex and difficult issue to solve.
The concrete industry is responsible for as much as 11% of global and 1.5% of domestic greenhouse gas emissions. For perspective, the steel industry is responsible for 8% globally and 2% domestically and the airline industry is around 2.5% globally. Two primary reasons have led to the concrete and cement industry being at the center of the embodied carbon discussion. The first is that the production of ordinary portland cement requires both significant energy (heat) and involves a chemical reaction that results in direct CO2 emissions (process emissions). The second and more important variable is the volume of concrete that the built environment requires. A commonly referenced statistic is that concrete is the most consumed man-made material on earth, or alternatively, the second-most consumed material after water. The built environment has become dependent on the use of a large volume and uninterrupted flow of concrete and each unit of concrete delivered to a project site is contributing greenhouse gas emissions to the atmosphere.
Embodied Carbon in Your Specifications
Approximately 90% of the emissions associated with concrete come from the production of cement, with the remainder from other constituent materials and transportation and placement during construction. For this reason, most reduction strategies involve reducing the amount of cement or exploring alternative cements, pozzolans, and supplemental cementitious materials (SCM). Prescriptive approaches to reducing the emissions of concrete typically involve requiring the cement quantity to be reduced or requiring a minimum amount of SCM use. A project may choose to balance performance and embodied carbon by specifying a range of SCM content, with the lower end of the range attempting to reduce embodied carbon in a prescriptive way and the higher value ensuring reasonable performance and strength gain is provided by the mix supplier for the contractor. It should be noted that it has been shown that an SCM specification requirement does not directly correlate to a reduction in embodied carbon but may be a strategy for projects without the necessary data to implement a global warming potential limit.
The best way to ensure a project meets a desired embodied carbon target is to specify global warming potential targets for each class of concrete. This concept is relatively new for the industry and typically requires some amount of education for the design and construction team to successfully implement. Like other performance-based criteria, it also requires data to be submitted to the structural engineer by the mix supplier to ensure conformance. The availability of this data (see sidebar on page 14) has been growing exponentially but is still a new concept for some suppliers and may not be available for every mix. A significant amount of lead time may be required if you are targeting a significantly lower value than the national or regional benchmarks as established by the NRMCA. There are inherently two components to specifying a global warming potential target. One is transparency and data, creating the information engineers need to understand what the emissions are for a mix and inform decision-making.
The second is to push the industry to develop lower-carbon emitting solutions. We should be leaning on the innovativeness of the supplier to produce new mixes both with the tools and materials they have available and looking for new materials and procedures to meet the demands of project specifications.
To meet an embodied carbon goal, along with performance and life safety criteria already demanded of suppliers and contractors, it becomes even more essential than ever to allow flexibility and control for the ready-mix supplier through an overall performance-based approach to the concrete specifications.
Summary
The best way to achieve an embodied carbon goal, or other project requirements for your concrete scope, is through the use of performance-based specifications. They allow for specification of a direct measure of embodied carbon: a global warming potential limit supported by an environmental product declaration. For detailed supplemental information on all the concrete mix design properties discussed in this article, refer to the Performance-Based Concrete Specification Guidance: Concrete Class Table published on the NCSEA website by the NCSEA Sustainable Design Committee. Take this document and start to have discussions with your project teams. Better yet, start having conversations with your local concrete suppliers about their capabilities when it comes to low embodied carbon concrete. For the ideas proposed in this article to be successful, it requires increasing our ability to specify the appropriate tests and trust the data that we receive. It also requires suppliers to understand the testing requirements and early communication among all stakeholders to optimize mixes and perform additional testing as required to meet the needs of the specification without resorting back to traditional prescriptive based approaches. ■
About the Author
Michael Lyons is a Senior Project Engineer and the Structural Sustainability Lead at Martin/Martin Consulting Engineers. He is passionate about educating structural engineers on embodied carbon and sustainability and is an active member of the NCSEA Sustainable Design Committee.
References
EPA (2024). Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2022 U.S. Environmental Protection Agency, EPA 430R-24004. https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gas-emissions-and-sinks-1990-2022.