Production and placement of concrete is responsible for an estimated 7 percent of total carbon emissions, worldwide. Recently, significant policy and legislative efforts have been undertaken to limit the embodied carbon associated with concrete and other construction materials. This action has taken place at the municipal, state, and federal levels simultaneously, leading to varied and inconsistent approaches to limiting the embodied carbon of concrete.
To provide a standardized approach to reducing carbon in concrete construction, American Concrete Institute has introduced ACI CODE-323-24: Low-Carbon Concrete—Code Requirements and Commentary. ACI Code-323-24 was developed by the concrete and sustainability experts on ACI Committee 323 to limit carbon emissions associated with concrete production and placement.
The first edition of the Code, published in November 2024, has a narrow scope focused on up-front embodied carbon emissions for cast-in-place concrete systems with design strengths between 2,500 and 8,000 psi. The Code was written so that it may be applied to any project using concrete materials and is not specifically focused on building design. It is expected that future versions of the code may expand to examine more aspects of sustainability, further life-cycle stages, and a broader range of concrete systems.
The code is entirely material technology agnostic, meaning that it does not require or limit the types of technologies that can be used in concrete mixtures. Instead, the Code provides a method for developing a performance requirement for limiting the global warming potential (GWP) of concrete for a particular project. The methods for achieving that GWP are left up to the Licensed Design Professional (LDP) and their team.
ACI Code-323-24 aids in the specification of lower-carbon concrete
Code-323-24 takes a modified carbon budget approach to reducing carbon emissions associated with concrete materials. The Code contains provisions for LDPs to determine a weighted GWP performance requirement for the concrete used in their projects. This weighted approach allows engineers the flexibility to use concrete systems that may be needed for specific applications without sacrificing performance and constructability requirements. Therefore, if an LDP needs to use a system with a relatively high GWP, they can offset this by using a low GWP system in another area of the building. This provides LDPs with better tools than what are available in the more common carbon limit per concrete class approaches.
The Code follows a three-step process. First, users must determine the project type they are completing, i.e. building, bridge, pavement or hardscape, or other structure (tanks, underground structures, etc.). For Step 2, based on the type of project and the size of the project construction, LDPs will then determine the appropriate compliance path: Tier 1, Tier 2, or Tier 3. For Step 3, the engineers will need to produce the appropriate information required by either Tier 1, Tier 2, or Tier 3 compliance paths.
The Tier 1 compliance path, currently reserved only for large construction projects, will require LDPs to identify the weighted GWP limit for the cast-in-place concrete used on their project. This weighted GWP limit will be a percentage of the local GWP benchmark, as defined by the adopting jurisdiction. The Code also allows adopting jurisdictions to set their own reduction factor, based on locally available technologies. LDPs will be required to source environmental product declarations for example concrete mixtures to be used on the project, and list the strategies that they will use to achieve lower embodied carbon in the concrete.
For Tier 2 compliance, reserved for medium-sized projects, LDPs will be required to determine what the weighted GWP limit for their project would be if they were a Tier 1 project, but are not required to meet that reduction level. They will also have to document the strategies that they will use to achieve lower embodied carbon in the concrete used in their projects.
Finally, for Tier 3 work, or the smallest projects, the LDPs are required only to document any strategies that they will use to achieve lower embodied carbon in the concrete used in their project. This current approach places the most significant burden of compliance on only the largest projects for the initial version of the code. The code also allows an adopting jurisdiction to waive compliance with the code, on a case by case basis, if the LDP can show hardship in meeting the requirements.
Where using a reduced-carbon concrete is a standard
The road to Code adoption can vary depending on the state and country that may be interested in adopting it. The process of official adoption into state building codes can take several years; however, many groups have expressed an interest in the adoption of Code-323. The Code may also be adopted by the owner or LDP on a project-by-project basis.
Currently, many areas have adopted buy-clean policies and requirements that could benefit from the use of Code-323-24 as a tool for determining the GWP performance requirement for concrete. These include Washington State, Oregon, California, Minnesota, New York, New Jersey, Maryland, and Colorado. Bills are under consideration in Illinois, Pennsylvania, Massachusetts, Rhode Island, Connecticut, and Virginia.
Learn more about Code-323-24, including FAQs and webinars, by going to NEU: An ACI Center of Excellence for Carbon Neutral Concrete, www.neuconcrete.org. NEU’s resources allow you to review the best practices for reducing the embodied carbon of concrete construction. When you are ready to order, go to ACI and order ACI CODE-323-24: Low-Carbon Concrete—Code Requirements and Commentary.
Dr. Matthew Adams is associate professor and co-director of the Materials and Structures Laboratory (MatSLab) at the New Jersey Institute of Technology and chairs ACI Committee 323 – Low-Carbon Concrete Code.