Reducing Embodied Carbon in Concrete

Specifying Portland-limestone cement makes a significant difference
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Sponsored by LafargeHolcim
By Peter J. Arsenault, FAIA, NCARB, LEED-AP

Green Building Considerations Of Portland Limestone Cement

The strategies of using PLC and SCMs clearly can be used to reduce the environmental impact of cement and concrete. For design professionals seeking to include that benefit in a project and demonstrate its effectiveness, there are three areas that can be considered.

  • 2030 Challenge: The 2030 Challenge for Embodied Carbon is a good resource for information and assistance. The targeted CO2 reductions have been adopted and supported by a wide coalition of leading manufacturers, design firms, and experts. Collectively, this group is working toward increasing awareness about the issue of embodied carbon, developing life-cycle assessments (LCAs) and environmental product declarations (EPDs) for building products. For design professionals, it means this information can be used to make informed, low-carbon decisions. Among the growing list of adopters and supporters, concrete and cement manufacturers and suppliers are included. They can provide industry wide or company specific information on the embodied carbon content of their products.
  • LEED Credit Contributions: For projects pursuing green building certification such as LEED, the selection of materials has always been important throughout the different versions of its history. Currently, it is the use of EPDs and LCAs that have become the key for assessing carbon content and other environmental factors of the materials and products used in a building. Designers can use the information in these documents to compare different concrete products (i.e., PLC versus OPC, and other Types with SCMs) and make decisions about the most suitable choices for their building. The information can also be used as part of the submitted documentation needed for LEED certification in two potential categories. Site concrete, such as that used for paving, sidewalks, curbs, ramps etc. can help in the Sustainable Sites category when concrete is specified with lower embodied carbon. Building concrete, such that used in floor slabs, structural frames, foundations, etc. can help in the category of Materials and Resources. Either way, the key is to assess the amount of embodied carbon contained in the manufacture and delivery of PLC compared to OPC. That calculation then plays into the overall LCA criteria for the building.
  • LEED Innovation in Design: In addition to the standard credit and point listing in LEED, there is the possibility of demonstrating greater sustainability by showing innovation. The use of Portland-limestone cement and SCMs in a project may be eligible for credit when the cement replacement factors can be demonstrated to be greater than 40 percent. Check the criteria for this credit related to the version of LEED that is being used for a project to ascertain the specific documentation and performance requirements.

Specifying Portland-Limestone Cement In Concrete

Making a difference on the amount of embodied carbon in any particular building project starts with design to determine and economize the amount of concrete used to make it efficient and avoid any excess use. The focus is then on the specifications of the concrete used in the different locations of the project. The first step is to specify Type IL Portland-limestone cement as a one-to-one substitute for Type I ordinary Portland cement. Note that PLC is not always locally available, so it is prudent to check with concrete suppliers in the area for two reasons. First is to be sure that it can be supplied when specified. Second, if it turns out not to be available, to ask—why not? Architects and engineers can help drive the market in this case by demonstrating the demand for PLC and encourage cement suppliers to provide it. In essence, the more PLC is specified, the more it will become available.

Photo courtesy of LafargeHolcim

Specifying Type IL Portland-limestone cement into specific projects helps reduce the embodied carbon of those projects and helps advance the availability of Type IL (PLC) in specific markets.

In terms of the specification writing, it is a fairly simple process to incorporate PLC since it is a direct replacement for Portland cement. In a typical CSI or AIA Masterspec concrete specification concrete is covered in Division 03 00 00 with detailed sections used for cast-in-place (03 30 00) or pre-cast concrete (03 40 00) as well as other types of concrete and related work. These are some of the things to incorporate in order to achieve concrete with a lower carbon footprint.

  • The “Part 1 – General” section of the specification will still refer to ASTM standards for concrete but be sure to include reference to ASTM C595 for blended cements and not just ASTM C150 for Portland cement. The rest of the usual Part 1 conditions for scope, quality control, testing, etc. will still apply as customary.
  • The “Part 2 – Products” section of the specification is where the primary change occurs. Instead of calling for Type I Portland cement (or Type II in some cases), specify Type IL Portland-limestone cement. If the concrete for the project requires special properties, such as sulfate resistance or controlling the heat of hydration, then a qualifier is added to the IL designation. For example, sulfate resistance would be specified as Type IL (MS) while moderate heat of hydration would be specified as IL (MH). Other options are available based on conventional concrete specification protocols, too. Another option for reducing embodied carbon is to specify other ASTM 595 blended cements that use supplemental cementitious materials such as a Type IP (with fly ash or natural pozzolan) or Type IS with slag cement.
  • “Part 3 – Execution” is largely unaffected by the use of PLC or SCMs. The concrete should be transported, tested, placed, and finished the same as for any other concrete. If there are any installation limitations, they are usually inherent in the nature of the blended cement or the concrete mix and are the same regardless of whether OPC or PLC is used. In case of any concerns, it is prudent to check with the local supplier or with industry wide organizations such as the PCA.

In addition to the process for general building specifications, the steps described above are also appropriate for DOT construction using AASHTO specifications for roadway, drives, and bridge construction. It is also appropriate for Federal Aviation Administration (FAA) specifications for airport construction including runways. Finally, these same strategies can be used in Canadian specifications for general-use cement.

Regardless of the application or specification basis being used, the overall goal is to reduce the amount of clinker needed which thus reduces the embodied carbon but without compromising the structural or other performance characteristics of the concrete. Proper attention to the specification writing process for concrete can help achieve exactly this goal.

 

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Originally published in Architectural Record
Originally published in September 2021

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