A new report from the Concrete Sustainability Hub at Massachusetts Institute of Technology, Cambridge, probes statistical and scientific methods for measuring carbon dioxide uptake in hardened concrete and other cement-based products (CBP). It proposes a framework to account for carbon uptake in Product Category Rules (PCRs), documents key to creating verifiable environmental product declarations (EPDs).
Credible uptake calculations will contribute to EPDs offering a more comprehensive view of life-cycle carbon dioxide emissions metrics than current declarations for CBPs. The focus on uptake—where CO2 in the atmosphere reacts with calcium-rich cement hydration products and is permanently sequestered in CBPs—aligns with Carbonation, the fifth of five Portland Cement Association Roadmap to Carbon Neutrality levers, and the UN Intergovernmental Panel on Climate Change’s recognition of the process as a carbon sink.
“Accounting for Carbon Uptake in the EPDs of Cement-based Products” authors weigh four major factors behind the intensity and extent of CO2 sequestration in hardened elements or structures:
- Climate and exposure conditions, including local CO2 concentration and proximity to industrial sources; ventilation; plus, temperature, humidity and exposure to moisture. Higher temperatures generally accelerate carbon uptake; moderate moisture facilitates CO2 transportation within CBP pores.
- CBP types and properties, the latter including permeability and porosity properties.
- Binder system. Portland cement (clinker only) and blended cements have varying impacts on carbon uptake rates and volume.
- Geometry. Uptake primarily occurs at the CBP surface, where coatings, sealers or other treatment can reduce CO2 sequestration.
“A uniform methodology for computing the carbon uptake potential using equations for different levels is proposed to be used to provide consistent guidelines for the EPD development process such that there is flexibility for input variables at different levels,” authors note. “The consistency of this methodology would facilitate its use over a wider range of climatically diverse regions and applications. These advantages may help a PCR committee standardize the introduction of carbon uptake data in EPDs.”
Authoring “Accounting for Carbon Uptake” are CSHub’s Randall Kirchain, director, and Hessam Azari Jafari, deputy director, and Pranav Pradeep Kumar, postdoctoral associate; and, Mikaela DeRousseau, Data & Methodology manager for Building Transparency, Embodied Carbon Calculator (EC3) administrator. Assisting them are Carbonation Working Group members, including representatives of Heidelberg Materials, Salt River Materials Group, Cement Association of Canada, Canadian Precast/Prestressed Concrete Institute, Concrete Masonry & Hardscapes Association and Portland Cement Association. Findings in their interim report are primarily applicable to ready mixed concrete and cast-in-place surfaces or structures, but will inform forthcoming carbon uptake work on manufactured concrete, wet or dry cast.