Recarbonation at Project Level: Using BIM to Account for Concrete's CO₂ Uptake

Concrete absorbs CO₂ from the air throughout its life. EPDs already capture some of this, but only with generic assumptions. This article explains why the next step is to calculate recarbonation at project level, using the building information model (BIM), and what each part of the value chain needs to do to get there.

By Nicolas Antoniou and Dr Andrew Minson, Global Cement and Concrete Association

Photo: Pexels

What recarbonation is, and why it matters

Recarbonation is the natural process in which CO₂ from the atmosphere penetrates concrete and reacts with its hydration products to form carbonates. It partly "rebinds" the CO₂ released when clinker and cement are made, so it offsets some of concrete's gross embodied carbon.

It happens at several points in a concrete product's life:

• while precast elements are stored at the factory
• throughout the building or structure's service life
• after demolition, when concrete is crushed, stored, recycled or landfilled

The amount of CO₂ taken up is significant. It should therefore be recognised and quantified much more widely in concrete carbon assessments.

Where we are now: recarbonation in EPDs

Two European standards set the rules today. EN 15804 gives the core rules for construction product EPDs. It allows, but does not require, CO₂ uptake to be included, as long as assumptions are documented and any exclusion is clearly stated. EN 16757 adds concrete-specific rules. Its annex on CO₂ uptake by carbonation (Annex G in the current edition) sets out a calculation method based on carbonation depth, reactive CaO content, cement composition, exposure and service life. Uptake is considered in modules B (use), C3 (waste processing) and C4 (disposal), and in A3 for precast products stored before delivery.

The GCCA EPD Tool applies this method and offers producers three ways to calculate uptake: detailed, default and user-defined. In Version 6, each life cycle stage also accounts for the uptake already achieved in earlier stages. We explain the method, the formula and the default values in our article Recarbonation of Concrete: From EPDs to Whole-Life Carbon Accounting.

The limit of product-level accounting

The EN 16757 calculation depends on two kinds of input:

• Material parameters, such as binder composition, reactive CaO and strength. The producer knows these.
• Use parameters, such as exposure, exposed surface area and service life. The producer usually does not know these.

A cement or ready-mix producer writing an EPD cannot know whether a cubic metre of concrete will end up as an exposed facade, an internal slab covered by a raised floor, or a foundation below ground. So the EPD falls back on generic scenarios and conservative defaults. Precast producers are a partial exception, as they often know what their product will be used for.

These defaults are right for a credible, transparent EPD. But in a real project they can significantly underestimate, or less often overestimate, the actual uptake. The factors that drive uptake are exactly the ones the producer cannot see:

• Exposure: concrete exposed to rain carbonates at a different rate from sheltered or indoor concrete, and concrete in the ground or below groundwater behaves differently again.
• Surface finishes: paint, plaster and low-permeability coatings change how much CO₂ reaches the concrete.
• Geometry: thin elements with a large exposed surface per cubic metre take up more CO₂ than massive ones.
• Service life: carbonation depth grows with the square root of time, so a longer design life means more uptake.
• End of life: whether concrete is landfilled, reused or crushed and stockpiled before recycling changes C3 and C4 uptake.

Designers and clients know, or can specify, all of these. That is why the calculation belongs at project level.

What comes next: project-level recarbonation in BIM

A BIM model already holds most of the use information that EPDs lack. In a BIM workflow:

1. Each concrete element is modelled with its real geometry.
2. The model calculates surface areas, volumes and surface-to-volume ratios automatically.
3. The designer assigns attributes such as surface finish and design life to each element.
4. Exposure conditions are set for spaces and surfaces, so the model can work out the exposure class of each face of each element from its location.
5. The model pulls the material parameters for each element from the project's digital EPDs.
6. It then applies the EN 16757 method using the project's own use data instead of generic defaults.

The result follows the same standard method, but reflects the actual building: its geometry, its exposure and how it will be used.

What the designer gets

A BIM-integrated tool can report:

• Annual CO₂ uptake during the use phase, in kilograms or tonnes
• Total use-phase uptake over the design service life
• End-of-life uptake (C3 and C4), shown separately so the effect of recycling assumptions is visible
• An element-by-element breakdown, showing which elements contribute most and where better data would improve accuracy
• Sensitivity analysis, for example changing design life, exposure, surface finishes or product choice, while making sure durability and safety are not compromised
• Whole-life carbon for the project, combining embodied carbon, operational energy and recarbonation

This turns recarbonation from a fixed line in an EPD into something designers can see, test and act on.

Avoiding double counting

If an EPD already claims uptake for its default scenario, and the BIM tool then calculates uptake for the real scenario, the same CO₂ could be counted twice. Digital EPDs should therefore state clearly what recarbonation, if any, they already include, so project tools can replace it rather than add to it.

The missing link: data fields in digital EPDs

For this to work at scale, BIM tools need to read the material parameters straight from the EPD. Today, EPD formats have no dedicated fields for carbonation-specific parameters. BIM users would have to search each EPD by hand, which is slow and error-prone. Adding standard, machine-readable fields to next-generation digital EPD formats would close this gap. The GCCA EPD Tool already exports EPDs in the digital ILCD+EPD format, which offers a natural route for these fields.

Recommendations

Now: include recarbonation consistently in EPDs. Recarbonation is not generally included in North American practice, and some producers elsewhere choose not to report it even when they could. GCCA recommends that recarbonation is included in EPDs in all regions, using consistent whole-life methods.

Next: move the calculation to project level. Over the medium to long term, the sector should shift towards calculating recarbonation for each project in BIM, so that uptake reflects the actual use, exposure and geometry of each structure.

Specific actions:

• North American PCR committees and standards bodies should extend assessed lifecycle modules to full cradle-to-grave and include recarbonation. The EN 15804 and EN 16757 framework is well established in Europe but is not yet included in any North American standards or PCRs.
• Material producers should be encouraged, or required, to declare recarbonation in their EPDs, supported by training.
• EPD programme operators should define a standard set of machine-readable data fields for concrete recarbonation in digital EPD formats.
• Building LCA tool developers should support EN 16757 recarbonation inputs, so designers can assess uptake at building level.
• BIM software developers should enable project-level recarbonation calculations that draw directly on digital EPD data.

Conclusion

Recarbonation is a real and significant CO₂ uptake that belongs in whole-life carbon accounting. The current standards, and the GCCA EPD Tool, already handle it well at product level. But producers cannot know how their concrete will be used, so EPDs have to rely on generic defaults.

Combining producers' EPD data with designers' and clients' knowledge of the building, through BIM-enabled whole-life carbon tools, would make recarbonation visible, credible and something designers can act on. It would support the concrete sector's path to net zero and allow fairer comparisons between concrete and other materials.

To see how recarbonation is modelled in EPDs today, read our detailed article on the method, or request a demonstration of the GCCA EPD Tool.

References

CEN. EN 16757 Sustainability of construction works. Environmental product declarations. Product Category Rules for concrete and concrete elements.

CEN (2019). EN 15804:2012+A2:2019 Sustainability of construction works. Environmental product declarations. Core rules for the product category of construction products.

Global Cement and Concrete Association (2026). GCCA Industry EPD Tool for Cement and Concrete: LCA Model, International version, v6.0.

Stripple, H., Ljungkrantz, C., Gustafsson, T. and Andersson, R. (2018). CO₂ uptake in cement-containing products: Background and calculation models for IPCC implementation. IVL Swedish Environmental Research Institute.

The Concrete Centre. Carbonation of concrete.