Technical nuances around the embodied carbon of steel

Steel is an incredible building material that allows a whole host of architectural and engineering concepts to become a reality. However, it is renowned for being carbon intensive to produce due to the mining and smelting of its constituent parts. As a result, the structural steel involved with any given project can often account for a large proportion of its embodied carbon. Conversations around the inclusion of increased recycled content are commonplace, but are these straightforward to represent within carbon modelling?

Modelling a baseline design or conducting optioneering studies with varying levels of recycled content in structural steel is integral to performing embodied or whole-life carbon analyses. However, selecting the right material, specifically the Environmental Product Declarations (EPDs) (essentially a passport of embodied carbon data for a material), while adhering to relevant guidance can be more complex than it initially appears.

Traditionally, a widely used methodology when modelling the embodied carbon of structural steel involves using data from generic databases (in situations where no actual carbon data has been provided by the design team or contractor). This is because generic databases can provide incremental changes in recycled steel content, giving normalised increases or decreases in carbon factor. This can be seen in the graph below, which shows the relative drop in kgCO2e/kgSteel (modules A1-A3 – or the ‘product stage’ emissions) with an increase in recycled steel content.

*Results based on OneClick LCA 2018 UK steel data – Structural steel profiles, generic, XX% recycled content (only virgin materials), I, H, U, L, and T sections, S235, S275 and S355

However, with the ever-increasing granularity of available data that feeds into the RICS Professional Standard 2023 —the authoritative guidance for conducting Whole Life Carbon Assessments—accurately accounting for varying levels of recycled content in structural steel has become increasingly complex. This complexity is partly driven by the RICS PS 2023 itself, which mandates assuming that structural steel is composed of 60% Blast Oxygen Furnace (BOF) material and 40% Electric Arc Furnace (EAF) material (if no other design data is available).

This not only requires the use of multiple EPDs for a single volume of structural steel, but also opens the door for interesting and vital conversations with the steel industry to obtain accurate carbon factors for both BOF (assumed to be produced using coal derivatives, with a recycled content of 13%, as per RICS) and EAF steel (assumed to be produced using electricity, with a recycled content of 97%, as per RICS).

Conversations with a leading contractor have led to the adoption of 2.45 kgCO2e/kgSteel for BOF and 0.6 kgCO2e/kgSteel, which have been used to create a weighted assumption of 1.71 kgCO2e/kgSteel (with 47% recycled content).

The below graph is a continuation of the previous image but shows where this hybrid carbon factor sits within the often-used generic values.

If a similar methodology is adopted for modelling the RICS assumption for structural steel, care must be taken when updating a model to include planned increases or decreases in recycled content. If the model is passed between analysts within an internal team, or transferred to another organisation, and the new owner uses the generic dataset to increase the recycled content to 60-70% for example (from the baseline of 47%), the results will experience an increase in embodied carbon instead of the expected decrease.

The various software tools are all constantly working hard to stay aligned with the updated guidance, however the role of the analyst needs to extend beyond the blind-trust of EPDs and data that is presented to the industry and should include a deep understanding of both the guidance and sources of data. This will result in the production of accurate carbon models, and allow consultants to advise design teams more effectively.

In the meantime though, the best course of action is to list all assumptions around material specification and modelling methodology to encourage transparency, and consistency throughout the industry.