System boundaries as set by the BS 15978 determine the processes taken into consideration of the object of environmental assessment of construction projects as seen in the Figure above. Cycle phases are classified into stages of the material production and transportation to site, construction processes, in use, and end of life, as described in detail below. Further for simplicity, definitions for the purpose of this study carbon emissions associated with modules A1 to A5 are defined as capital carbon.
• A1-A3 (Cradle to Gate)
Product stage also known as ‘cradle to gate’ and modules A1–A3, are carbon emissions (kgCO2e) released during raw material extraction, processing, manufacture (including prefabrication of components or elements), and transportation of materials between these processes until the product leaves the factory gates to be taken to site.
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• A4-A5 (Construction process stage)
Modules A4 to A5 are associated with the embodied carbon released during the transport of materials/products to the site (A4), the energy usage due to activities on site (machinery use, etc.), and the carbon emissions associated with the production, transportation, and end of life processing of materials wasted on-site (A5) (O P & J J, 2020) (15804:2012, 2020).
• B1-B7 (Use Stage)
The usage stage includes the carbon emissions released due to use, maintenance, repair, replacement, refurbishment, and operational energy. Module B4 (replacement) is often the focus of the use stage when embodied carbon is being considered (O P & J J, 2020) (15804:2012, 2020).
• C1 – C4 (End of Life)
End of life stage, modules C1–C4 are emissions released during decommissioning, stripping out, demolition, deconstruction, transportation of materials away from the site, waste processing and disposal of materials (O P & J J, 2020).
The best practice for analysing the core values of a modern net-zero infrastructure project is by adopting a life cycle approach to the whole life span of stages A1-C4 from the extraction of raw materials and manufacturing component products of build assets to operation and disposal, and to the potential beyond End of life of assets.
A reliable and comparable environmental methodology as per BS EN 15978 should be adopted based on the study scope with the minimum required calculation including Module A1-A5 as outlined in (O P & J J, 2020). As stated in ‘How to calculate embodied carbon’ (O P & J J, 2020), 50% of emissions of a project are associated with the material selection of embodied carbon (A1-A3) where the most analysis should be done to identify hotspots early in the project.
LCA as defined by ISO 14040
As defined by ISO 14040 an LCA analysis should consist of the phases illustrated in Figure above. A partial LCA framework is established including elements such as the study goal, the system boundary, scope definition, inventory analysis, impact assessment, and interpretation of results.
Useful Reading
Calculating embodied carbon in the same rigorous way across all designs will allow meaningful comparisons to be made between structural schemes, developing our understanding of embodied carbon as well as how the industry can most effectively reach net-zero carbon.
This guidance is equally applicable to infrastructure and building projects. The calculation of embodied carbon must become a key part of every design process. Such efforts support the immediate need to reduce resource demand and increase reuse and recycling to enable a circular economy. This guidance will:
Help you select and specify materials that are efficient while ensuring they're safe and durable
Establish an agreed set of principles for the measuring of embodied carbon
Reduce the amount of carbon you use in your projects
Quantify the benefits of low carbon design to the client and society
Inform you about alternative ways you can move towards net zero carbon design
This guide has been translated into an open-source Excel estimator, The Structural Carbon Tool. The tool is free to download and can be used to quickly identify ways in which to minimise embodied carbon on your designs.
The guide supports legislation by UK Parliament to achieve net-zero carbon emissions by 2050 and forms a part of the response of The Institution of Structural Engineers to implement the necessary changes in the profession to respond to the climate emergency. IStructE has built a portfolio of guidance documents, FAQs and articles related to embodied carbon. The guide also supports the sustainability-related core tasks in The Structural Plan of Work 2020.