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Embodied carbon in construction is defined as the carbon emissions associated with manufacturing and transporting construction materials and the process of construction.

Embodied carbon in construction

Buildings account for 39% of global greenhouse gas (GHG) emissions: 28% from building operations and 11% embodied carbon from building materials and construction.

In some cases, embodied carbon can account for as much as half of a building’s total carbon footprint over its lifetime. This is largely due to carbon-intensive material manufacturing processes and large quantities of fossil fuels used before materials even reach the construction site.

1. Need to Understand Embodied Carbon

Unlike operational carbon, embodied carbon cannot be reduced in materials once a building’s construction is complete. As buildings continue to improve operational efficiency with increased sophistication, embodied carbon will form a larger proportion of the buildings’ overall lifetime carbon emissions. Although awareness of this subject is still growing, reckoning with embodied carbon is necessary for the construction industry as it works to mitigate climate change.

If nothing is done to tackle the issue of embodied carbon, it is unlikely that emission targets necessary to keep global warming within 2°C will be achieved. To meet ambitious climate goals, addressing embodied carbon has to be a part of the construction industry’s climate mitigation strategy.

2. How to Assess Embodied Carbon?

Identify the carbon “hotspots”—systems or materials that contribute the most to a building’s embodied greenhouse gas emissions. This would enable the project team prioritize and choose the materials that can make a huge difference.

Whole-building life-cycle assessment (WBLCA) is the most widely used method to assess embodied carbon, but other tools can be included to supplement this as a first step.

For rudimentary understanding and getting an idea for the carbon footprint in different materials, a few free resources are available on the internet. One may refer Bath Inventory of Carbon and Energy (ICE), which is a long-respected source of embodied carbon data, or the Quartz database, which comprises basic health-related and environmental-impact data on 102 common construction materials.

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Environmental product declarations (EPDs) can be referred to understand the carbon footprint of specific products. EPDs are based on “functional units” rather than weight, and generally provide the carbon footprint of a specific material or set of products rather than a generic baseline.

The best way to get a clear picture of how one product or system compares to another in the context of a construction project is to use WBLCA. The process analyses the impact of building materials from various aspects. For instance, a material’s global warming potential over its entire life cycle, from extraction and manufacturing through the landfill or recycling plant, can be ascertained.

3. How to Reduce Embodied Carbon?

The very first question to ask before beginning a construction project is whether new construction is really needed or the reuse and incorporation of salvaged building materials can get the work done. By avoiding the use of new building materials, it would be possible to avoid their impacts altogether.

It’s also important to think about the lifespan of new buildings before they’re built. It doesn’t seem feasible to emit carbon twice or three times by constructing a new building when the existing building could serve two or three different uses over its life. Design the building for future uses and deconstruction of systems in consideration to put materials to use for a second time in another building.

4. Optimizing Structural Systems

One of the most important takeaways from WBLCA is that structural systems are the major sources of embodied carbon in the building, with a composition of 80%. Therefore, the primary step for reducing the embodied carbon in a project is to focus on the structural system. Concrete, wood, and steel can all be optimized in different ways to reduce impacts.

4.1 Concrete and Cement

Concrete has a large footprint due to the carbon-emitting process used to make binder portland cement, one of its main ingredients. This process contributes to 5% of the global CO2 emissions. The embodied carbon of concrete in the projects can be reduced by replacing some cement with supplemental cementitious materials (SCMs) such as fly ash or blast-furnace slag.

There are several ways to reduce cement content, like less usage by specifying higher-quality aggregate or reducing the water content. The impact of concrete can also be reduced by establishing a direct dialogue with the ready-mix supplier and specifying the structural and environmental requirements.

4.2 Steel

In terms of weight, steel has a significantly higher embodied carbon footprint than concrete; one ton of steel contributes to approximately a ton of greenhouse gas emissions. Production of steel is responsible for 6.6% of global greenhouse gas emissions, which is comparatively higher than portland cement.

Though developed nations have adopted electric arc furnaces (EAF) for the production of steel, several countries are still using basic oxygen furnaces (BOF), which is a dirtier technology. The use of EAF along with a cleaner electrical grid has resulted in a 36% reduction in the industry’s carbon footprint since the 1990s.

Approaches for reducing the quantity of steel in construction include the adoption of composite design, where the steel and concrete slab work together to reduce the size of the beams. Moreover, the choice of lateral system can also have a big impact on the quantity of steel. Braced frames with diagonal braces require less steel than moment frames. It is advised to get a structural engineer involved early on in the project for reducing embodied carbon.

4.3 Structural Wood

The application of wood instead of concrete or steel elements provides major carbon benefits. Wood products sequester carbon, whereas steel and concrete are made by burning fossil fuels. Inclination towards building with mass-timber structural products like cross-laminated timber (CLT) has seen a drastic rise, partially due to presumed lower embodied carbon impacts.

FAQs

What is embodied carbon?

Embodied carbon in construction is defined as the carbon emissions associated with manufacturing and transporting construction materials and the process of construction.

What is the best way to assess embodied carbon?

Whole-building Life-cycle assessment (WBLCA) is the most widely used method to assess embodied carbon.

How can the use of cement and concrete be optimized to reduce embodied carbon?

There are several ways to reduce cement content, like less usage by specifying higher quality aggregate or reducing the water content. The impact of concrete can also be reduced by establishing a direct dialogue with the ready mix supplier and specifying the structural and environmental requirements.

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