How to save 1 million tonnes of CO2

While the buildings sector has made significant strides in reducing operational carbon emissions, we must now focus on the challenge of reducing embodied carbon. By thinking differently about some of the most carbon intense elements of a building, we believe it is possible to remove as much as 1 million tonnes of CO2e from the UK building sector annually, that’s equivalent to the annual CO2 uptake of 40 million trees.

The term embodied carbon is used to denote the global warming potential (GWP), measured in equivalent carbon dioxide emissions (CO2e), attributed to all materials and processes that go into the materials and components of a project.

In a whole life cycle assessment (LCA), we consider both the upfront embodied carbon that is associated with all the materials and components up to practical completion, in addition to the in-use embodied carbon associated with replacement and maintenance as well as operational carbon.

Whole life cycle assessments are key to ensuring holistic solutions, and in a buildings structure the upfront embodied carbon is generally the most important component to focus on.

In a typical building the majority of upfront embodied carbon is in the building structure, typically accounting for 50-70% of total emissions. Whilst construction technology has progressed massively, almost all building structures are still made of concrete or steel, and to a lesser extent masonry and timber. It is hard, though, to find a building that does not use any concrete, at the very least in the foundations.

Concrete is the most used construction material on earth. In the UK it is estimated that we use almost 36 million tonnes of concrete in building structures every year(1). This equates to 4.5 million tonnes of CO2e, around 1% of the UK’s total carbon emissions(2).

Concrete in building structures presents a huge opportunity for us to reduce embodied carbon. At Ramboll we have developed a three-step philosophy to reduce carbon in building structures in the UK:

1. Challenge the brief: Challenging the brief is about testing the requirements of the building structure and making sure that we have explored opportunities to reduce material use.

For concrete structures this can make a huge difference, and might involve finding a more efficient grid, reducing load or changing the way the façade is supported.

For example, reducing a structural grid from 9m x9m to 8m x8m can save 15% CO2e/m2 per slab.

2. Refine the design: Once we are confident that our brief meets the client’s needs without any unintentional carbon intensive requirements, we focus on refining the design. Through more rigorous analysis, careful application of existing codes and pushing structural utilisation we can make further savings. Rigorous application of codes alone can reduce design loads by 5-10%.

3. Cut the carbon: Finally, we focus on ensuring that our material specifications are optimised to meet technical requirements while minimising carbon. For concrete this involves an active process to review the structure to identify the opportunities to reduce the carbon in each element and concrete mix.

Cutting the carbon in concrete requires a detailed understanding of the material, what it is made up of and ultimately how we can refine it to reduce emissions. According to our estimates, the UK building sector alone could save one million tonnes of CO2e by using this approach.

Cement must be the focus of attention when it comes to reducing carbon. Cement is the active ingredient in concrete, typically accounting for 12% by weight, but it accounts for up to 85% of the embodied carbon.

Engineers mainly consider strength and durability when specifying the quantity of cement in concrete. However, engineers typically specify a minimum cement content with the final mix being determined by the contractor and their supplier.

Whilst a contractor must supply a concrete which satisfies the permanent requirements of the engineer, they are also focussed on the ‘fresh’ properties of concrete. As such, in addition to the engineer’s requirements they are also focussed on the rate of strength gain as well as the consistence, or flow, of the concrete.

These latter requirements can significantly increase the cement content over and above what is required for strength alone. For this reason, cement content may be as much as 50% higher than the engineering requirements demand.

There are many different types and blends of cement. The most common cement is called Portland cement, or CEMI. It is made by crushing limestone and clay, heating it to 1450° degrees and grinding into a fine powder.

The process changes the chemical structure of the limestone, resulting in a reactive material which, when mixed with water, will form a strong and stable matrix, locking together the aggregates in concrete. The production process is very carbon intensive: for every tonne of Portland cement there is approximately 860kg of eCO2(3).

It is rare, however, to use pure CEMI concrete. It is generally blended with supplementary cementitious materials (SCM), typically waste products from other industries. These SCMs offer varying degrees of reactivity and alter the fresh and permanent properties of the concrete. As most SCMs are far less carbon intensive than Portland cement they present an easy way for us to reduce the carbon in the cement.

Up to 80% of Portland cement can be replaced by SCMs under current technical codes, though it may not always be appropriate to do so. The two most common SCMs are Ground Granulated Blast Slag (GGBS) and Fly Ash (FA), produced by steel blast furnaces and coal-fired powered stations respectively.

The potential of these SCMs is thus limited by our need to also address the carbon intensity of those industries. Furthermore, there is insufficient supply of GGBS and FA to address the global challenge and the industry needs to rapidly investigate alternative SCMs.

There are also new cements that are manufactured using different materials and processes, which react differently when acting as a binder. There are even cements that use carbon dioxide as the reactant, locking in carbon as they cure.

These alternatives offer promising future opportunities; though there is much work to be done before they are commercially viable and can be used at scale. It is important, however, that we support these technologies by providing low-risk opportunities for their use to boost knowledge development and experience in the use of these new materials.

When the UCL Institute of Neurology and Dementia Research Institute is completed in 2024 it will be a world class research facility. Located in the centre of London, this 17,000m2 building will house cutting edge research and imaging facilities. Ramboll have designed an in situ concrete frame to provide the vibration performance and flexibility needed to meet the requirements of the highly complex research that the building will house.

Through the application of our three-step process and working closely with the contractor and their subcontractors Ramboll have been able to include a host of measures which will result in significant reductions in the carbon intensity of the carbon frame compared to typical industry figures.

• Using a feature arched soffit for part of the main office area, which keeps the concrete in compression and uses less concrete and steel than an equivalent flat slab solution.
• Using post-tensioning solutions where appropriate.
• Allowing the concrete to develop strength at a slower rate in the substructure – thereby reducing cement demand.
• Using high quantities of cement replacements, particularly in the foundations, which account for 25% of the concrete.
• Working with the contractor and supplier to limit total cement content through early engagement.
• Exploring opportunities to test levels of cement replacements not normally adopted in flat slab construction.
• We are also investigating whether we can use novel concretes in non-critical external works areas, such as the landscaping paving build-up and hard standing areas.

This suite of measures will result in a weighted average carbon intensity of 97kg eCO2/t , a 23% reduction against typical concrete. In total, this represents a saving of 490 tonnes of carbon, equivalent to taking 400 cars off the road for a year.

With a focus on embodied carbon this project demonstrates that it is possible to significantly reduce embodied carbon in structural concrete by using a systematic approach. If we can bring down the typical carbon intensity across building structures in the UK by a similar proportion, we estimate we can reduce carbon emissions by more than 1 million tonnes per year.

We already have the technology and expertise available to significantly reduce embodied carbon in structural concrete — but it takes systematically identifying savings and a targeted collaborative approach to really maximise the carbon reductions.

Whilst this alone will not take us to a net zero position, it will make a significant contribution and, importantly, establish the processes that allow us to identify next steps and unlock future technologies.

References

1. Concrete volume calculated based on cement estimates in Shanks et al, using a 12% cement content.
2. Assumes 126kg eCO2/tonne. UK Government carbon data for 2018.
3. Predicted based on concrete specification and discussions with subcontractor. Excludes reinforcement.