Tackling embodied carbon

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What actions is the industry taking to reduce embodied carbon and what are the restrictions?

Steel production from the Port Talbot Blast Furnaces accounts for a fifth of Wales’ annual carbon emissions (or 1.5% of the UK’s[1]). Therefore, the announcement of their designated closure in January 2024, to be replaced by more efficient electric arc furnaces (EAFs), when viewed in isolation, could be seen as a win for sustainability. However, the much-publicised impact on job losses reveals the complicated nature of such closures. One less noteworthy but significant side-effect, is the impact this will have on the UK’s supply of ground granulated blast furnace slag (GGBS).

GGBS is a byproduct of blast furnace steel production which is separated and sent away to be used as a secondary cementitious material (SCM) or cement replacement, as a means of reducing the embodied carbon of concrete through reduced energy-intensive production of virgin cement, which currently equates to roughly 860kgCO2 per tonne of Portland Cement[2]. This is sometimes seen as a ‘get out of jail for free card’ for some designers who, when assessing embodied carbon emissions, view GGBS as a means of avoiding implementation of meaningful carbon saving measures. UK demand for this material currently outstrips supply, with much of the supply being imported from elsewhere on the continent, and the closure of the Port Talbot furnaces will further deepen this dependency.

In September however, IStructE published an appraisal on the availability and usage of GGBS[3], summarising that global demand currently outstrips supply by 800-1,200% and therefore concluding that unless technically required, GGBS should only “be used in proportions cognisant of global constraints.” This therefore begs the question, what is the construction industry doing to actively reduce embodied carbon emissions without simply relying on the presumption that GGBS will solve our issues, and what are the obstacles preventing further decreases in embodied carbon? 

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Concrete

An update to the British Standard for concrete, BS 8500, was released in November 2023, which increased the range of lower carbon concretes that can now be specified within the UK. Formerly only a single SCM could be specified, which tends to take the form of either GGBS or pulverised fly ash (PFA), a byproduct from coal-fired power stations, both sources of which are being phased out as part of decarbonisation. Under the new standard, multiple SCMs can now be used within concrete. Notably this includes the use of limestone and calcined clay, both widely available materials, therefore not only will their inclusion amount to a greater decrease in carbon emissions, but they shall begin to alleviate some of the demand on GGBS & PFA. It should also be noted that the UK holds approximately 50 million tonnes of PFA stockpiles[4] as a legacy of its coal burning power stations that could also be utilised in the short-term, however this also carries with it environmental caveats with the associated emissions as a result of processing and transporting the finite material.

Other low carbon concrete technologies exist, though these still tend to comprise GGBS or PFA, such as Cement-free Concrete (Cemfree)[5] or Earth Friendly Concrete (EFC)[6]. Other GGBS/PFA-free materials are however appearing on the horizon but still remain some years away from large-scale utilisation within the industry, however they signal some of the exciting innovations that the construction industry could be set for, including biogenic concrete[7], recycled concrete paste[8] and Cambridge Electric Cement[9].

Steel

Basic oxygen furnace (BOF) steel, produced from a blast furnace, remains an exceptionally carbon intensive material, but one that remains widely used on construction projects across the UK. The closure of the Port Talbot Blast Furnaces however, means that the availability of this steel is set to be temporarily diminished leading to dependence on imported steel until the EAFs, proposed both there and at various UK sites, come online later this decade.

BOF steel is therefore likely to be sourced from overseas, with consequent higher transport-related carbon emissions. Low carbon alternatives include importing EAF steel from neighbouring countries with a decarbonised grid, such as the Nordics, or reusing steel from other projects. The latter is dependent upon the supply of material, which currently cannot be guaranteed. Therefore, greater effort needs to be spent salvaging materials sourced from demolition and making them more available and open for use. Increased  collaboration between design teams will also raise awareness of the potential availability of resources for use on other construction projects.

Timber

Due to the biogenic sequestration captured within the material, timber remains one of the prime solutions for pursuing a low carbon design and there exists examples of large-scale timber buildings being successfully constructed both in the UK, such as the Black & White Building in Hackney[10], and more widely abroad, such as in Scandinavia where the world’s largest timber building is being constructed (the current largest is the Ascent MKE building in Milwaukee, USA). Its usage in the UK however, also brings about inherent challenges that have stymied its mass usage, most notably fire regulations.

Understandably following the Grenfell Tower disaster, fire regulations were tightened to prevent a repeat disaster occurring, however this greatly inhibited the applications in which timber could be used within construction. Residential buildings over a certain height cannot use timber within the external walls of the building and the use of timber as a structural component in large developments requires an extensive testing process to be undertaken which can be long, expensive, risky and disruptive to construction programmes, all of which tend to put off prospective developers. However, as more projects endeavour to utilise timber within buildings, safety and testing standards can be more widely shared for use by other design teams.

Simply put though, there isn’t enough timber to satisfy our demand should all developments suddenly be constructed out of timber, plus the production of dry sawn timber also equates to c.80% wastage from the original tree[11]. Furthermore, in order to account for biogenic sequestration in whole life carbon (WLC) assessments, it must be demonstrated that the material can be recovered and reused in lieu of disposal at the building’s end of life. This leads nicely on to the final type of materials to discuss.

Reused materials / the circular economy

Designing timber to be re-usable ensures its sequestered carbon remains within the materials system where it can be used on further projects for as long as possible. Therefore, the same view needs to be adopted with other materials whereby existing buildings are seen as an ‘urban mine’ and a potential materials depot as opposed to simply being something requiring removal or crushing for use as fill. Designing buildings to be adaptable, to extend their services lives, and designing for disassembly upon the end of their serviceable life ensures that their materials stock can be repurposed, thereby negating the need to produce new virgin materials.

The difficulty in obtaining these materials boils down to several factors:

  • Identifying where materials can be sourced from;
  • The increased deconstruction time that this imposes on developers;
  • The processing and storing of said materials;
  • Testing and obtaining the necessary insurance to ensure materials meets the required technical standards; and
  • Collaboration between stakeholders to raise awareness of when materials become available.

Both significant financial and carbon related savings can be achieved through opting for re-use over sourcing virgin materials, however the industry needs to be better structured to facilitate this. This includes the creation of a central materials sharing database for projects where excess materials are available or outlining what materials are likely to come available as a result of disassembly, and when these will be obtainable.

There will never be a single silver-bullet solution to all our embodied carbon issues, and no one size fits all approach that can be copied and pasted across each development, but solutions do exist outside of our standard approach to designing buildings that can go further to reducing carbon on a typical development and the future is looking optimistic with the range of solutions on the horizon. However, we cannot be static and simply wait for these solutions to arrive. That is why a collaborative approach to sharing information and data is one of the most easily and immediately implementable actions we can enact, and it will begin to pay-off in the long-run. This begins with ensuring that all projects are uploaded to Built Environment Carbon Database (BECD), to give the industry better coverage of WLC performance to aid with benchmarking and sharing best practice designs and methodologies.

The reduction of operational emissions has been something that we have been doing for decades, however the focus on embodied carbon emissions is a more recent, yet just as pressing, challenge that the industry is going to have to better grapple with. The speed at which we need to overcome this challenge relies on every stakeholder playing its part and assisting one another.

For more information, please contact Cameron Parker

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