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How do we optimise structure for low carbon?

Steve Toon

For optimum efficiency we must tailor our calculations to the specific structural design, says AKT II design director Steve Toon, offering five guiding principles to interrogate your design

Five considerations affecting embodied carbon in structures.
Five considerations affecting embodied carbon in structures.

At the start of projects, we often feel there is an expectation for a simple answer, a one-size-fits-all solution to achieve the lowest carbon solution.

But there isn’t a silver bullet – not yet at least.  There is something unique about each project brief, design, procurement and delivery. 

The closest we get to it is a mantra: ‘Think more, use less’.

The diagram above shows the five considerations that fundamentally affect embodied carbon in a structure. Projects must assess these aspects as a minimum (the issue is complex) to ensure the optimum solution has been rigorously explored. The five are grid; massing/geometry; environmental strategy; material specification; and materiality.

If ‘less is best’ then ‘nothing is most’ – that is, put retrofit first.  AKT II pushes this agenda on projects to test its efficacy. The initial brief for AHMM's Angel Building, for example, anticipated a full rebuild, but strategic thinking by the client and design team placed retrofit as the optimal response.  However, this article focuses on cases where there are elements of new-build.

Chart 1: Carbon intensity set against weight of various structural materials
Chart 1: Carbon intensity set against weight of various structural materials Credit: AKT II

Less is best

In simple terms, less embodied carbon on a like for like basis means less material. But here is the first conundrum, all materials – even overtly similar ones – are different when embodied carbon is considered (for example, steel – EAF, BOF, recycled content, energy sources in production, etc). We need to develop an understanding of the carbon intensity of the materials we use, in addition to the structural properties used in engineering design – something engineers acquired from those first lectures at university. 

On a simple, elemental basis, ‘embodied carbon = material quantity x carbon Beta factor’.  We summate all of these individually to determine the total embodied carbon value of a structure.  The tendency is to define quantity by volume or weight, but this misses the inherent mechanical properties of any given materials. In addition, when we start to relate carbon to strength or stiffness (as opposed to m3 or weight), a very different and more informative picture emerges. 

On a carbon per weight measure, steel is up to 15-20 times more carbon intense than concrete and up to 7-10 times more so than timber (see Chart 1 above).  But from a carbon per strength angle, the ratio is much closer on all three materials (see Chart 2 below).  This suggests that there are no standout materials in respect to carbon intensity if each material is applied structurally in its optimum and most efficient manner.

Chart 2: Carbon intensity set against strength of various structural materials-giving a more informed design perspective
Chart 2: Carbon intensity set against strength of various structural materials-giving a more informed design perspective Credit: AKT II

Once we consider materiality carefully, the need to reduce redundancy of structural systems and to remove excess material – perhaps provided for ease of design, construction, coordination or interfaces of finishes etc – becomes evident. Taking the reliable concrete flat slab as a case in point, we can eliminate up to 40% of the concrete volume by removing essentially un-needed material. The resulting form may be slightly deeper, morphing into a once oft-used ribbed or waffle floor system.  So why don’t we still build with these forms? Perhaps because they can take longer to construct, may be more expensive and may provide a more challenging finishes, interface etc. But as designers we must encourage our clients and those who build our designs to also rise to this challenge.  This example is by no means structurally optimised, like some of the bespoke shell and tie systems recently prototyped, but if our industry cannot make the small leap back to a ribbed slab to reduce material, how will it make the quantum leap forward to what can really be achieved?

Relative concrete volume / material use of a flat slab verses various ribbed and waffle slab geometries.
Relative concrete volume / material use of a flat slab verses various ribbed and waffle slab geometries. Credit: AKT II

Must be fit for purpose

We must promote solutions and materials that are fit for purpose, that are used in an appropriate way and that will endure. There isn’t a single material available to us yet that is suitable for optimising carbon footprints of our building fabric in all typologies, or for all uses and scales. Some promote timber as meeting all these criteria, and while timber is a highly suitable material for many scales and uses, it is not a silver bullet that can address the climate emergency issue for the built environment. 

System boundaries and overlapping edges

When investigating the timber, lumber and forestry industries it is eye-opening to see the small proportion of the total felled biogenic material (that is, sequestered carbon) from any individual tree or plantation that actually makes it beyond a 10-15 year life before being sent to landfill or incinerated, resulting in its sequestered carbon being re-released.  Research estimates that as little as 20% of a trees total biogenic carbon is likely to make it into medium-long term structural timber use and be sequestered away for the foreseeable future. Bearing in mind that this timber had sequestered its carbon over 20-50 years, we do need to ask ourselves whether trees should be left to grow?  Re-planting only meaningfully removes carbon at a suitable rate 15-30 years after planting.

UK demand for construction timber is around three million tonnes  per annum. To meet that demand, 21% of the UK would need to be forested for sustainable harvesting, compared to the 5% there is now, so there is a limit to how much timber can be used as an alternative to other materials. In mid-December last year the government announced the publishing of a route map promoting use of structural timber in the UK.  This is intended to resolve anomalies in whole life carbon data for timber and fire/insurance issues – which is welcome news to bring some consistency to this part of the agenda.  

In dynamic carbon systems like timber, time is a critical consideration. To reduce the impact of time and peripheral carbon release on the carbon to atmosphere calculation and system boundary, we need to find alternatives to slow growing timber – such as bamboo. While bamboo needs significant processing to be used in sufficient size and form to replace mass structural timber, its growth rate is so rapid by comparison and its peripheral waste so low that it can significantly out-perform mass timber on the sequestration calculation. Preliminary studies also suggest that it will perform equally well as a structural material.  

Designers, along with our clients and procurement teams, need to create the demand so that technical assessments are in place and the material supply is available for use. 

Structural optimisation and performance assessed solution to fine-tune total carbon measures in design.
Structural optimisation and performance assessed solution to fine-tune total carbon measures in design. Credit: AKT II

Ill-informed and conflicting planning / statutory legislation

There are immense complexities in the system boundaries of the Environmental Product Declaration (EPD) process, and if we don’t look beyond the confines of our own project we can inadvertently make ill-informed decisions that make our projects shine only on paper. This is how UK legislation (or lack of it) leads us to a lack of wider consideration and an inability to make direct comparisons. 

There is no statutory UK-wide criteria on the assessment of embodied and whole life carbon. Although the RIBA, LETI and other bodies have produced guidance set around the RICS modules, there is evidence that some local authorities and councils are tackling this issue in reactive or prescriptive ways. Although well-meaning, these approaches can be ill-informed, and consequently may skew local policies or decisions that conflict with Local Plans or other planning/regeneration policies. Efforts need to be made to ensure that schemes submitted for planning are assessed on ‘like for like’ basis, through an auditable, transparent framework.

Diminishing material resources-and finding alternatives

There is much debate surrounding limited and diminishing supplies of GGBS, which is used as a replacement for carbon-intense Ordinary Portland Cement (OPC) in concrete production. Globally, there is only enough GGBS produced from raw steel production to cover about 15% of the annual OPC replacement demand. So once again we must consider the relevance, or otherwise, of the project specific system boundary.  In the UK, and London in particular, use of GGBS is common and its rate of OPC replacement is often high (well over the 15% global average potential). However, there are discussions around limiting the carbon benefits that can be recognised where more than 15% replacement is used, with the aim of evenly distributing GGBS carbon benefits globally. This is perhaps an honourable and even logical argument, yet at a project scale it does not promote conscious consideration in design and specification as to ever lower carbon solutions. This approach will ultimately make it more difficult to deliver lower targets of, say, 500kgCO2e/m2 A1-A5 in densely developed inner city buildings and increased density are, after all, beneficial sustainable outcomes.  So what can we do?

The Seratech carbon capture and OPC process.
The Seratech carbon capture and OPC process. Credit: AKT II

Conclusion: A collective direction will bring change

Designers need to encourage clients to push for alternative materials and/or technologies that have the raw supply material in abundance; such as bamboo, for instance.  There are also current and emerging alternatives in concrete production, such as calcined clays. These are available as OPC alternatives, are commonly used in Europe yet have barely touched the UK market.  And the evolving carbon capture technology that Seratech has developed and is scaling up uses base materials of olivine and limestone, which are abundant in the UK and globally.

As an industry, we need to be the catalyst to get these technologies working at scale so the built environment can benefit from them. AKT II has been supporting and helping Seratech promote its technology, with the goal of getting UK industry to provide the funding and demand it needs to produce at scale, so that the potential of its system can be realised globally. We live and operate in a global market that responds to demand, so if we create demand for these alternatives then supply will emerge. All it needs is a catalytic kick-start from designers. 

What do we do in the interim?  One answer would be to stop and wait. But this stymies the demand the market needs. The simple answer remains to think more, and still use less.



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