Concrete and climate-compatible design

vaulted roof slab project under construction it s a clever design but it s not that ambitious compared to some of the great shell structures by felix candela for example2

If the economics of your project make concrete irresistible, there are still ways to minimise its carbon impact. Steve Webb and Liam Bryant explains how.

So, you’ve pushed and pushed (as has the engineer, hopefully) to develop a low-carbon scheme, but the concrete frame industrial complex approach won and your timber and hempcrete has been cast aside in its favour.

Unfortunately, this is the story of too many projects, with logistics and cost uncertainty around ‘green’ construction creating barriers that only an experienced team and an ambitious client can overcome.

So, what happens next? 

Too often the design spirals deeper into the mire, with climate aspirations sacrificed on the altar of economic efficiency, and we end up with uninspired designs that we’ve produced for decades, with high embodied carbon and limited innovation. But don’t lose hope, there’s plenty we can do to salvage the design and perhaps find concrete a place in ‘green’ design.

Re-use
It is now widely accepted that re-using existing assets is the best thing an architect or engineer can do for the planet.  So when starting a scheme, we should look to re-use elements on site, from full retention of the existing building, re-use of foundations or simply recycling materials and finishes. This is predicated on specifying suitable surveys and testing, to understand the opportunities, then working with the existing fabric, rather than against it.

Often minor tweaks to the proposals,­ ­either in footprint or sequencing, can hugely simplify the works, reducing demolition and new structure by retaining key elements. This is when good communication is critical, as each discipline brings its own knowledge, and open discussion and challenging ‘negative’ decisions are essential.

Once you’ve decided what to demolish, try to find opportunities to re-use the ‘waste’ material. While re-use isn’t carbon negative, it generates significantly less impact than using new materials. The engineer should be looking for record drawings and specifying testing early to ensure that this opportunity isn’t missed.

Reduce
Once the scope is finalised, recover some of that initial lost ground through the design, with clever framing, and ensuring that the architecture responds to the structure, rather than forcing the engineering to accommodate complex and inefficient demands.

At its simplest, moving from a 6m grid to an 8m grid will increase the slab thickness from around 225mm to 275-300mm, a 30% rise in concrete. Similar problems arise from setbacks and transfers.

These decisions cascade down the building, with columns, beams and foundations all working harder to support an overweight slab, further raising embodied carbon and cost.

This doesn’t force us to sit in a bleak block, with a grid of columns marching into the distance. Designers should embrace the benefits of concrete; it can be cast in any shape and is great in compression. We only need to look to classical architecture to appreciate curves. 

The industry is obsessed with flat slabs, mainly due to the high cost of skilled labour, but this must change if we are to ‘solve’ the climate crisis. One Webb Yates project has a 200mm thick vaulted slab spanning more than 10m, a low carbon and efficient way to span the space. 

Mitigate 
Finally, once design opportunities have been exhausted, we should be specifying low-carbon concrete mixes that have high ­proportions of cement replacement and ­r­ecycled aggregates. 

While a critical part of sustainable design, too often this ‘greenwashes’ a carbon intensive design, skipping the steps above.

Cement replacements (GGBC/PFA etc) aren’t truly carbon neutral, if you’re buying it, it’s not ‘waste’, but they can significantly reduce the embodied carbon of structures. This can offer eCO2 savings of ~40% compared to ‘baseline’ concrete, but realistically the saving compared to ‘industry standard’ is probably half that.

A froth of companies are pitching ‘new’ products as CEMfree or Carbicrete that promise low/negative embodied carbon, but these are typically based on the same GGBS framework. Unfortunately, this isn’t a long-term fix as supply is limited, and the UK is already reliant on imports. Studies suggest that there is only enough GGBS/PFA to replace up to 20% of world cement demand and production of this ‘waste’ is dropping.

To address the climate crisis, we must embrace the opportunities of Reduce and Re-use, bringing creativity, innovation and good design to ‘green’ concrete, not jumping to the easy promise of ‘carbon-negative concrete’.

This article was originally published on The RIBA Journal