Making buildings: The concrete slab with a third less CO2

House Made By Many Hands, by architect Cairn, is a Victorian renovation and extension in Hackney, London. As part of its low-impact design approach, it has pioneered the use of limestone calcined clay cement concrete – LC3 – which generates 30-40% less CO² in production relative to standard Portland cement. RIBAJ spoke to Cairn director Kieran Hawkins and engineer Peter Laidler, director of Structure Workshop, about the specific product and the home’s bigger idea.

What’s the background to the project?

Kieran Hawkins: This was a dream client in that it was clear from the outset that they wanted a low-carbon build; they put a call-out on social media for a Hackney-based architect to help them with it and selected us. They were conflicted about an extension at all as they were environmentally conscious and aware that any construction work went against any low-impact ethos.

At 77m², it’s the smallest project we’ve done. The ground floor was lowered partly to get better head heights at the rear – but crucially it was needed for planning. To ensure adequate daylight for the neighbour adjacent to the side return, we needed a low roof line, which meant dropping the floor. The rear extension ended up being 390mm lower than the original ground level.

After doing trial pits we discovered that the house sat on over 2m of unstable made ground which ruled out any idea for screw piles and a raised timber floor. Once we knew we’d be using a concrete slab, that’s when Structure Workshop suggested using LC3 concrete. This was the first project we’d worked on where the groundworks strategy was the driver from the start. The learning process for us was in understanding this in the context of the construction’s overall carbon emissions.

How did you come to decide on the use of LC3 concrete?

Peter Laidler: Structure Workshop has been trying to drive down carbon use in our projects. Two of our staff had been looking into supplementary cementitious materials (SCMs) like GGBS but the problem is that 95% of it has been used and so has no further potential to reduce global emissions. Calcining limestone into clinker needs temperatures of 1400°C and releases CO₂ in the chemical reaction. Calcining clay occurs at 700°C, doesn’t release CO₂, and clay is a globally abundant material. Portland cement accounts for 8% of global emissions; replacing it with LC3 could reduce that to 5% overnight.

As far as we’re aware, its use here is the first time LC3 has been employed in a construction project in this country. We used Futurcem LC3, by Italian producer Cementir, and brought it from Denmark. It’s a mix of limestone, 15% clinker and calcine clay. Here, Aggregate Industries now appears to be making a similar low-carbon mix called ECOPact Terra.

You used timber beams instead of steel to support the side return walls. How easy was this to achieve in practice?

KH: The issue was really the headroom we needed to maintain. Using softwood would have meant that we either needed bigger section sizes or more columns in an already constrained area, so we decided on an FSC-certified hardwood, which has a sacrificial charring layer in the event of fire. Where things became tricky was in its certification as a loadbearing structure, as the beams’ sapele timber doesn’t have standard UK strength grading. There’s only a small number of graded timbers in the UK so if you are not using one of those then you need independent certification for use on your project. It meant TRADA coming to site and writing a report on the specific piece of timber – which the contractor had already bought – for sign off. Building Control had Structure Workshop’s calculations based on a specified timber grade of timber but it in turn needed TRADA’s report to sign off the whole installation. The longest span is 3.5m and the beams and flitch plate details are lovely.

PL: Hardwood was both a structural and aesthetic choice. Here, it was specified as D30 (under BS5756), defining its hardwood nature and the flexural strength, making it stronger as well as stiffer than softwood and glulam – and  more beautiful! Most engineers associate the D30 classification with oak, but as oak takes about a year an inch to season, it may have been difficult to source a piece this size here, hence the eventual use of uncertified sapele.

Tell us a little about how you formed the hempcrete walls?

KH: One strategy for reducing carbon was to take away lining layers, so no boxing out of beams – in fact, no plasterboard was used at all. All the structure was expressed and the insulation was the finish. Here we used a softwood frame against the party wall and hand-applied hempcrete over the course of a day – even the client mucked-in! We ran ply shuttering up to the frame and rammed hempcrete in the void. After a couple of hours we’d it pull away, move the shuttering up and ram in another layer to give a strata-like finish. The Lithuanian builders had never used it but they loved it. One worker said it was the best site he’d worked on as there was no steel grinding or insulation cutting with dust everywhere. It was just timber and hemp, the site was quiet and relaxed and air quality was lovely. 

How did you get way with having no doors on the ground floor?

KH: With a fire suppression mister system – it was a necessary evil to achieve the openness. Without that we’d have needed to have compartmentation and the client was happy to spend £7,000 of the budget on that and not on fire-rated joinery or fripperies like expensive kitchen appliances. They sourced most of the furniture, fittings and equipment for the project second-hand from eBay.

What was the biggest challenge in the process?

PL: From an engineering point of view, what I liked was the willingness to tweak the design to allow it to work in harmony with the building. People tend to have idealised views about what they want but that didn’t happen here; if we want to drive down carbon use, we have to challenge those idealised expectations.

KH: The project is so much richer as a result because there is a narrative in everything that you see. It had four or five low carbon strategies embedded in it and we employed all of those, but if we could embed just a couple in any domestic retrofit project, that would be a big win.