Using concrete strength sensors on Allies and Morrison’s King’s Cross apartment building enabled contractor Laing O’Rourke to increase the proportion of GGBS and cut 34 tonnes of embodied carbon
Allies and Morrison’s Capella is one of the last buildings to complete in the 20-year King's Cross regeneration programme. Facing onto Lewis Cubitt Park, the 14-storey tower contains 120 market and 56 socially rented flats in a range of studios, duplexes and split-level units. On the 12th floor, five two-storey homes with private rooftop gardens animate the skyline.
The most striking concrete elements of the building are the wavy, pale green precast panels, which form part of a simple, light-toned palette alongside pale brick, green-framed glazing and white precast balconies. But behind the facade, the concrete frame also has an interesting story to tell – not least how the use of heat sensors during the curing process enabled the contractor to accelerate striking times and increase the proportion of GGBS.
A concrete frame was specified for durability and acoustic reasons, to offer privacy to the 176 individual households and shield them from the noise of surrounding trainlines. Working with structural engineer Ramboll and main contractor Laing O’Rourke from early in the design process, Allies and Morrison explored different framing solutions, balancing embodied carbon and cost, as well as the buildability and logistics issues of delivering the final parts of a masterplan. “It was a very, very tight site and, being the last piece in the puzzle, there was not much space to store material,” says Arpad Toth, project architect at Allies and Morrison.
Although this may have tipped the scales in favour of design for manufacture and assembly (DfMA), other variables supported an in-situ approach. “It was a really interesting exercise,” says Toth. “The structural engineers did a brilliant job creating a comparison document that assessed the impacts in terms of slab thickness, spans and every other factor you can think of. We looked at post-tensioned systems, hollowcore planks, delta beams. This fed into the process in a very useful way. From design stage two, we were able to plan ahead but also keep some flexibility in the programme in case a better solution presented itself later.”
In the end, the team settled on a hybrid, with in-situ flat slabs up to level one and on the top floor, and precast lattice slabs with an in-situ topping on the intermediate levels. “With lattice slabs, the formwork is built in, so it’s a very efficient way to build. It saves a lot of time in terms of building formwork, it reduces material wastage, and you need less space for storage.” The frame was completed with 622 precast columns and 414 twin walls, all supplied by Laing O’Rourke’s DfMA arm, Explore Manufacturing.
The busy and congested location continued to influence the construction, with tight and strictly monitored deadlines. To improve efficiency on site, SmartRock sensors from concrete supplier Heidelberg Materials were attached to the reinforcement of the in-situ concrete elements, to generate real-time data on the curing process. Assessing in-situ strengths, as opposed to testing cubes poured on site at the same time, accelerated the striking time and shutter turnaround, so the construction team could move on to the next stage more quickly.
The data also enabled a change to the concrete mix design so that a greater proportion of the cement was replaced with GGBS. This lowered the embodied carbon content of the concrete by 10kg/m3, saving more than 34 tonnes of CO2e.
The finished in-situ concrete has been left exposed in the circulation cores, as well as feature stairs and some common areas. Phenolic-faced ply formwork was used, leaving a smooth surface. “We were very mindful of superfluous finishes,” says Toth’s colleague Angie Jim Osman, a partner at Allies and Morrison. “We wanted to make sure that the concrete could actually be the end product. So in some areas we decided to just paint signage on.”
The structure stacks vertically but has a complex arrangement of layouts, with more than 10 basic apartment types, and over 30 variations in all. This is reflected in the distinctive building form, with deep setbacks for terraces and roof gardens and a staggered elevation on the western side. The massing has also generated a high proportion of dual-aspect homes, allowing generous light levels into the building and views out to the green space in front. “Particularly for the social rented apartments, six out of seven on every floor are dual aspect, which is an amazing result,” says Toth.
The articulated, open facades and different material treatments also help to diffuse the backdrop to the park. The curtain-like precast panels play an important role here, reflecting the greenery and lending a softness and sense of movement to the building exterior. Breaking up the expanses of brickwork was also a material-efficient solution, says Osman. “We used hand-formed bricks laid on precast panels, so we had two masonry materials back-to-back. We thought, why not just use the concrete as a facade material in some places to limit the doubling up?”
The architects worked closely with the precast manufacturer to develop the corrugated pattern, which was cast using a bespoke flexible form liner. The colour, which comes from green-grey stone aggregate as well as pigment, also required a number of test samples. “The manufacturer created a visual mock-up of a panel, which was incredibly useful in terms of understanding the finish, how it reads at different distances, how the joints work, and how it transitions to other surfaces such as brickwork. We tried to include as many tricky details as we could get in,” says Toth.
Allies and Morrison has been central to the massive regeneration of King’s Cross since 2000, when developer Argent first took on the challenge of reimagining the land around north London’s grand Victorian rail terminals. As well as masterplanning the 67-acre area with Porphyrios Associates, it has designed several buildings, including the offices at Two Pancras Square and the refurbishment of the German Gymnasium into a restaurant. “It's so rewarding to actually have a masterplan built in our lifetime,” says Osman. “Often these things take so long. For some of us to be able to work on the masterplan, then design and deliver buildings, it's been quite remarkable.” With the completion of the Capella building, Allies and Morrison is now close to signing off. Its facade of curtain-like precast concrete seems a fitting final act before leaving the stage.