Bjarke Ingels has always cultivated a reputation as a trailblazer – the freewheeling, globetrotting creator of vertical film studios, twisting bridges and rooftop ski slopes. Now, for his own practice’s headquarters in Copenhagen, he has done something less showy but arguably more radical, completing the first building in Denmark made from calcined clay concrete.
Situated at the head of the Sundmolen pier on Copenhagen’s North Harbour, the Bjarke Ingels Group (BIG) HQ is an unambiguously concrete structure. Seven storeys high, it takes the form of a distorted cube, with one side sliced off at an angle as the pier narrows to a point. The frame is comprised of 28 “mega-beams” – enormous sections of exposed concrete, 20m long and 4m high. These are stacked alternately with expanses of full-height glass and meet like interlocking fingers at the corners of the building. On each elevation, two of these solid sections, triangular in shape, protrude from the frame. These support terraces and a 140m-long stairway that winds around the building from the ground to the roof.
The building has been described as “neo- brutalist” and certainly captures the defiant spirit of 1960s modernism (“historically ugly” was the verdict of one local politician). But it’s hardly an outlier in the North Harbour’s post-industrial landscape of repurposed siloes, warehouses and apartment buildings. Concrete was the obvious solution in the harsh marine climate, with the building facing out onto the Oresund strait separating Denmark and Sweden.
“The design intent was for it to be raw and industrial looking,” says Andrea Hektor, director of BIG Engineering, the practice’s structural team, and BIG CPH Sustainability. The original plan was to assemble it from off-the shelf precast concrete elements, which are used extensively throughout Denmark for everything from housing to car parks. “We wanted to take something very standard and then use it in a very non-standard way,” she explains.
However, the sheer scale of the elements made mass production unviable. Instead, the mega- beams were cast in situ, but with a similar build- up to a precast sandwich panel, with a layer of insulation between a loadbearing inner leaf and a thinner outer leaf.
This meant that both interior and exterior surfaces could be left exposed, in keeping with the brutalist aesthetic. The fact that casting on site offered less control over the eventual finish was seen as an added benefit. “If there were blemishes, we were happy to accept that.” The switch to in-situ concrete also gave BIG a greater say over the mix. The designers were anxious to reduce the embodied carbon of the frame, but also wanted to move beyond accepted approaches such as using fly ash or GGBS as supplementary cementitious materials (SCMs). As by-products of industrial processes not used in Denmark, neither were locally available.
In the role of its own client, BIG was in a unique position to explore novel solutions and take on the risk of adopting them, potentially helping to establish a wider market. So, instead, the team specified concrete made with calcined clay as an SCM. In Denmark, calcined clay concrete had been used on civils projects such as bridge piers, but never on a building, let alone a piece of statement architecture.
As a binder, calcined clay has a number of advantages over cement. Clay calcinates at 700-800°C, compared with 1,500-1,600°C for limestone, the main raw ingredient of cement clinker. Because it doesn’t contain carbon, no carbon dioxide process emissions are released during the chemical reaction. As a result, calcined clay generates roughly a quarter of the carbon emissions of limestone-based clinker. In Denmark, it also has the advantage over more common SCMs of being an abundant natural material.
The mix specified for the BIG HQ, marketed in Denmark as Futurecem, replaced 30% of the clinker with calcined clay. BIG says this has reduced the embodied carbon of the concrete by 25%. “We could have reduced the embodied carbon more by replacing cement with fly ash or GGBS,” says Hektor. “But how do you ever push the industry towards greener solutions – solutions which are not reliant on other high-emissions industries – if we don’t try alternatives?”
As it had never been used in a building context, BIG undertook a series of additional tests to verify the Futurecem’s performance. “There were concerns about whether it cured at the same speed as conventional concrete, and it had never been tested for longer-term freeze-thaw effects in this sort of environment,” says Hektor. “Because we were the client and there was the desire to make it more sustainable, the additional time spent on testing was considered an acceptable risk. With another client, it would have been a longer conversation.”
BIG also built a scaled mock- up of one of the mega-beams to get a better understanding of the concrete’s workability and aesthetics. The density of reinforcement needed in the thinner outer leaf required a self compacting mix, resulting in a couple of minor adjustments to the formulation.
These included the addition of a curing agent, which ensured that the curing time was unaffected by the calcined clay. In keeping with the utilitarian spirit, standard steel formwork was used, leaving a smooth finish. The tone of the concrete is slightly browner than usual, which the architects were happy with. Apart from an anti- graffiti sealant, it has been left as struck.
The industrial feel continues inside the building, which aims to recreate an open warehouse-type space across multiple levels. At 4,800m2, it is double the size of BIG’s previous studio, an old industrial hall in the Norrebro neighbourhood with high ceilings and a single workspace. The challenge was to maintain those visual and physical connections in a far smaller floor plan of roughly 40m x 40m and within a building height of 27m. The answer again involved structural innovation.
A single central column rises through the full height of the building. Described by the architects as its “totem pole”, this is made from different varieties of granite and marble, the type of stone changing on each level.
In-situ concrete beams – cast from the calcined clay mix – radiate like 20m-long spokes from this column to the perimeter structure. On top of these sit the floorplates, or more accurately, half-floorplates. Each level is like an oversized mezzanine, occupying just half the available space and cutting across the central column from different angles. This generates a variety of oblique sightlines across the whole building.
The floors are made of off-the-shelf prestressed double T-plate units, typically used for car parks. These were precast from a standard cement- based mix. The T profiles are 600mm deep and span up to 17m. “We spent a lot of time doing vibration analysis to make sure there weren’t any issues with bounciness,” says Hektor.
A black steel sawtooth staircase zigzags through the heart of the space, but the rest of the services, including lifts, toilets and vertical risers, have been pushed to the north end of the building behind a full-height wooden book wall. This is part of a strategy to reduce visual clutter, allowing the views over the harbour to take centre stage. The soffits are unadorned, apart from simple strip lights. The external stair acts as a fire escape and eliminates the need for sprinklers. Acoustic felt, placed between the T profiles, is precisely colour- matched to the concrete.
The building is primarily heated and cooled by geothermal energy. This is provided via a network of 222 precast concrete energy piles, each 14m deep. A heat pump increases or decreases the supply temperature as needed, and any surplus heat is stored in the piles. The system is designed to deliver 132MWh of heating a year – 84% of the building’s needs – with the remaining demand met through district heating. With its open layout and thermally massive concrete structure, the building lends itself to natural cooling, but any additional needs will be covered by the geothermal system. Electricity is also generated by a rooftop PV array.
BIG says the building will be energy-positive in operation, and it has achieved DGNB Gold, the second highest ranking in Denmark’s rating system for ecological, economic and sociocultural sustainability. The development also contributes to the area’s biodiversity, replacing a former parking lot with a 1,500m2 park of native, wind-tolerant shrubs and trees. But its main contribution to sustainability is arguably something less showy. By taking on the risk of testing out a new product, BIG has helped to establish a domestic market for a viable long-term cement replacement.