Ghost in the shell

Amin Taha uses the latest digital 3D modelling and casting techniques to bring a ‘mis-remembered’ London terrace back to life. By Andy Pearson

A Victorian building that has been missing from the end of an otherwise symmetrical brick-fronted Palladian terrace for over 70 years is back – re-imagined in concrete. Almost every detail of the original building has been recreated in its intricately crafted facade: sash windows and ornate lintels, a decorative frieze, the roof-line balustrade and even a slate-covered roof are all present in a terracotta-coloured concrete shell. Inside the level of detailing is, if anything, even more fastidious: here skirting, dado rails, cornices and even Anaglypta wallpaper adorn the facade’s inner leaf.

This extraordinary concrete monument, which fronts London’s Upper Street, is architect Amin Taha’s realisation of the building he imagines would have completed the terrace. “The original building was bombed during WWII and then demolished. There is nobody alive that remembers it, so all we’ve got to go on are photographs and the building on the other end of the terrace,” he says.

To create his memorial to an imagined past Taha has turned to state-of-the-art construction techniques. The starting point was a laser cloud-point survey of the vacant plot at the terrace’s northern end, along with a detailed survey of the existing brick building that the new one would adjoin to ensure a seamless transition between the two. The facade of the missing building’s symmetrical opposite at the southern end of the terrace was also surveyed in detail.

Taha then developed the design as a detailed digital three-dimensional model using information from the laser scans, combined with visual elements gleaned from studying historical photos of the building. External mouldings, facade features and cornice details were also interpreted and digitally modelled based on the survey. “Given the wholly intact nature of the block and good condition of its detailing, our approach has been to remember, or rather to mis-remember the missing piece,” Taha says. “We used educated guesswork, but there is no way that it will be the exact representation of the original building.”

To enable his mis-remembered facade to be constructed, the three-dimensional model was subdivided into a series of digital surface tiles covering the entire facade, both inside and out. Each tile contains all the information needed to enable a detailed impression to be created by robot arms manipulating a router to hollow out a mould from a block of expanded polystyrene. These moulds are then used to line shuttering to cast the facade. More than 300 individual polystyrene moulds were needed to line 450m2 of formwork.

The routing system was trialled in the Bartlett School of Architecture’s workshops. Mostly it worked well. However, in some places a glitch in the translation of the digital model into the digital manufacturing coordinates resulted in some details becoming distorted. Rather than attempt to resolve the issue, Taha has, instead, exploited the technology’s shortcomings: “Areas likely to fail were encouraged to do so,” he explains. “As a result some facade details are skewed or lost altogether, which helps to reinforce the notion of mis-rememberance.” To perfect the terracotta-like appearance of the concrete, the architect worked closely with concrete supplier Hanson. The colour was created using pigments added to the mix, and impressively, it incorporates subtle variations to give an aged appearance to add to the illusion that the facade has itself stood the test of time.

Because the facade is load-bearing, steel reinforcement was placed between the shutters, and sheets of rigid thermal insulation were inserted to create a thermally broken double-skinned wall. The facade was cast in 1.2m-high horizontal bands; the walls are up to 0.5m thick in places depending on the depth of detail. A self-compacting additive was used to ensure that the concrete filled the mould – a challenge as the formwork made it impossible to use a vibrating probe effectively in places.

Once the concrete had cured, the formwork was removed and the concrete was soda-blasted to remove any trace of the formwork and to add a texture. “We worked with the contractor on site who had to gently grit-blast to partially reveal the aggregate before sealing the concrete,” says Taha. This clear sealant penetrates the surface to create a water-resistant but breathable finish. The completed concrete shell is designed to stand as an independent structure. “In the same way we mis-remember the past, the concrete tells the story of not being quite exact,” says Taha. “All of the errors such as form slippage, grout loss, discolouration are all part of the fuzzy nature of our memory, like an old pair of jeans that reflect their life with wear and tear. The imperfections in the detailing remind us that memories are often adjusted and imperfect.”

On the ground floor, the facade encloses a shop for furniture retailer Aria, who was also the client for this £3.5m project. On the upper floors, the shell supports five floors of apartments. It is in the apartments’ windows where the juxtaposition between Taha’s imagined facade and the demands of a modern development are most apparent. The apartments’ floorplates pay no respect to the floor levels of the past. As a consequence the windows cut into the concrete monument where needed, without heed to the facade’s neoclassical detailing, so that they appear as random openings.

Ventilation to the apartments is provided by small concrete panels built into the facade, which pivot open. The glazed apartment windows do not open, however – just as the concrete windows of the imagined building will always remain closed.
Architect Amin Taha Architects
Structural engineer Webb Yates
Contractor Talina Building

‚ÄčImages: Timothy Soar