The pattern makers

Before we get stuck into the nitty gritty, a quick English lesson. Concrete is a word taken from the Latin concretus, meaning ‘hardened’ or ‘hard’. It’s a tough, course material, used to build robust structures. Lace, meanwhile, is a fabric of fine threads of linen, silk or cotton, a delicate tissue used to make dresses and handkerchiefs. Combine these two materials and what do you get? Well, you could end up with one of the most uncomfortable dresses in history. Or you could end up with a tough but delicate external cladding material just like that covering Nottingham’s new contemporary arts centre building.

Located in the ancient heart of the city, Nottingham Contemporary features more than 1,100m2 of green, double-scalloped, prefabricated concrete panels, most of which are ‘etched’ with an intricate historic lace pattern. Nottingham’s industrial heritage is founded on lace, it pioneered the lace making machines that took production from cottages to factories in the 19th century and its Lace Market became the hub of the lace industry – hence architect Caruso St John’s decision to replicate it on the exterior.

But for cladding contractor Trent Concrete, transferring the ornate pattern onto the face of the panels proved a tricky procedure requiring an accurate moulding technique and careful installation to prevent damage to the fragile design.

‘Replicating the intricacy of the lace pattern in the concrete was quite a headache,’ says David Walker, managing director of Trent Concrete, which manufactured and installed the cladding. ‘We have used rubber mat moulds before, but not with a design this detailed.’

When it is completed this September Nottingham Contemporary will be both impressive and controversial. Caruso St John’s design was influenced by the New York and Berlin warehouses used by artists as flexible ‘found’ space and as such it has very few windows. ‘Concrete box’ accusations from the public were levelled at the council when the design was unveiled and to some replicating the historic lace on the cladding might seem a bit arbitrary.

The building also takes its inspiration from the late Victorian industrial buildings of the nearby Lace Market. The total 3,000m2 floor area comprises four galleries, education rooms, a study and resource centre, a cafe-bar and a shop, while a two-storey performance space for staging events rises from the centre of the building.

Nottingham Contemporary stands on a sloping site on what was once the centre of the old Saxon city: its market place, town hall and fort. Before building work began, the main railway line into Nottingham Victoria station crossed the site through a cutting, and sandstone caves – used during the industrial revolution to house lace factory workers unable to get accommodation – were present beneath the castle and around the site.

‘After an archaeological investigation the groundworks team filled in the railway cutting to give us a flat platform to work from,’ says Paul Goodwin, project manager for main contractor Sol Construction. ‘From there we put up the in situ concrete frame, which rises to street level. On top of that sits a steel frame. Inside the building is fairly conventional – metal stud partition walls, plasterboard and timber floors – but the felt roof’s impressive with 133 roof lights and then there’s the signature precast cladding.’

The lace design imprinted on the cladding is an exact copy of a cherry blossom pattern found in a small book of samples uncovered during construction of a nearby Marks & Spencer. The book was discovered preserved in a time capsule buried under the foundation stone of the Nottingham Corporation Water Works offices by an unknown Victorian in 1847.

The cladding’s journey to site began at Derby University where the pattern was first laser cut into a piece of wood. The wood was then sent to Germany to be transferred onto a durable rubber mat by mould supplier Reckli. Back at Trent’s Nottingham factory, a special mix of green self-compacting concrete was poured against the mat, left to set, then de-moulded the following day.

In total, 93 scalloped panels ranging from 4m to 11m tall and all 2.4m wide have been cast on this project. ‘We used self-compacting concrete because it flows easily, gets into all the nooks and crannies of the mould and doesn’t need vibration to ensure the fine detail is achieved,’ says Walker. ‘After setting, the mould is rolled from the face of concrete from either end like a carpet.’

We have used rubber mat moulds before, but not with a design this detailed

David Walker, Trent Concrete

Green pigment was used to colour the concrete, and dilute hydrochloric acid was applied to the exterior surface after de-moulding to create a stone-like texture. Bespoke grey plinth panels, designed for installation along a slope running down the side of the building and at its base, were also manufactured, but using a different aggregate, sand and cement mix. These were also polished to give a marble effect.

Installing the panels also brought challenges. The site is confined, with little space for materials storage, so Trent had to use Sol’s lone tower crane – and during the final stages a mobile crane – to lift panels directly from its trucks onto the building.

The lifting capacity of the cranes dictated the weight of panels Trent was able to manufacture: in this case, the heaviest weighed 11.5 tonnes. The vertical height of the panels was also limited by driving height restrictions and could not exceed 4.1m, which meant the 11m-tall panels had to be laid on their side on the back of the trailer.

‘Just-in-time deliveries meant we could avoid congesting the roads by lifting the cladding straight off the back of the trailer and on to the building,’ says Walker. ‘We didn’t have to close off any roads, apart from when we took out the tower crane, and because the panels were manufactured offsite it speeded up the programme and maximised available space on site.’

Trent developed special transport, handling and installation techniques on this job to ensure the lace patterns were not damaged. Purpose-built metal frames were used to carefully turn completed panels onto their edges for safe transport. And once delivered on site a ‘shoe’ made out of a steel frame was used to turn panels 90 degrees before they were lifted vertically via chains on hooks attached to sockets in the back of the panels and lowered onto the steel frame for fixing.

Lifting the longer panels, which came to site on their edges, onto the frame was much more tricky and meant using a complicated system of chain blocks and pulleys to rotate the panels mid-air into a vertical attitude before they could be lowered by crane onto the frame.

Each cladding panel is fixed to the frame via a system of two supports and four restraints. Two corbels protruding from the rear of each panel sit on the steel beam to provide support, while sockets in the back of the panel are bolted to the steel frame to secure it. ‘Concrete’s usually weak in tension and strong in compression so corbels are often positioned at the bottom of the panel, which sits on the steel frame, to keep the concrete in compression,’ says Walker. ‘But here there was no room for bottom corbels so panels are supported on the top beam instead. The restraints ensure there is no chance of cracking,’ he adds.

The polished concrete plinth panels rest on corbels at the base, however. While CM was on site Walker noticed a problem with the installation of one plinth panel. A discrepancy between the alignment of the frame and the panel meant one of its corbels had nothing to rest on for support. A remedial steel fixing plate had been bolted to the panel to compensate for the poor detailing.

‘You see this sort of thing more on other jobs,’ says Walker. ‘Often we get involved in a project after the steel frame has been procured and fabricated, so frequently we come to site and say: “Hang on, there aren’t any plates, slots and angles,” so a lot of money has to be spent on remedial fixings. But at Nottingham Contemporary we sat down with the steel contractor at the start of the project to work on the detailing.’

From trailer to frame each panel takes four Trent operatives just 40 minutes to an hour to install. When CM was on site the outer skin was almost complete with just the gold metallic strips running in the recesses between panels to be fitted. ‘We’re really happy the cladding wasn’t damaged,’ says Goodwin. ‘The absence of windows definitely helped because window fixers tend to climb all over the cladding during installation.’

As for Walker, he’s comfortable with his new role as both subcontractor and tailor: ‘After all, cladding a building is just like buying a suit,’ he says. ‘The person buying the suit [the architect] mixes and matches materials to get the look they’re after and it’s our job to create the fabric and stitch it together on the building.’