Whirl pool

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Over the shoulders of Sue Barker and Gary Lineker, a number of intriguing structures competed for our attention this Olympic summer. There was the tangled height of the Orbit tower; the cupcake of the Olympic stadium. But perhaps most eye-catching of all was the incredible Zaha Hadid-designed aquatics centre. With its swirling roof flanked by enormous stands of temporary seating, it looked like a vast stingray gliding among the teeming thousands of visitors to the Olympic park.

It remains one of the architectural stars of the show, and is also an impressive demonstration of the sheer versatility of concrete, and its ability to perform across a surprisingly wide variety of applications. Beneath the centre, for example, are immensely strong reinforced concrete transfer structures that carry the weight of the building across deeply submerged power lines. Above ground, the giant, swirling, exposed concrete walls perfectly complement the astonishing curves of roof. And, right at the centre of the action, the graceful lines of the diving boards – as smooth as a Tom Daley plunge – show off concrete’s capacity to create sculptural shapes to the finest of tolerances.

The grace of the finished product belies little of the monumental task that faced contractor Balfour Beatty back in 2008. To make way for the aquatics centre, 11 industrial buildings had been demolished, four prehistoric skeletons removed and 160,000 tonnes of polluted soil dug out. Providing a stable base for the building was never going to be easy but, as project director Stuart Fraser explains, it was the buried power lines that posed the trickiest problem. “Obviously we had to avoid any loading on the power tunnels,” he says. “The solution involved an extensive system of piles, pile caps and transfer structures to support the weight of the building.”

This involved sinking some 1,800 continuous flight auger piles around the site and “bridging” over the tunnels with the two transfer structures – effectively enormous reinforced slabs 3m thick. The volume of the larger one was 1,832m³.

Piling started in August 2008, and in April 2009 the transfer structures were created during the largest concrete pours undertaken anywhere on the Olympic park. These provided a secure base for the building’s north-east and north-west cores, each of which would act as a support for the centre’s famous steel and timber roof.

“The roof is supported at only three points – from the two cores and a supporting wall at the southern end,” says Fraser. “No transfer structure was needed for this, because, to the south, the power tunnels diverge, leaving enough room in between for the piling and pile caps to support the wall directly. The north-west core is an oval shape containing lift shafts and staircases, and so it had to be high quality in appearance. This was our first opportunity to get the kind of finish we were looking for from the concrete.”

This was not as straightforward as it might seem, as building for the Olympics required very high environmental targets to be met. A variety of concrete mixes were used, but if all the Olympics sustainability promises were to be kept, overall a very high proportion of recycled aggregate and cement replacement was needed. In the end almost 100% of all aggregate used was recycled and the use of ground granulated blast-furnace slag (GGBS) reduced cement requirements by 50%.

Balfour Beatty worked closely with concrete contractor Morrisroe to arrive at a workable mix that both met environmental requirements and provided a top-class finish. “We experimented a lot before we got it just right,” says Fraser. In the end, the team settled on a mix with 40% GGBS replacement cement content and 76% china clay stent secondary aggregate from Cornwall, which went on to be used for most of the visual concrete areas.

Once the cores were complete, Morrisroe turned its attention to creating the training pool situated behind the northern cores and beneath the land bridge that carries pedestrians from the nearby railway to the venue’s entrance. The merging of pool and bridge meant a new set of considerations as far as the concrete was concerned. Sara Klomps, project architect at Zaha Hadid Architects (ZHA), explains: “While the centre itself has a 60-year design life, the training pool forms part of a bridge structure and must have a 120-year design life.”

This, she says, involved more reinforcement, and a special high-strength visual concrete mix for the training pool walls and ceiling, the welcome zone ceiling and the roof supports. The concrete mix here was based on a cement replacement of 60% GGBS with 10% stent secondary aggregate.

The most striking feature of the training pool is its beautiful coffered ceiling. This was poured in sections as a 1.2m-deep slab but using special formers to create the attractive petal-shaped lighting troughs. These both dominate the design and decrease the ceiling weight. Fraser adds: “The roof spans over 25m, but its depth, together with the use of high-strength concrete and 40-50mm high-tensile bar reinforcement makes it immensely strong and self-supporting.”

Finally, and with the roof structure complete, the competition pools themselves could be built. As you might expect, a key requirement was that they should not leak. According to Tony Henry, contract manager with Morrisroe, absolute care was taken to achieve this: “We started pouring the pools in 5m x 5m bays – small pours to reduce any shrinkage and cracking. This proved too slow however, and we were able to speed up considerably by switching to 10m x 10m pours and using a resin-injected grout to seal any small cracks that did occur.”

Ever careful, Balfour Beatty filled the pools early to test for leaks, and only when the water stayed put was the go-ahead received to drain the pools – and to affix the first of the 850,000 tiles.

The finished product has never been short of admirers. Culture secretary Jeremy Hunt described it as “a moment of architectural genius” and International Olympic Committee president Jacques Rogge thinks it “a masterpiece”.

 Now the Games are over, at some point soon the huge temporary stands will be removed. The spaces they leave will be filled with glazed facades and, with its dimensions less strained and natural light flooding in, the true beauty of the centre will finally be revealed.

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