It may sound strange, but concrete in a floor slab is a workshy material – all the the middle bit does is keep the bits at the surface apart. So why not replace it with something lighter? Thomas Lane reports on the Danish invention that does just that
BubbleDeck is a load of balls that have just been lobbed on to an unsuspecting UK construction products market. More specifically, they are air-filled plastic balls sandwiched between two layers of reinforcing mesh, and they are the basis of a structural flooring system.

The advertised advantage of BubbleDeck is that it will eliminate some of the problems associated with flat-slab floors. Flat slab is itself a space-efficient way of creating floors because it does not need supporting beams, which leaves more room for services and the people who use them. But there is a downside: flat slab has a weight problem. In fact, it is so heavy that it has a significant effect on the economic performance of the building that it is used in. This is because the greater the mass of a building, the more revenue-generating space is taken up by its structure, and the more concrete and steel is needed for its foundations.

The makers of BubbleDeck – there are a number of manufacturers throughout northern Europe – are hoping that their product will be able to exploit this weakness of solid concrete slabs.

They claim that it is as stiff as solid concrete but is only two-thirds of the weight. And because the floor has less of its own weight to support, it can be used in either longer or thinner spans than conventional flat slabs. The thinner option was taken by the designers of the Millennium Tower in Rotterdam, which allowed them to add two extra floors to the 34-storey building – as well as cutting the cost and carbon dioxide associated with 500 lorryloads of concrete.

The system helps designers in another way as well. Conventionally reinforced slabs have to be carefully designed to accept service voids, so once the slab is cast, the location of those voids is fixed. With BubbleDeck, however, holes for services can be punched through where needed after installation.

The idea behind BubbleDeck is simple enough. Plastic balls are trapped in concrete in the middle of the slab. This means that they displace large volumes of concrete, but have little effect on the slabs' structural strength. There are no fewer than three ways of installing the system. First, it can be fitted in situ with the reinforcing mesh and balls being installed on site. This would mean that conventional shuttering would be needed to contain the concrete when it is poured.

Second, a fully precast slab can be installed. BubbleDeck says the advantage of this method is that the finish of the concrete surfaces is good enough to be ready for painting. This would be ideal for buildings in which the surface of the concrete is left exposed to increase the thermal mass of the structure and help to regulate the temperature.

A halfway house solution is semi-insitu installation. This method is ideal for fast-track projects. The lower half of the slab, which contains the reinforcement, is precast and the balls and the upper layer of reinforcement sit on top (see picture on page 55). The units can be craned in as lightweight modules, and their size is limited only by the transportation method. Once they are in position, the modules are tied together and concrete is poured to finish the floor. This is similar to the fast-track Slimflor system, where corrugated steel supported on steel beams forms a permanent shutter for the concrete. BubbleDeck has the edge here, though, as no space-hogging beams are required to support the floor, and it can be used in concrete-framed as well as steel-framed buildings.

Although the Romans were the first people to use clay pots to create voids in concrete, BubbleDeck is of more recent provenance.

It was invented in Denmark by civil engineer Jörgen Breuning. The product was first tried on the Continent in 1998 and is currently being used in Germany, Holland, Switzerland and Italy. John Massey, a director of BubbleDeck UK, says the system will be licensed to concrete suppliers here as it can be produced easily from existing plant.

Structural engineers are enthusiastic about the BubbleDeck system. "I think it's great; it's a very simple idea," says Arran Chadwick, a director of structural engineer Atelier One. "It's efficient when compared with ordinary systems, which has got to be a good thing. I couldn't see anything wrong with it, and I would like to use it somewhere."

Chadwick reckons, however, that nothing can compete with post-tensioned precast slabs, which are very cheap and ideally suited to rectangular buildings. However he does think that BubbleDeck's flexibility would make it a frontrunner for a non-orthogonal building, such as one with a kidney-shaped footprint.

David Dexter, director of structural engineer David Dexter Associates, is positive about the possibilities of BubbleDeck: "It's a really good idea," he says. "Anything that reduces the weight of the concrete slab is a good thing." Dexter differs from Chadwick as he thinks the system is ideal for commercial buildings with a rectangular structural grid. Either way, it looks as if it will not be long before the first BubbleDeck systems are being installed in the UK.

Why you don’t need the bit in the middle

BubbleDeck can do away with the concrete in the middle of a slab because that bit of the structure doesn’t really do anything useful. This is because the stresses that affect a floor are concentrated at its top and bottom edges. When a load is applied to the floor, it bows downwards. This squeezes the upper part of the slab and stretches the lower part – or, more technically, it puts the top into compression and the bottom into tension. In the middle, where the forces fade into each other there is no stress. The design takes this into account by using thick steel reinforcement at the base of the slab to resists the tensile forces –steel performs well in tension – and a layer of concrete – strong in compression – on the upper edges. All the middle of the slab does is to keep the two stressed zones apart – the further they are from each other, the stiffer the slab becomes. This can be seen to good effect in a steel I-beam, in which the primary function of the vertical part is to separate the flanges.

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