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Pay a visit to Block F at the Alexandra Gate housing development in Newport, south Wales, and you could be forgiven for thinking you’ve been transported to the surface of an alien planet in an old episode of Star Trek. Looking out across the almost-completed first floor you are confronted by a sea of plastic bubbles, which glisten in the morning sun and baffle the eye with their beguiling colours.

What you’re actually looking at is BubbleDeck, a lightweight hollow concrete slab system that is being trialled on the project by housebuilder Redrow. Designed to match the performance of a traditional in situ concrete solid slab, BubbleDeck purports to simultaneously halve its weight, potentially offering numerous design and cost benefits.

Already tried and tested on several projects elsewhere in the EU, the product is a relatively new concept in the UK, but one that’s gaining acceptance (one of the Construction Manager of the Year winners used it on a teaching block in Newcastle see . At Alexandra Gate it’s being trialled on several luxury apartment blocks which form part of a large new housing development to include cheaper housing for keyworkers and first-time buyers.

‘We’re weighing up the pros and cons of BubbleDeck to determine whether we expand its use or revert to a more traditional method,’ says Redrow’s site manager Derek Bates. The project’s structural engineer, Betts Associates, specified the system mainly for the promised speed of construction.

Each semi-precast BubbleDeck element comprises a sandwich of permanent formwork, including bottom mesh reinforcement, a centre filled with plastic air-filled bubbles and top mesh reinforcement. ‘The bubbles eliminate the need for concrete in areas of floor with no carrying effect,’ says BubbleDeck MD Paul Harding. ‘A 280mm-deep BubbleDeck is equivalent in strength to a 340mm in situ deck, whilst reducing deflection and enabling larger spans.

The system’s top mesh reinforcement is prefabricated in the element, holding the bubbles in position and providing a safe platform for operatives to work on as they install loose site reinforcement.’

Construction of each 400m2 floor at Block F is carried out in five stages. First, temporary propping is placed under parallel beams at 1.8m centres. Next the BubbleDeck elements, most measuring 7.8m x 2.4m, are lifted into position by crane. Then loose site reinforcement is installed around columns, core walls and tolerance joints. Perimeter shuttering is then installed before concrete is poured into the slab, with temporary supports left in place for three to four days.

Each separate BubbleDeck element is being installed in just four hours, as opposed to a traditional solid panel, which might take two and a half to three days. It takes about a week to install a complete floor and another week to finish it off with columns and core walls ready for the next floor to be delivered.

When CM visited the site, the first floor was in place and operatives from concrete frame contractor Michael J Gallagher were finishing the steel fixing of loose bars, ready for pouring to begin.

‘The whole idea is to eliminate the need for formwork and steel fixings, which are already cast into the unit,’ says MJ Gallagher’s senior contracts manager, Bob Miles. ‘It pretty much does what it claims to do.’

MJ Gallagher’s site manager, Darren Archer, says a big advantage is that the system removes the need for extensive falsework too, with a simple skeleton required to support the slab elements. ‘Once the slab has set you just drop the falsework down and that’s it,’ he says. ‘It’s also quicker and cleaner as the BubbleDeck we’re using has a precast concrete soffit, so you haven’t got to remove sheets of ply and it doesn’t require the finishing or face work associated with exposed ceilings it has a really good finish.’

Miles has noticed that less labour is required to complete the job. ‘If we were doing this traditionally we’d probably be turning it out in the same time, but with a lot more resources. We’re only using three men as opposed to five or six,’ he says. ‘We do quite a lot of work for housebuilders. Residential house frames tend to have lots of columns to ensure clear rooms you don’t want a column in the middle of room this product enables longer spans and less columns. It has definite advantages.’

These advantages include reducing the building’s overall weight, which could mean less foundations. On this job three columns were removed per floor, which meant nine piles, each costing perhaps £1,500-£2,000, could be removed from the foundation design.

Unlike a typical solid concrete slab installation, concrete is poured in one go – about 40m3 per floor. ‘On most jobs you pour in one bit and then get working on the next bit, it’s more cyclic,’ says Bates. ‘Here the cycle is one floor. Everything’s prepared, then we pour it in about two hours. When you don’t have much space, as on this site, you can also unload the elements straight from a lorry onto the building, which gives operatives a walking deck straight away.’

The small site does pose problems, however, Bates admits: ‘This type of site maybe doesn’t lend itself as economically as it would if you had a large rectangular building where you could lay vast areas of BubbleDeck quickly. On a large area I can see great advantages in it. You could get all the slabs installed in basically in half a day or so, then there’s just the steel fixing to attach.’

Fixing can be a chore too, says Bates. ‘It’s been quite intensive, with some problems threading the bars through the bubbles. But you have to weigh that downside against the speed with which we can lay the majority of it in one go. The operatives are still learning how to install it too, so we should see things improve as each floor goes on.’

Block F was about five weeks into construction when CM visited in October and is scheduled for completion around July next year. And there are several more projects on site and on the drawing board.

For instance, Harding says BubbleDeck has just been selected for the new Salisbury Law Courts building, where it will be used to achieve some unusual spans and load transfers, with construction due to begin in mid-January 2008. cm