An innovative void forming system that halves frame weight is getting its first UK use. Mark Hansford reports from Poole in Dorset.
Anyone willing to pay up to £1.75M for a three bedroom flat in Dorset would have to consider it pretty special. So it is not surprising that when the new owners of 28 luxury flats in Poole move in they will - at least for a short time - be living in a building like no other in the UK.
That is because Gleeson Homes' £10M Mirage development overlooking Poole harbour is being constructed with thousands of plastic balls - some up to 405mm in diameter - embedded in the concrete deck.
The system, known as BubbleDeck, uses the balls as void formers and works as a composite two-way hollowbody floor slab. The balls reduce non-working dead load while retaining biaxial strength. The system enables deck weight to be reduced by as much as 35%, with total frame weight reduction capable of reaching 50%. The resulting reduction in load on foundations can be as much as 4.77kN/m 2. And because it is semi-precast, site labour is minimised and construction is speeded up.
With all these benefits it is perhaps surprising the system has taken so long to break into the UK market. After all, the technique dates back to Roman times, when terracotta pots were used to reduce dead load. In more modern times BubbleDeck is now an established system in mainland Europe, with the Millennium Tower in Rotterdam serving as a beacon for its worth.
Completed in early 2000 and standing 132m high excluding antenna, the Millennium Tower is Rotterdam's second tallest building. By using BubbleDeck engineers were able to add three floors to the 34-storey building without increasing height. The tower contains 2,500m 3of air, has 6,000t less concrete and saved 500 concrete truckloads to site. And with the increased prefabrication, site labour was reduced, allowing the tower to be erected in one third the time compared to traditional methods.
'They were taking just four days per floor as opposed to the original 10, ' says BubbleDeck managing director Daniel Ptacek.
Savings on the three storey Mirage development being erected by contractor Civils are bound to be modest in comparison.
The volume of concrete displaced by the balls increases exponentially as the balls get larger, so it is only on deeper, longer slabs that BubbleDeck truly proves its worth.
At Poole only a 1,600m 2,470mm deep transfer slab at ground floor level comes into this category, with 405mm balls displacing 0.172m3/m 2and reducing loads on the foundations by 4.29kN/m 2.Elsewhere, the 180mm balls being used in the 240mm thick slabs for the 800m 2upper two storeys are the smallest at which it makes economic sense to use the system.
And even then, by displacing 0.0763m 3/m 2and cutting foundation load by 1.91kN/m 2, it is only cost neutral in terms of materials costs. But there are many other benefits, claims Ptacek.
'On the Poole job they are getting faster construction, the quality of a prefabricated solution, just in time delivery, bigger spans and a reduction in temporary props, and much simpler form working than is needed for traditional void forming, ' he says.
With the BubbleDeck semiprecast module concreting takes place in two steps. In the factory the balls required for slabs up to 7m long and 2.8m wide are positioned precisely and trapped inside reinforcement mesh which is then set into a 60mm thick concrete 'biscuit'.
This pre-cast concrete layer replaces the horizontal part of the formwork, so all that needs to be laid on site is edge formwork. At the factory each module is tagged with a unique reference number to allow contractors on site to drop each into its specific position. Once the entire deck is in place T12 splice bars are positioned across joins and concrete is then poured in two stages to create a continuous two-way slab.
Laying out a complete floor takes just two days, with two further days needed to cast the concrete. Work began on site in October and remains broadly on target for civils completion in June.