Following the design of the one-way, hollow core slab in the 1950s, after many years of the absence of innovative ideas in the construction industry, the Danish Ministry of Housing held a design competition in 1997 with the aim of bringing about a starting point that would lead to the development of flexible and sustainable structures.

Unfortunately, the competition did not lead to any breakthroughs. However, the idea intrigued Jorgen Breuning, who at the time had been focusing on special building systems, in particular shell constructions.

Soon after, Breuning invented Bubbledeck technology, the first true bi-axial hollow flat slab, which fulfilled the demands for flexibility, sustainability and material reduction within the construction industry.

Bubbledeck technology may be simply described as an extremely efficient flat slab system, which reduces construction costs and project times considerably. These savings in time and money were made through the complete elimination of beams in concrete structures, as well as through the reduction in the number of structural, load-bearing walls required.

Further savings in the amount of concrete used are brought about through a 33 per cent reduction in the dead weight of the slab as a result of the introduction of 'bubbles' (recycled, hollow plastic balls) that are placed within the very core of the slab, leading to a considerable increase in weight-strength ratio when compared to traditional slabs. These savings in load also mean that smaller-sized foundations are required, which in turn mean less excavations and less setting out.

Savings in construction time are significantly made by transferring the production of Bubbledeck slabs from the work site to the factory floor. Every part of the Bubbledeck slab can be delivered on site in a precast state, eliminating the need for onsite formwork, and only requiring temporary propping.

The precast units contain most of the steel reinforcement, thus further reducing the onsite steelwork placing and tying normally required. This results in savings of time and substantial labour costs.

These all-round savings will significantly increase the eco-friendliness of the local construction industry, especially when one considers how much CO2 is generated from the manufacturing of concrete, the quantity of which is reduced.

A reduction in noise and air pollution will also come about since less work is carried out on site and less truck deliveries are required over a shorter timeframe.

Due to the saving in dead weight within the slab, larger spans can be achieved. This means that a much more flexible internal layout is possible, with increased spacing between load-bearing walls and columns.

The fact that beams are eliminated from the structure means a significant reduction in floor height, allowing further storeys to be constructed within the permitted building height. The elimination of beams also facilitates the passing of services.

Studies comparing a typical Bubbledeck structure with the most efficient local structural techniques show impressive savings.

For a small structure (total floor area of 500 square metres), savings in construction time have been estimated to be in the region of 50 per cent, while savings in costs were about 5-10 per cent. For larger structures (total floor area of 5,000 square metres), savings in time and costs were found to be in the region of 25 per cent.

A good example which shows how efficient Bubbledeck is, as opposed to traditional structural techniques, can be shown by studying the construction of the Millennium Tower in Rotterdam. At 34 storeys, it is the second highest structure in Holland.

This structure was originally designed with hollow core slabs. However, late in the design stage it was discovered that the use of Bubbledeck technology would bring about considerable cost and time savings.

Adopting Bubbledeck slabs also reduced the structural floor zone depth due to elimination of beams. Half-way through constructing the structure, it was decided to add another two floors. This could be done within the overall height of the originally proposed building due to the ability of Bubbledeck slabs to reduce structural floor height.

Eight-hundred-square-metre floor plates were erected and completed in half the usual time - four instead of the normal eight days it would have taken to construct a comparable structure using hollow core slabs.

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Andrea Sammut is a civil engineer with an M.Sc. in Structural Engineering from the University of Manchester.

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