Bubble Deck: a revolutionary method of designing two-way slab systems in Europe
by:
Mustafa Mahamid
Bubble Deck is a hollow two-way
slab system that has been used in Europe for the last 10 years in lieu of
conventional two-way and one-way slab reinforced concrete slab systems. The
bubble deck system is compos
ed
of top and bottom welded steel mesh and hollow plastic ball that replaces the
concrete that has no structural function. The geometry created by the plastic
balls and the mesh width, as shown in the Figure, provides an optimized
statically stable concrete construction with a maximum utility of both moment
and shear zones.
The reinforcing mesh keeps the balls in exact position while the balls control the level of reinforcing, shape the air volume, and stabilize the spatial lattice making it ready for concreting to obtain a two-way hollow slab system.
The main advantage of this system is in reducing the weight of the structure of approximately 35% that eliminates the constraints of high dead load and short spans. The weight reduction results in smaller column sizes and smaller footing sizes, or up to 50% longer spans that result in fewer columns and footings in the building. The weight reduction also, has a huge impact on earthquake forces and seismic design.
Punching shear is one of the major limit states in designing reinforced concrete slabs. Experiments have shown that shear capacity of bubble deck is 72-91% of the shear capacity of solid slab and areas of high shear require special attention, therefore the punching shear issue was resolved by omitting few balls in the critical area around columns to provide full shear capacity by providing full slab depth multiplied by the perimeter of the critical section around column to resist shear forces and moments.
As green credentials are becoming highly required these days to obtain the LEED certification, this system may provide some credits due to its contribution to reducing environmental impact by saving in material up to 50%, using recycled plastic balls used in the middle of the slab, 1kg of plastic balls replaces 100 kg of concrete, less energy consumption both in production and transportation, less emission-exhaust gases from production and transportation, and no waste generation-100% recycling.
Regarding constructability, the slab system is constructed by dividing the overall floor area into a series of individual elements, either 3 meters or 2.4 meters wide. These elements include the top and bottom reinforcement mesh, sized to suit the specific project. The top and bottom reinforcement is joined together by vertical lattice girders with plastic balls to fix the top and bottom mesh in their optimum position. This is known as “Bubble-reinforcement” sandwich which is then cast into bottom layer of concrete to provide permanent formwork within part of the finished slab depth. This is done off-site making it ready to be transported to the construction site.
On site, a series of individual elements are placed and stitched together with loose reinforcement across the joints between the elements. Splice bars are placed above the precast layer and between the bubbles to connect the individual elements together and prepare the slab for the final concrete pour.
The bubble Deck system has been tested in the Technical University of Denmark, Denmark, Technical University in Delft, Technical University in Eindhoven, TNO, Dycore Verwo, Netherlands, Technical University of Darmstadt, Germany.
The system has proved to be
highly successful in Europe for the last 10 years, and has been used in Canada
for the last three years.
In the meantime, a local US consulting firm is investigating the slab system, discussing it with architects around the country, testing it for UL rating, and in the process of submitting the technical information to get it certified by the International Code Council.