A voided concrete slab is a system that removes the non-working or excess concrete from the structural slab and replaces it with void formers. The voided slab system was invented by Jorgan Breuning of Denmark about 20 years ago.
The idea of creating engineered voids within a solid slab system helps to reduce the overall weight of the slab system. One of the main disadvantages of conventional solid slabs is their weight which not only imposes strain on itself but also puts a huge burden on the building and the foundation. The voided slab system offers a huge opportunity to reduce the cost and environmental impact without compromising with the performance of the structure.
Void formers are employed to make voided slabs to eliminate unnecessary concrete. It hence reduces the weight of the slab and improves the structural capacity and span length of the slab.
This article explains some of the important properties and features of the voided slab system.
These void formers are usually made out of plastic or any recycled substances. Voided slab incorporates an array of rigid void formers that contain air. The voided slabs rely on these voids created within the concrete during the time of casting. The arrangement creates an array of hollow boxes in the slab that support the flat surface. The system hence provides support in both directions and safely transfers the load to the vertical member. It is hence called voided biaxial slabs or biaxial slabs.
It is arranged using temporary supports. Steel mesh is commonly used to create a framework as they create optimal geometric proportions between the voids, concrete, and the reinforcement. The void formers are placed in the slab in a grid arrangement.
A voided concrete slab system can be constructed using either of the following void formers:
All the above-mentioned void formers for concrete slabs are technologies developed within the past 20 years. They vary in terms of the shape of the formers used, the limit of the span of the slab, and the type of substance used for void formers.
A bubble deck system uses plastic balls or bubble forms, while the u-boot system makes use of truncated-pyramid-shaped void formers. Irrespective of the type of void formers used, they bring a drastic decrease in the dead weight of the slab.
With the increase in the thickness of the slab, the dead-weight also increases. The increasing need for sound and vibration resistance for buildings with a growing number of occupants is the reason behind the demand for concrete slabs with a large thickness. This increase in weight needs enlargement of the foundation and the column size. To avoid these limitations of conventional solid concrete slabs, the voided slab system is preferred. Altogether, it makes the construction consume a large amount of concrete and steel reinforcement.
Another issue faced by traditional construction of slabs is the maximum span. Due to the relatively high density of the solid horizontal slabs, their span is limited. To overcome this issue, more reinforcement is designed for the slab. The use of voided slabs helps to reduce the weight of the slab without compromising its structural strength. It also helps to design a thicker slab to support more load over a longer span.
The strength parameters of the voided slab system in comparison to the solid slab system are explained below.
Shear strength depends on the effective mass of the concrete. Usually, it is measured to be 72-91% of the shear capacity of the solid deck. Hence, in the case of voided slabs, the areas with high shear loads need special attention. For example, the areas around the columns are subjected to heavy shear loads. In these areas, balls (void formers) can be eliminated to achieve full shear capacity around the columns. Hence, in shear critical areas, the slab is designed the same way as a solid concrete slab.
Different studies conducted between bubble deck and solid deck showed that the bending stress in the bubble deck slab is 6.43% lesser than that of solid slabs.
Based on finite element analysis conducted on bubble deck and solid slabs, it was found that the deflection of bubble deck is 5.88% more than solid slabs. The stiffness of the bubble deck slab is less because of the hollow portion.
Punching shear is of great concern for the voided concrete slabs due to their reduced weight. It is always determined whether the applied shear is greater or lesser than the bubble deck capacity. If it is more than the permissible limit, then the void must be omitted around that area and the design is rechecked for the new solid section. If it is still lesser than the needed value, then additional shear reinforcement must be provided.
This was concluded by studying the bubble deck and solid deck in three different variations. Based on the cost analysis conducted for the Town Hall building in Den Hang, it was found that the bubble deck was 5 to 16 % less expensive than a solid concrete deck. It gave a total saving in the order of 20%. Two variants of bubble deck were compared and concluded that a bubble deck building is less expensive than the traditional system.
The advantages of voided slabs are:
A voided slab is a system that removes the non-working or excess concrete from the structural slab and replace it with void formers.
A voided concrete slab system can be constructed using either of the following void formers: 1. Bubble Deck 2. Cobiax 3. U-Boot Beton 4. Bee Plate System 5. Airdeck