Voided Concrete Slab System: Its Working, Properties, and Advantages

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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.

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 idea of creating engineered voids within a solid slab system helps to reduce the overall weight of the slab system. The voided slab system provides 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.

Working of Voided Slab System

Voided slab incorporates an array of rigid void formers that contain air. These void formers are usually made out of plastic or any recycled materials. 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.

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The void formers are placed in the slab in a grid arrangement. 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.

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

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 material 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.

Need for Voided Slab System

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. With the increase in the thickness of the slab, the dead-weight also increases. This increase in weight requires enlargement of the foundation and the column size. Altogether, it makes the building consume a large amount of concrete and steel reinforcement. To avoid these limitations of conventional solid concrete slabs, the voided slab system is preferred.

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.

Properties of Voided Slab System

The strength parameters of the voided slab system in comparison to the solid slab system are explained below.

Shear Strength of Voided Concrete Slab System

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.

Bending Strength of Voided Concrete Slab System

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.

Deflection of Voided Concrete Slab System

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 of Voided Slab System

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 required value, then additional shear reinforcement must be provided.

Cost of Voided Slab System

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. This was concluded by studying the bubble deck and solid deck in three different variations. Two variants of bubble deck were compared and concluded that a bubble deck building is less expensive than the traditional system. It gave a total saving in the order of 20%.

Advantages of Voided Slab System

The advantages of voided slabs are:

1. Weight reduction of slabs is achieved by the voided slab system. This would help to achieve longer spans, lighter foundation, and smaller columns.
2. The voided slab system provides design freedom in building construction. It helps to provide an open floor plate with lesser numbers of columns and beams. The system hence prepares the building to adapt to future changes.
3. A voided slab system reduces the amount of concrete and steel reinforcement consumed by the building. This system brings savings in the form of material and cost.
4. This system simplifies formwork by the elimination of the beams and related labor. This hence saves time and brings a positive impact to the project schedules.
5. The void formers used to construct voided concrete slabs are recycled materials. Hence, this construction can contribute towards LEED certification.
6. Voided concrete slab system helps to achieve life-cycle benefits by increasing the flexibility of the building. The system helps the building to meet the ever-changing demands, thereby extending the useful life of the building. The structural depth of the voided concrete slabs can be reduced by incorporating post-tensioning systems.

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