What Is A Voided Biaxial Slab? Properties Of Voided Biaxial Slabs

What Is A Voided Biaxial Slab? Properties Of Voided Biaxial Slabs

What Is A Voided Biaxial Slab?

Voided biaxial slabs, also known as biaxial slabs or voided slabs, are a type of concrete slab that has air-filled voids in order to reduce the amount of concrete needed and decrease the environmental impact of construction.

These voids also make the slab lighter allowing for up to 50% of the slab volume to be removed. This not only lowers the load on structural members but also allows for increased weight or span due to the reduced self-weight of the slab.

One of the main problems with traditional solid slabs is their weight, which can cause strain on the slab itself and also put a lot of pressure on the building and its foundation.

To address this issue, voided slab systems have been developed which include engineered voids within the solid slab structure. These voids help to reduce the overall weight of the slab system.

Voided slab systems offer the potential to reduce costs and environmental impact while still maintaining the structural performance of the building. Void formers are used to create the voids within the slab which helps to remove excess concrete and reduce the weight of the slab. This also increases the structural capacity and span length of the slab.

How Does Voided Biaxial Slabs Works?

A voided slab is a type of concrete slab that contains air-filled voids, or empty spaces, within it. These voids are created by using rigid materials, such as plastic or recycled materials, called void formers.

The void formers are placed in a grid pattern using temporary supports and steel mesh to create an optimal geometric balance between the voids, concrete, and reinforcement.

The resulting array of hollow boxes within the slab supports the flat surface and helps transfer the load to the vertical members, making it a biaxial slab. There are several types of void formers that can be used in the construction of a voided slab, including Bubble Deck, Cobiax, U-Boot Beton, Bee Plate System, and Airdeck.

These systems differ in the shape and material of the void formers, as well as the span limit of the slab. The use of void formers significantly reduces the weight of the slab.

Properties Of Voided Biaxial Slabs

Voided biaxial slabs have several properties that influence their performance. These properties include shear strength, bending strength, deflection, and punching shear. The shear strength of voided slabs is typically lower than that of solid slabs, with the effective mass of the concrete typically being 72-91% of the shear capacity of a solid deck.

Bending stress in voided slabs is typically 6.43% lower than in solid slabs. Deflection in voided slabs is typically 5.88% higher than in solid slabs due to the lower stiffness of the hollow portion. Punching shear is a particular concern in voided slabs due to their reduced weight.

If the applied shear is greater than the capacity of the voided slab, voids may need to be omitted in certain areas and the design recalculated for a solid section. If the required shear value is still not met, additional shear reinforcement may be necessary.

Advantages Of Voided Biaxial Slabs?

Voided slabs are a type of concrete slab that have hollow spaces, or “voids,” within them. There are several advantages to using voided slabs in building construction:

  • Weight reduction: The presence of voids in the slab allows for a lighter overall structure, which can result in longer spans, lighter foundations, and smaller columns.
  • Design freedom: The voided slab system allows for more open floor plans with fewer columns and beams, giving architects more flexibility in the design process.
  • Reduced material and cost: The voided slab system uses less concrete and steel reinforcement, leading to savings in materials and cost.
  • Simplified formwork: The elimination of beams in the voided slab system simplifies the formwork process, saving time and improving project schedules.
  • Sustainability: The void formers used in voided concrete slab construction are often made of recycled materials, which can contribute towards LEED certification.
  • Life-cycle benefits: The flexibility of voided slab construction allows buildings to adapt to changing demands and extend their useful life. Additionally, the structural depth of voided concrete slabs can be reduced through the use of post-tensioning systems.

Applications Of Voided Biaxial Slabs

Concrete slabs with a large thickness are in high demand due to the need for sound and vibration resistance in buildings with a growing number of occupants.

However, these thick slabs also increase the dead weight of the building, requiring larger foundations and columns, and using more concrete and steel reinforcement.

An alternative to traditional solid concrete slabs is the voided slab system, which reduces the weight of the slab without sacrificing structural strength, and allows for the design of a thicker slab that can support a greater load over a longer span.

Additionally, traditional slabs have a limited span due to their high density but the use of voided slabs can help overcome this limitation by requiring less reinforcement.


In conclusion, voids in biaxial slabs are an important element of modern building construction. They can offer increased strength and flexibility to the structure, while reducing cost and labor by using less material.

Voided biaxial slabs also have a stronger bond than traditional pre-stressed concrete structures. With the proper design, these slabs can provide a strong yet economical foundation for any structure.

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