Voided Slab


The first voided slab was designed and constructed in 1950s. At that time, the performance of the voided slab was similar to one way slabs. The cavities formed had a cylindrical cross section. In the 1990s, the idea of using spherical and cylindrical cavities in Europe has grown, and today it has become one of the most commonly used ways to construct the concrete slabs.

Theory of Voided Slabs

The need to cover the large spans in structures without the middle columns, and to reduce weight and reduce the dimensions of the structural elements on the other, have created different systems of building slabs. The first option is to use a beam-slab system rather than a flat slab system. In this case, due to the presence of beams around the slab, the effort to reduce the slab thickness is the most important factor in reducing the weight of the building. The need to intermediate beams in long spans, problems due to the reinforcement of beams, as well as the shape of beam’s drops and the relatively high volume of concrete and reinforcement used in this system are known as disadvantages of this method.
In addition to the beam and slab system, the use of flat slabs has been considered due to ease of implementation and architectural advantages. But to cover the long spans, it is necessary to increase the slab thickness to increase the cross-sectional strength in bending and shear and increase the inertia of the cross section to prevent long-term deformation. As the slab thickness increases, the dead load also increases and increasing its load increases the deformation and creates larger bending and shear forces. In the voided slabs, with the formation of permanent holes in the concrete section, the weight of the slabs will decrease with increasing thickness. The idea of ​​using voided slabs has two general objectives: Benefits of Flat Slabs decreasing slab weight while increasing its thickness.
For example, consider a slab with a thickness of 30 cm, which uses cubes with dimensions of 50 x 50 x 16 cm to create holes in concrete. If these cubes are spaced 12 cm apart, their sections and plan are shown in the following figure.

In this case, the inertia of the hollow slab is about 90% of the flat slab with a thickness of 30 cm, while the weight is equal to the slab with a thickness of 20 cm. It provides the possibility of covering large spans using a flat slab system.
Various types of formworks can be used to create permanent voids. But in general, these forms are in the form of two groups:
- Spherical or elliptical shapes
- Cube shapes (U-boot)
ArmeGroup has used cubic shapes as a system used in voided slabs. The use of cubic shapes against the spherical forms has the following advantages:
- Transport of cube formworks is easier than spherical ones. The cubic formworks are mounted on each other and get smaller volumes.
- Cubic formworks installation does not need not to any metal cage or reinforcement