WHAT IS WAFFLE SLAB? CONSTRUCTION PROCEDURE AND ADVANTAGES, DISADVANTAGES OF IT.

WAFFLE SLAB

  • A waffle slab is a concrete slab made of reinforced cement concrete with concrete ribs which are running in two directions. It can be shown from below side and slab has a plain top. Due to the underneath ribs, It is also called ribbed slab. Waffle slab are preferred for longer span which are greater than 12 m. They are much stronger than flat slabs, flat slabs with drop panels, two-way slabs, one-way slabs, and one-way joist slabs.
  • A waffle slab is flat on the top. The top portion of the slab having a thickness of 100 to 120 mm. Below portion of the slab is made of ribs which are intersected to each other. The thickness of these ribs are generally 100 to 200 mm and overall depth of slab is 300 mm to 600 mm Dependent on the span of slab. These ribs create small square blocks. These square blocks has sized of 600 to 1200 mm depending on design. Sometimes the ribs creates triangular blocks.
  •  The ribs of waffle slab are formed by the ready made mould, on which the concrete would be poured. After the concrete has been set, the mould will be removed. This structure is designed to be more solid when used on longer spans with heavier loads. This type of structure, because of its rigidity, is recommended for buildings that require minimal vibration, like laboratories and manufacturing facilities. It is also used in buildings that require big open spaces, like theatres or train stations.
  • Due to the complex formwork of it, It becomes more expensive than the regular slabs. But it depends on quantity and area of waffle slab. This kind of slab designs consume less steel and less concrete in compare to regular slab which are designed heavy for long span.


Construction procedure :

  • A waffle slab can be made in different ways but generic forms are needed to give the waffle shape to the slab. The formwork is made up of many elements: waffle pods, horizontal supports, vertical supports, cube junctions, hole plates, clits and steel Bars. First the supports are built, then the pods are arranged in place, and finally the concrete is poured.

This process may occur in three different approaches, however the basic method is the same in each:

1. In situ : Formwork construction and pouring of concrete occur on site, then the slab is assembled.
2. Precast : The slabs are made somewhere else and then brought to the site and assembled.
3. Pre-fabricated : The reinforcements are integrated into the slab while being manufactured, without needing to reinforce the assembly on site. This is the most expensive option among others.

  • To construct a waffle slab in-situ conditions, formwork should be necessary to support the slab. But some special tools are required for the form work in case of waffle slab.
  • Formwork tools required in the construction of waffle slab are:
  1. Waffle pods
  2. Horizontal supports
  3. Vertical supports
  4. Wall connectors
  5. Cube junctions
  6. Hole plates
  7. Clits
  8. Steel bars
  • Horizontal support and vertical supports are arranged first and they are fixed in position by the connectors. At the edges wall connectors are used to provide connection between wall and slab. The horizontal beam supports are connected by small beam connectors which form square like shape in which pods are going to be placed.
  • The pods are generally made of plastic and they are available in different sizes and different shapes. Size selection of pod depend upon the requirement and span length. For longer span large number of pods are required. Same size should be used for one complete slab.
  • Similarly beam connectors and cube junctions are also available in different sizes based on the suitability of pod sizes.
  • Thin concrete slab is provided on the top and after its hardening pods and frameworks are removed from the bottom. Thus, the waffle like shape appears at the bottom surface.
  • Cube junctions are used to fix the corners of pods with the frame work. After fixing the formwork, reinforcement is placed in the two directions of the slab and then concrete is poured in the gaps which are called as ribs after hardening.
Characteristics of waffle slab :
  1. Slab depth is typically 80 mm to 120 mm thick.As a rule of thumb, the depth should be ​1⁄24 of the span.
  2. The width of the ribs is typically 100 mm to 200 mm, and ribs usually have steel rod reinforcements.
  3. The distance between ribs is typically 915 mm (3 ft).
  4. The height of the ribs and beams should be ​1⁄25 of the span between columns.
  5. The width of the solid area around the column should be ​1⁄8 of the span between columns. Its height should the same as the ribs.
  6. Volume of concrete used is very less compared to others.
  7. Waffle slabs are generally suitable for flat areas. 
  8. It has good finishes and robustness. Fairly slim floor depth and fire resistant. 
  9. It has excellent vibration control. 
  10. It uses 30% less concrete and 20% less steel than a raft slab.
Advantages of waffle slab :

The waffle slab floor system has several advantages:

1. It is a good resistant of vibration. Hence it is better for buildings that require less vibrations.
2. Bigger spans can be achieved with less material, being more economical and environmentally friendly.
3. The waffle pattern or grid system gives aesthetic appearance.
4. It can bear greater load with longer span.
5. It is compatible for slab which are connected to less column.

Disadvantages of waffle slab :

The waffle slab floor system has several disadvantages:

1. Greater quantities of formwork materials are needed, which can be very costly.
2. Waffle slabs are thicker than flat slabs, so the height between each floor must be greater to have enough space for the slab system and other building services.
3. Waffle slabs are preferred for flat topographical areas not sloped sites.
4. It is not suitable in windy area due to low sustainability against heavy winds.
5. The construction of waffle slab required highly skilled workmen and strict supervision.

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