A waffle slab or two-way joist slab is a concrete slab made of reinforced concrete with concrete ribs running in two directions on its underside. The name waffle comes from the grid pattern created by the reinforcing ribs. Waffle slabs are preferred for spans greater than 40 feet (12 m), as they are much stronger than flat slabs, flat slabs with drop panels, two-way slabs, one-way slabs, and one-way joist slabs.

The waffle slab is mostly found in entryway of hotels, malls, restaurants to obtain nice realistic view and set up artificial lighting as well as airports, parking garages, commercial & industrial buildings, bridges, residence and other structures for which additional strength is essential.

There are empty holes in the slab while detaching the formwork. Initially, PVC trays alias pods are arranged on shuttering. After that, reinforcement is arranged among the pods along with steel mesh at the top of the pods. Then the concrete is filled. Once the concrete is settled, the formwork is eliminated except PVC pods. It produces hollow holes in it where the hole is sealed at one end.

Advantages of waffle slab

1- The larger span of slab and floor with less number of columns.

2- load carrying capacity is greater than the other types of the slab.

3- Savings on weight and materials.

4- Good vibration control capacity.

5- Attractive soffit appearance when exposed.

6- Lightweight.

7- Vertical penetrations between ribs are easy.

8- Economical when reusable formwork is used.

9- Fast and speedy construction.

Disadvantages of waffle slab

1- Require greater floor-to-floor height.

2- Requires special or proprietary formwork which is costly.

3- requires strict supervision and skilled labour.

4- Difficulty in maintenance.

5- Not suitable in a highly windy area.

Characteristics of a waffle slab

1- It is suitable for flat sites.

2- No need to provide beam excavation.

3- No need to utilize controlled or rolled fill.

4- Cardboard slab panel or empty formers are applied.

5- Slab panels are provided on 1-metre grids (roughly).

6- Trench mesh or separate bars can also be applied.

7- The depth of slab varies from 85 to 100 mm.

8- Width of inside beams ranges from 110 to 200 mm.

9- There is a nominal concrete volume.

10- Shrinkage of the slab is lesser as compared to stiffened rafts and footing slabs.

11- It consumes 30% less concrete as compared to a hardened raft.

12- It consumes 20% less steel as compared to a hardened raft.

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