What is sunken slab?
The sunken slab is the slab that is cast at a certain depth below the normal floor level. The extra depth is used for laying pipes and utility ducts. After that, the depth is filled with sand and other lightweight materials until the normal floor level.
It is normally kept at the depth of 1 foot to 2 feet from the normal floor level. It is very rarely used in construction. The level difference accommodates the pipeline and helps to conceal the pipeline when the floor is finally finished.
Advantages of sunken slab
- It helps to lay out the plumbing as well as electrical wiring.
- It is used for fitting the toilet and washroom below the normal level of the floor so that water will not come out from the bathroom or toilet.
- It makes a noise-proof environment due to sound coming from pipes of water.
Disadvantages of Slab
- Leakage may occur below the slab.
- The area below the slab becomes low.
- Need proper design for the safe sunken slab.
- Need more skilled manpower to construct sunken slabs.
- Waterproofing materials are required in this slab which makes it more costly.
Sunken slab details while designing
- A sunken slab must contain a steel rod more than 10 mm on the floor.
- The thickness of the slab should be kept above 5 inches.
- The spacing between the steel rod should not be more than 6 inches to 8 inches for safe design.
- The slab should contain a beam at two sides as a support so that the load on the slab will be distributed to the beam and finally to the column.
- The slab rod should be extended up to the full width of the beam on which it is supported.
- The lapping of the rod should be avoided between the sunken slab.
- A crank bar should be used for connection joint between beam and slab if the area of the slab is large.
- The pipe should not be used in large volumes which makes the concrete weak.
- The proper clear cover should be provided by using the block below steel rods.
Filling materials used in Slab
- Broken red clay bricks
- Coal Lump
- Brick jelly
- Lightweight cinder
- Broken fly ash bricks
How to waterproof sunken slab?
- At first, the surface of the sunken slab should be cleaned well.
- The side of the drainage pipe should be properly sealed with the help of cement and mortar.
- The cement and mortar should be mixed with waterproofing chemicals.
- Pore pipe should be below the surface level of the sunken slab so that there will not accumulation of water.
- After drying of cement the tar coal should be pasted on the surface thoroughly.
- After that, the filling materials are filled and PCC is done above it.
- The surface level of the slab is kept about 10 mm to 20 mm below the normal level after PCC.
I hope this article on “Sunken slab details” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Ranjeet Sahani