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Last edited 09 Sep 2020
Concrete slabs in buildings
A slab is a structural element, made of concrete, that is used to create flat horizontal surfaces such as floors, roof decks and ceilings. A slab is generally several inches thick and supported by beams,columns, walls, or the ground.
Concrete slabs can be prefabricated off-site and lowered into place or may be poured in-situ using formwork. If reinforcement is required, slabs can be pre-stressed or the concrete can be poured over rebar positioned within the formwork.
There are several different types of slab, including:
- One-way: Supported by beams on two opposite sides, carrying the load along one direction.
- Two-way: Supported by beams on all four sides, carrying the load along both directions.
This type of slab has longitudinal voids/cores running through it, which decrease the weight of the slab, as well as the amount of concrete required. They can also function as service ducts. This type of slab is generally reinforced with longitudinal rebar, and can achieve long spans, making it suitable for office buildings, multi-storey car parks, and so on.
This type of slab contains square grids with deep sides, resembling a waffle shape, often used where large spans are required without the interference of columns. Waffle slabs can be capable of supporting a greater load than conventional slabs.
This is a type of shallow foundation, typically formed by a reinforced concrete slab that covers a wide area, often the entire footprint of a building. It spreads the load imposed by a number of columns, walls, and so on, over a large area, and can be considered to ‘float’ on the ground in a similar way to a raft floating on water. It is often used for lightly-loaded buildings on weak or expansive soils such as clays or peat.
Composite slabs are typically constructed from reinforced concrete cast on top of profiled steel decking, (re-entrant or trapezoidal). Slabs are most commonly made of concrete because of its mass and stiffness which can be used to reduce the floor's deflections and vibrations, and achieve the necessary fire protection and thermal storage. Steel is often used as the supporting system underneath the slab due to its superior strength-weight and stiffness-weight ratio and ease of handling.
 Related articles on Designing Buildings Wiki
- Building foundations.
- Cast-in-place concrete.
- Cellular raft foundation.
- Concrete floor.
- Domestic floors: Part 1: Construction, insulation and damp proofing.
- Floating floors in buildings.
- Floor slab.
- Floor void.
- Pad foundation.
- Strip foundation.
- Types of floor.
- Types of raft foundation.
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