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Concrete slabs are structural elements used to create flat horizontal surfaces, like floors, ceilings, and roof decks. They support and transfer the loads safely from columns to walls.

They are commonly made of reinforced concrete with their top and bottom surfaces parallel to each other. Their thickness is dependent on the load imposed on them and can be supported either by beams, columns, walls, or constructed on the ground (slab-on-ground).

This article explains the important features and structural behavior of concrete slabs used in building construction.

Loads on Concrete Slabs

Concrete slabs are generally assumed to carry uniformly distributed loads. The uniform loads for which slabs are designed are dead loads and imposed loads. The dead loads include the self-weight of the slab, floor finishes, walls, grills, etc.

The design of slabs for imposed loads and live loads is based on the type of the building. For residential buildings, a live load of 2000 to 3000 N/m2 is used as per Indian standards. The imposed loads considered for various types of buildings can be referred from IS 875-Part 2 (1987).

Construction of Concrete Slabs

Slab-on-ground
Figure-1: Slab-on-ground

Generally, concrete slabs can be ground-bearing or suspended. A slab is ground-bearing if it is constructed directly on the foundation while a suspended slab spans between the supports and is designed to resist bending moments based on the magnitude of the load and the length of the span. Concrete slabs can be prefabricated off-site and assembled at the site, or they are constructed in-situ using formwork.

Suspended Concrete Slabs
Figure-2: Suspended Concrete Slabs

Types of Concrete Slabs

There are several types of concrete slabs widely used in construction but the most general ones used in building construction are:

  1. Conventional slabs
  2. Flat slabs
  3. Hollow-core slabs
  4. Waffle slabs
  5. Composite slabs

1. Conventional Slabs

Conventional concrete slabs are supported with beams and columns. They transfer load to the supporting structural elements. Conventional slabs are mainly classified into one-way slabs and two-way slabs (Figure-1).

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One-way slabs are slabs that are supported only on two of their opposite sides. They transfer loads along the longer direction. Two-way slabs are slabs that are supported on all four sides. They transfer loads along both directions.

2. Flat Slabs

Flat slabs are slabs that are supported directly by the columns without the use of beams. They are easy to construct and require less formwork compared to conventional concrete slabs.

Flat Slabs
Figure-3: Flat Slabs

3. Hollow Core Slab

Hollow core slabs are constructed to have longitudinal cores or voids within them. This reduces the overall weight of concrete, unlike conventional concrete slabs. These types of slabs consume less concrete and can achieve longer spans. These are best suitable for multi-story car parkings and office buildings.

 Hollow Core Slabs
Figure-4: Hollow Core Slabs

4. Waffle Slabs

Waffle slab possesses a waffle-shaped geometry. It is designed to have square grids with deep sides without the interference of columns. It is a good choice for the construction of longer spans. Waffle slabs take a greater load compared to conventional slabs.

Waffle Concrete Slabs
Figure-5: Waffle Concrete Slabs

5. Composite Slab

A composite slab system involves constructing a reinforced concrete cast on the top of the steel decking as a supporting system for the above concrete slab. The system gains superior strength-weight and stiffness-weight ratio.

Composite Slabs
Figure-6: Composite Slabs

Structural Action of Concrete Slabs

The structural behavior of concrete slabs subjected to loads is controlled by the geometry of the slabs and the direction of their reinforcement. The dead loads and imposed loads coming over the slabs are distributed over their support system.

The behavior of slabs can be explained by taking into consideration the conventional concrete slabs: one-way and two-way slabs.

Structural Behaviour of One-Way Slabs

One-way slabs have the ratio of longer dimension to shorter dimension greater than or equal to two. It is supported at two opposite edges, as shown in Figure-7. The loads acting on one-way slabs are transferred between the adjacent beams or supports. When a one-way slab is supported on all its four edges, the load is transferred along the longer direction other than the shorter direction.

Due to the loads, the one-way slab bends along the shorter span, as explained in Figure-7 (a). Hence during the design, the main reinforcement is provided along the shorter span and distribution reinforcement is provided along the longer spans. A common example of a one-way slab is a cantilever slab used for verandahs.

Structural Behavior of One-Way and Two-Way Slabs
Figure-7: Structural Behavior of One-Way and Two-Way Slabs

Structural Behaviour of Two-Way Slabs

Two-way slabs are supported on all their four sides and the ratio of their longer dimension to shorter dimension is less than two (Figure-7 (b)). Hence, the load is carried in both directions of the slab.

Two-way slabs undergo bending along the shorter and longer sides; hence the main reinforcement is provided in both directions. They are commonly used in the construction of floors of multi-story buildings.

FAQs

What is a concrete slab in building construction?

Concrete slabs are structural elements used to create flat horizontal surfaces like floors, ceilings, and roof decks. Slabs are flat sections with their top and bottom surface parallel to each other. They are commonly made of reinforced concrete, and their thickness is dependent on the load imposed on them.

What are the loads taken by concrete slabs?

The loads taken by concrete slabs are dead loads and imposed loads. Dead loads include the self-weight of the slab and other weights coming over it. Imposed loads are live loads that are provided based on the occupants using the slab space. It is different for residential and commercial buildings.

What is the structural behavior of two-way slabs?

Two-way slabs under loads are subjected to bending along their shorter and longer spans. Hence, the main reinforcement is provided along both directions of a two-way slab.

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Reinforcements for Slabs-on-Ground Crack Width Control -As Per ACI

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