Slabs: Types [Conventional , One Way , Two Way , Pre-stressed]

What is a Slab in Construction?

A slab is a structural component that is usually made of concrete, which is used to create flat horizontal surfaces.
Its main function is to transfer the load by bending in one or two directions. Reinforced concrete slabs are used in roofs, floors, ceilings, and as the decks of bridges.

The floor system of a structure can take many types such as in situ slabs, pre-cast slabs, ribbed slabs, etc. Slabs may be placed on either steel or monolithic concrete beams, walls, and even straight over the columns. There are different types of slabs in construction.

Conventional Slab

The slab which is supported on Beams and columns is called a conventional slab. It is supported either on walls or on beams and columns. In this slab, the thickness of the slab is less whereas the depth of the beam is large. In conventional slabs, the load is transferred to either walls or beams and after that from beams to columns. It needs more shuttering work as compared to the flat slab.

In the conventional type of slab, there is no requirement for providing column caps. The thickness of the conventional slab is 4 inches to 6 inches is recommended if the concrete will receive heavy loads.
Conventional concrete slabs are square in shape. Reinforcement can be the single or double layer in conventional slab depends on load, horizontal direction bars are called main bars and vertical direction bars are called distribution bars.

The conventional slab is categorized into the following two types based on their length and breadth,


One way slab

The one-way slab is supported by beams on the two opposite sides to carry the load in one direction. When the ratio of the long side (length) to the short side (breadth) is equal to or greater than 2, the equation will be (l/b > 2), it is considered a one-way slab because this slab will bend in one direction only i.e. in the direction along its shorter span. In a one-way slab, main reinforcement is provided in a shorter span and distribution reinforcement is given in the longer span.
This sort of slab is an economical, good option for commercial and residential projects. It’s even more economical than other types of slabs. The main thing to restrict one-way slabs is that they do not have the same length features as other slab types.


Two Way Slab

The slab is supported on beams on all four sides and the loads are carried by the supports along both directions is called two-way slabs. In this slab, the ratio of the long side (length) to the short side (breadth) is less than 2, the equation will be (l/b<2). These slabs are probably to bend along both spans.

In a two-way slab, the load is transferred in both directions to the four supporting sides, and for this reason, reinforcement is given in both directions. This type is generally used in multi-storeyed buildings, where floors need to support the weight of other floors.

Flat Slab

The flat slab is a reinforced concrete slab supported directly by concrete columns or caps. A flat slab is also called a beam-less slab because it doesn’t have beams. It is supported directly by columns or caps. Here, the loads are transferred directly to the columns.
The function of this sort of slab is to provide a plain ceiling surface, which will give the best circulation of light. They are generally used in hotels, parking areas, commercial buildings, or structures where beam projections are not ideal for height controls or even architectural view.
The flat slab is easier to construct and needs less formwork. This is one of the types of concrete slabs.


Hollow Core Slab

It is a type of precast slab where cores are placed. The cores can function as service ducts and the slab self-weight reduce considerably and structural performance improves. The cores also have sustainability benefit as it reduces the quantity of material used. These types of slabs are Precast and it is used where the construction needs to be done fast. This reduces the amount of labor and time needed for the project. Just use a crane to lift and install the hollow core slabs.


Hardy slab

Hardy slab is constructed by hardy Bricks. Hardy blocks are hollow bricks and made up of concrete. These blocks are used to fill portions of the slab. Hardy slabs save the volume of concrete and therefore the own weight of the slab is reduced. Hardy slab is used where temperatures are very high. To resist the temperature from the top of the slab thickness is increased.

It is economical for spans of length up to 5 meters, and it minimizes the quantity of concrete below the neutral axis, and moderate live loads shall be imposed. These kinds of slabs are commonly seen in Dubai and China.


Waffle Slab

This type of slab contains square grids with deep sides. It is also called a grid slab. Waffle slab is majorly used at the entrance of hotels, cinema halls, auditorium, Shopping malls, and Restaurants for good pictorial view and to install artificial lighting.

Its primary function is to carry heavier loads and longer span than flat slabs as these systems are light in weight and can be used as both ceiling and floor slab. They are also used where there is a restriction on the depth of beams for getting clear height.

First of all PVC pods are positioned on shuttering then reinforcement is provided between the pods and steel mesh is given at top of the pods and after that pour with concrete. After concrete sets, the formwork takes out and PVC pods are not removed. This forms a hollow hole in it in which the hole is closed at one end. The concrete waffle slab is usually used for industrial and commercial buildings while timber and metal waffle slabs are used in many other construction sites. This is one of the types of concrete slabs.


Based on the shape of Pods waffle slabs are categorized into the following types:

  1. Triangular pod system
  2. Square pod system

Pre-stressed Concrete Slab

1) Pre-Tensioned Slab

The slab in which steel is tensioned before placing the concrete is called a pre-tensioned slab. The slab has the same features as post-tensioning.


2) Post-Tensioned Slab

The slab in which the steel cables or tendons are tensioned after concreting is called the post-tensioned slab. Reinforcement is given to resist compression. In this slab, the reinforcement is replaced with steel cables or tendons.
This type of slabs break the natural weakness of the concrete in tension and makes better use of its strength in compression.


Precast concrete slabs

Precast concrete slabs are casted and cured in manufacturing plants, and then supplied at the site for erection. The most significant benefit of the precast concrete slabs is that, as they are produced in plants, their efficiency is enhanced and higher quality control is achieved over the on-site concrete slabs.
Furthermore, it is observed that precast concrete slabs are more affordable than cast in situ concrete slab. Though you spend more on erection, you save a lot in formwork.
Type of most commonly used precast slabs is the channel and double-T types.

Slab on Grade

The slab which is casted on the surface of the earth is known as a Ground slab. It is used on the Basement floor or at the plinth level. Slabs on grade can be plain or reinforced.
It is suitable for a well-compacted stable ground rock or constrained sand that is not affected by dampness, and soils that do not go through even minor movement due to wetness.


Sunken Slab

Residential and commercial buildings may need a sunken slab in washrooms or around other fixtures. These construction situations require a slab that is below the floor level and fits water and drain lines. A sunken slab typically has a slope and is below grade enough to fit a shower base or other fixture so that the top of the fixture is flush or closer to flush with the floor. Use a construction level to establish the recess needed to accommodate both the slab and the fixture. Care must be required to prevent leakage issues as the water pipes are hidden below the ground.
Proper waterproofing and treatment of the slab are provided to prevent leakage or dampness.


Pitch Roof Slab

Pitch roof is an inclined slab. This sloping style typically uses thin tiles that are lightweight and easy to install. This roofing material commonly requires steel or wood framing to support it. It isn’t ideal for big spans without support. This weight saving decreases the wood or steel structural demands resulting in significant cost savings. Tile-sheets are tailor-made for each project providing labor cost savings and decreased site waste.