Slab is one of the most important elements In a structure. Learn about different types of slabs in this article.
What Is Slab?
Slabs are flat, horizontal structural elements made of reinforced concrete that receive the load and transfer it through the beams to the columns and to the footings to the soil below. Slabs are used in both load-bearing structures and framed structures.
In a load bearing structure, the load is transferred from the slab to the load-bearing walls. The thickness of the slab varies from 100 mm to 500 mm. There are 20 different types of slabs in practice which are discussed in this article.
The methods for slab construction vary from each other. Based on the support condition, a slab can be a simply supported slab, cantilever slab or a continuous slab.
A slab can be a floor slab or a roof slab. The live load acting on the floor slabs is greater than the roof slabs. Thus, the floor slabs are heavily reinforced when compared to the roof slabs.
Types Of Slabs
1. Conventional Slab
The slab which is supported by beams and columns, is called conventional slab. In conventional slab the thickness of the slab is small but depth of the beam is large. The load is transmitted from slab to beam and then from beam to column.
These types of slabs are provided with reinforcement where the horizontal bar is called main reinforcement and vertical bar is called distribution bar. Conventional slabs are two types.
- One way slab
- Two way slab.
a. One Way Slab
One way slab is a conventional horizontal slab rectangular in shape and supported by beams on two of its four sides. The slabs are supported on beams opposite to each other. Therefore, the deflected shape is cylindrical.
In one way slabs the longer span is more than twice of the shorter span. The ratio of longer span to the shorter span is greater than two. As a result, the shorter span is subjected to bending.
The main reinforcements are provided in the shorter directions to resist the bending and the secondary reinforcements are provided in the longer direction.
Application: Veranda slab.
b. Two Way Slab
Two-way slabs are also conventional slabs mostly rectangular in shape and supported on all four sides of a beam. In two-way slabs, the longer span is less than twice the shorter span. The ratio of longer span to shorter span is lesser than two. The deflected shape of the slab is like a dish or saucer shape.
In this case, both the shorter and longer directions are subjected to bending. Therefore, the main reinforcements would be provided in both directions based on the magnitude of bending. The two-way slabs are economical to a size of 6m x 6m. Also, the quantity of steel used in two-way slab is more than the one-way slab.
Most apartments, multi-storeyed buildings.
3. Flat Plate
Unlike conventional slabs, in flat plates, the loads are not transferred from the slabs to the beams. In flat plates, the loads are transferred directly to the columns. The flat plate system may be one-way or two-way depending on the design plan.
Underground parking, multi-storey buildings
4. Flat Slab
Flat slabs are the modified version of flat plates with a column head and/or a drop panel cast monolithically with the slab. There are no beams present in the flat slabs but the drop panels and column heads will transfer the loads smoothly to the columns.
The drop panels are square or rectangular in shape and increase the shear capacity of the slab. The drop panels add deflection to the slab and thus minimize the deflections.
The column heads are provided below the drop panels and they are mostly sloping to meet the column dimensions. A flat slab may have either column head or drop panel or both.
The flat slabs are mostly popular in unconventional structures without column symmetry. The column heads and drop panels act as a special beam confined to that particular space. However, the formworks become complicated due to the need for column heads and drop panels.
These types of slabs are used in underground parking and unsymmetrical buildings.
5. Waffle Slabs Or Grid Slabs
Waffle slabs are lightweight slabs with hollow grid-like systems on their soffit. The hollow grid system reduces the self-weight of the slab without compromising its structural stability.
The grids are spaced at equal intervals and they transfer the load from above to the beams. The space between the ribs is sometimes treated as a beam and reinforced to increase the flexural rigidity of the slab.
Due to their lightweight, they can span long distances with ease. The waffle slabs may have a grid system or a girder system where the bands of beam run throughout the slabs. The grids can be or any shape – square, rectangular, triangular etc.,
These types of slabs are used in public buildings, museums, airports, etc.,
6. Ribbed Slab
Ribbed slabs are very similar to waffle slabs but often mistaken for being the same. A ribbed slab has wide grids or bands of beams in the soffit of the beam whereas a waffle slab has deeper corresponding parts.
The ribbed slabs like waffle slabs are flexible, light in section, economical and can be constructed for longer spans. They have the same advantages and disadvantages as the waffle or grid slab.
Public buildings, museums, galleries.
7. Hollow Deck Slab
Hollow deck slabs are special types of prefabricated concrete slabs that use the fundamentals of the stress developed in a section for the design. The concrete takes care of the compression while the steel takes care of the tension. There is no specific use for the concrete in the tension zone other than to provide a solid section.
Taking this principle into account, the hollow deck slab does not provide concrete in the tension zone. It is provided with minimum concrete to hold the steel reinforcements in place. The hollow deck slab can be prestressed or non-prestressed.
Bridges, wall panels, multi-storey prefabricated buildings.
8. Bubble Deck Slab
Bubble deck slabs are hollow slabs where the concrete in the tension zone is replaced by recycled high-density polyethylene or HDPE plastic balls. Bubble deck slabs use the same principle as the hollow core slab. The concrete in the tension zone of a section has no structural importance.
The removal of that concrete will not affect the performance of the structure in resisting the loads acting on it. Bubble deck slabs can reduce the amount of concrete in a slab from 40% to 60%. This reduces the dead weight of the structure and also saves on costs.
Bubble deck slabs are mostly used in public buildings, high rise buildings.
9. Hardy Slab
Hardy slabs are special types of slabs consisting of hardy bricks placed in a uniform fashion inside them. The hardy bricks are hollow blocks made of concrete. The depth of the hardy bricks is usually at least 200 mm. Therefore, the depth of the slab is increased.
The formworks are placed over which the hardy bricks are placed in rows with space between them. This space between the hardy blocks is treated as small beams. Reinforcements are placed within these ribs and concrete is poured and the slab is cast monolithically. It is mostly used in regions of high temperature.
Hospitals, office buildings, residential buildings, and public buildings.
10. Dome Slab
Dome slabs are hemispherical in shape and are used mostly to enhance the view of a structure. In ancient times, most structures were built in such a way that only compressive forces act on the structure.
Before the invention of steel, there were no tensile materials known to man to resist tension. Therefore, the load was transferred through axial compression as in domes. In the modern era, the discovery of steel has led to thin sections of domes.
These concrete slabs are preferably used in monumental buildings, museums, meditation halls, galleries, and religious structures.
11. Pitched Roof Slab
The reinforced concrete pitched roofs are sloping slabs mostly preferred in high rainfall regions. The most common shape of a pitched roof is the triangular shape. Proper design must be made beforehand to ensure the smooth construction of the pitched roofs.
These types of concrete slabs are used in resorts, public buildings, and restaurants.
12. Post-tensioned Slabs
Prestressed slabs are commonly used in the construction of prefabricated buildings. Post-tensioned prestressed slabs have ducts that carry high-strength steel tendons in addition to conventional reinforcement.
The steel tendons are prestressed and anchored at the ends after the concrete slab has hardened. This is called post-tensioning. The post-tensioned slabs are mostly precast and are of various shapes.
13. Pretensioned Slabs
Pretensioned prestressed slabs are prestressed before the concrete hardens. The high-strength tendons are pulled and the slab is cast. After the slab has sufficiently hardened, the prestressing strands are cut loose and the stress is transferred to the slab.
Pretensioned slabs are similar to post-tensioned slabs except for the prestressing part. They have the same advantages and disadvantages. However, post-tensioned slabs are widely used because of their ability to be cast in a shorter period of time.
14. Prefabricated Slabs
Prefabricated slabs are cast in the industry or site and transported to the place of construction. The prefabricated slabs can be of the following types:
- Pre-tensioned slabs
- Post-tensioned slabs
- Hollow core slabs
- Ribbed slabs
High-rise buildings for fast construction.
15. Arched Slab
Arched slabs are mostly found in bridges. Arches like domes, transfer the loads through axial compression. The arches are provided in the bridges to resist the wind loads acting on the bridge. In ancient times, arches were built in masonry, but modern developments are giving birth to reinforced curved arch slabs.
These types of slabs are used in bridges, buildings, and tunnels.
16. Cantilever Slab
Cantilever slabs, as the name suggests, are fixed at one end and free at the other end. The deflection in cantilever slabs is greater than the other slabs due to its support condition. One great example of cantilever slabs is Balconies.
The main reinforcement is provided in the span that is protruding outside. The secondary reinforcements are provided throughout the length of the slab. In RCC work, cantilever slabs should not be extended more than 4 feet.
Cantilever slabs are mostly used in residential buildings.
17. Low Roof Slab
Low roof slabs are also called lofts and are provided at the sill level of a building. The low roof slab is a cantilever slab that is provided above the doors and windows for storage purposes. The low roof slab is constructed during the construction of the lintel beam.
Residential buildings for storage.
18. Sunken Slab
Sunken slabs are the slabs that are provided below the bathrooms and washrooms. The level of the slab is reduced below the bathrooms so as to separate the grade between dry areas and wet areas. The sunken slabs shall also be used to provide cover for sewage pipes and fittings.
Sunken slabs are provided in buildings with identical floor plans. The level of the slab can either be increased or decreased based on the need. Special care has to be taken to check water leakage in the pipes.
These types of slabs are used in bathrooms, toilets, washrooms, and utility areas.
19. Slab On Grade Or Grade Slab:
Slab on grade slab also known as the grade slab is constructed on the ground level and mostly supported by the soil or earth below it. Most of the ground floor slabs are grade slabs.
Before the grade slabs are constructed, the base layer of the earth should be properly compacted to prevent uneven settlements in the future. The base layer consists of well-compacted gravel and moisture barriers.
The depth of the slab should be at least 100 mm and it should be increased towards the edges to increase the structural integrity of the slab. Unlike conventional slabs, grade slabs don’t always require reinforcements but will require expansion joints.
These types of concrete slabs are used for open parking, and ground floors.
20. Composite Slab
An element is said to be composite when it is made up of two or more materials. The composite slab is made up of concrete and steel decking.
The steel decking of trapezoidal, rectangular or re-entrant shape is also used as a formwork over which the reinforcement cages can be placed and concreted. The introduction of the steel decking increases the torsional stability during buckling.
Steel structures, Tall buildings.