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.
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.
There are 20 different types of slabs in practice which are discussed in this article.
Types of Slabs:
1. Conventional Slab (One way slab & Two way slab)
3. Flat plate,
4. Flat slab,
5. Waffle slab (Grid slab),
6. Ribbed slab,
7. Hollow deck slab,
8. Bubble deck slab,
9. Hardy slab,
10. Dome slab,
11. Pitch roof slab,
12. Pre-tensioned slab,
13. Post-tensioned slab
14. Prefabricated slab,
15. Arched slabs,
16. Cantilever slab,
17. Low roof slab,
18. Sunken slab,
19. Slab on grade,
20. Composite slabs.
Miscellaneous Types Of Slab:
Floor slab
Kitchen slab
Lofts
1. Conventional Slab:
Types of conventional 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 the four sides on 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 slabs is more than the one way slabs.
Application:
Most apartments, multi-storeyed buildings.
Advantages of Conventional Slabs:
i) Easy to construct.
ii) It does not require skilled labours.
iii) Easy to install electrical, mechanical and plumbing services.
iv) The formwork are simple.
Disadvantages:
i) Becomes very deep for higher loads.
ii) During the curing period, the formwork cannot be removed.
iii) The concrete in the tension zone has no structural performance but it is provided to simplify the construction.
Application:
Multistorey buildings, framed residential apartments etc.
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.
Advantages:
i) Simplified formwork since no beams are involved.
ii) The height of the floor can be minimized due to the absence of the beams.
iii) Increased floor to floor height.
i) Structural depth is reduced.
v) The mechanical and electrical installations can be installed easily without bending them.
vi) The flat soffit of the beam is easy to construct.
Disadvantages:
i) Increased risk for deflection.
ii) Due to the absence of the beams, special design considerations should be made to take care of the shear forces.
iii) Reduced stiffness.
iv) Large spans cannot be constructed.
v) Brittle partitions like the brick masonry are not suitable for flat plates.
vi) Increased column size.
Applications:
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 minimise 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.
Advantages:
i) The height of the building can be saved.
ii) The floor to floor height can be reduced.
iii) Shorter construction time if big table formwork is used.
iv) The mechanical and electrical installations can be installed easily without bending them.
Disadvantages:
i) Increased risk for deflection.
ii) Large spans cannot be constructed.
iii) Brittle partitions like the brick masonry are not suitable for flat slabs.
Applications:
Underground parking, unsymmetrical buildings.
5. Waffle Slabs Or Grid Slabs:
Waffle slabs are lightweight slabs with hollow grid-like systems on its 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 the light weight, they can span to 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.,
Advantages:
i) Lighter sections and therefore less self weight leading to economic foundations.
ii) Long spans can be constructed.
iii) The construction materials used for the slab can be saved.
iv) Very economical when modular coordination is followed for the form works.
v) High load carrying capacity than conventional slabs.
vi) Less use of concrete leads to less CO2 emissions.
Disadvantages:
i) The reduced depth of the slabs between the grids increase the risk of fire transmission.
ii) The construction of waffle slabs is also tedious since they are demanding special and sophisticated formwork.
iii) The mechanical and electrical installations are very difficult to install because of the complex design in the soffit.
iv) The height between the floors is increased and thus not economical.
v) It is not suitable for buildings subjected to sudden vibrations or constant vibrations.
vi) Not suitable for regions with high wind velocities due to the low self weight.
vii) Skilled workers are required.
viii) High maintenance.
ix) Not economical for small projects.
x) Difficult to repair.
Application:
Public buildings, museums, airports, etc.,
6. Ribbed slabs:
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 of the waffle or grid slab.
Applications:
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.
Advantages:
i) Lighter sections and therefore less self-weight leading to economic foundations.
ii) Long spans can be constructed.
iii) The materials used for the slab can be saved
iv) Prefabrication lead to faster construction.
v) It can be fully loaded immediately after placing.
vi) It gives a fuller and pleasing appearance from below.
Disadvantages:
Not suitable for regions with high wind velocities due to the low self weight.
ii) Skilled workers are required.
iii) Connections are difficult and tedious in a precast structure.
iv) Cranes are required for the handling of the slabs.
v) It is time consuming if it is a Cast in-situ construction.
vi) Not economical for small projects.
vii) Difficult to repair.
Applications:
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 of 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.
Advantages:
i) Lighter sections and therefore less self weight leading to economic foundations.
ii) Long spans can be constructed.
iii) The materials used for the slab can be saved.
iv) Very economical when formworks are repeated for every floors.
v) High resistance to explosions.
vii) Less use of concrete leads to less CO2 emissions.
vii) The recycled plastics are used thus reducing the plastics in the landfill.
Disadvantages:
i) The construction of bubble deck slabs is tedious since they demand special and sophisticated formworks.
ii) It is not suitable for buildings subjected to sudden vibrations or constant vibrations.
iii) Not suitable for regions with high wind velocities due to the low self weight
iv) Skilled workers are required.
v) Deflection is more
vi) Punching shear capacity is less
vii) Not economical for small projects
viii) Difficult to repair
Application:
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 it. 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.
Advantages:
i) Lighter sections and therefore less self weight leading to economic foundations.
ii) Long spans can be constructed.
iii) The materials used for the slab can be saved.
iv) The hollow space improves the insulation for sound and heat.
v) Less use of concrete leads to less CO2 emissions.
Disadvantages:
i) The construction of hardy slabs is tedious since they demand special and sophisticated formworks.
ii) Skilled workers are required.
iii) Deflection is more.
iv) Not economical for small projects.
v) Difficult to repair.
Applications:
Hospitals, office buildings, residential buildings, 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.
Advantages:
i) Gives a pleasant look.
ii) Increased service area inside the building.
iii) Increased height improves the natural ventilation and lighting.
Disadvantages:
i) Skilled labours are required.
ii) The vertical expansion of the building is not possible.
iii)The construction time is more.
iv) High maintenance.
v) The formwork shuttering is tedious.
Applications:
Monumental buildings, museums, meditation halls, galleries, 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 designing must be made beforehand to ensure the smooth construction of the pitched roofs.
Precautions:
i) The mix design of concrete should not be tampered.
ii) Proper formworks with sufficient support should be provided.
iii) The concrete has to be placed from bottom to top.
iv) The tiles, if placed on the slab shall be custom made to reduce wastage.
v) If tiles are not placed, weather protection coats should be used.
Advantages:
i) Gives a pleasant look.
ii) Increased thermal performance.
iii) Saves materials due to less depth of the slab.
iv) Increased service area inside the building.
v) Increased height improves the natural ventilation and lighting.
Disadvantages:
i) Skilled labours are required.
ii)The formwork shuttering is tedious.
iii) The vertical expansion of the building is not possible.
v) The construction time is more.
Applications:
Resorts, public buildings, 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 the 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.
Advantages:
i) Lighter sections can be achieved.
ii) Longer spans.
iii) Reduced thickness in the structural member cuts the footing size.
iv) High load carrying capacity.
v) Faster construction.
vi) Less deflection.
Disadvantages:
i) Uneconomical for small projects.
ii) Connections must be sealed properly.
iii) Increased risk of corrosion.
iv) Bursting forces may be high.
13. Pretensioned Slabs:
Pretensioned prestressed slabs are prestressed before the concrete hardens. The high strength tendons are pulled and the slab is casted. After the slab has sufficiently hardened, the prestress 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
Advantages:
i) High quality control can be observed in factories.
ii) Less labours are used for the construction.
iii) It is very economical if the same moulds are used for repetitive floor plans.
iv) They can be dismantled and reassembled wherever possible.
v) Construction time is reduced.
vi) Formworks are not needed.
vii) The slabs will be of uniform quality.
Disadvantages:
i) Cranes are needed for erection.
ii) Uneconomical for small projects.
iii) It may get damaged during transportation and erection.
iv) During transportation the length of the member is limited.
v) Connections are complex.
Applications:
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 the modern developments are giving birth to reinforced curved arch slabs.
Application:
Bridges, buildings. tunnels.
16. Cantilever Slab:
Cantilever slabs, as the name suggests are fixed at one end and free at the other ends. The deflection in cantilever slabs is greater than the other slabs due to its support condition. One great example of the 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.
Advantages:
i) It improves the elevation of the building.
ii) It does not require any support on the other side.
iii) It is easier to construct.
Disadvantages:
It is not economical since it requires heavy sections.
The deflection is more.
The uplift at the fixed support has to be taken into account while designing.
Applications:
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.
Applications:
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 for checking water leakage in the pipes.
Application:
Bathrooms, toilets, washrooms, utility areas.
19. Slab On Grade:
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 is increased towards the edges to increase the structural integrity of the slab. Unlike conventional slabs, the grade slabs don’t always require reinforcements but will require expansion joints.
Advantages:
i) Does not need tedious formworks and scaffolding.
ii) Easy to construct.
Applications:
Used for open parking, ground floors.
20. Composite Slabs:
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.
Advantages:
i) The thickness of the slab can be reduced.
ii) Reduction in self weight also reduces the footing cost.
iii) No need for extra formworks.
iv) Economical.
v) Increased resistance against deflection and torsion.
vi) Longer spans can be constructed.
vii) Speedy construction.
Disadvantages:
i) Vulnerable to corrosion.
ii) Skilled labours.
iii) Vulnerable to fire.
Applications:
Steel structures, Tall buildings.
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