Concrete Slabs and its Types

    Hi there, Welcome to "thecivilengineer18.com".

   There are various structural members in a Concrete structure like the Footing, Column, Beams, and Slab. But Slabs are the first member that takes up the load from the floor of a framed concrete structure or Load-bearing Structure. So there are specific aspects that need to be followed while designing and constructing the slabs and there are various types based on their dimension and position.

    There are various types of slabs but in this article, we discussed the Conventional type of slabs as well as unconventional beams and their classification..

Conventional Beams

What are conventional beams?

    These are the slabs that were used for most of the cases in the residential and commercial buildings where there is no special designs were applied. Simply these were the traditional slabs / normal slabs / actual slabs

    So, Conventional slabs are the slabs where the reinforcements are connected with the beams of the framed structure so that the loads get transferred to the beams then to columns, and then to the footing where it is transferred to the ground.

    Conventional Slabs were classified based on their dimensions and bending into

1. One way slab
2. Two-way slab

One Way Slab.

    In one way slab the bending occurs only in one direction i.e. the bending occurs in the shorter span(Length), Hence the main reinforcement is given in the shorter span and the distributor reinforcement is given in the longer span. To easily find whether a slab is one way to use this formula.

Ly/Lx > 2

where,

Ly - Longer Span(length) of the slab

Lx - Smaller Span(Length) of the Slab

    From the above formula, if the value is Greater than or equal to 2 then we can say that it is a one-way slab.

    These slabs were used mostly in Portico's & for Less or low load-bearing areas.

Two-Way Slab.

    In a way slab, the bending occurs only in both directions i.e. the bending occurs in the shorter span(Length) as well as in the longer direction, Hence the main reinforcement is given in the shorter span as well as in the longer span. To easily find whether a slab is one way to use this formula.

Ly/Lx < 2

where,

Ly - Longer Span(length) of the slab

Lx - Smaller Span(Length) of the Slab

    From the above formula, if the value is less than 2 then we can say that it is a two-way slab.

    These slabs were mostly used in apartments and multi-storeyed Building

Unconventional Beams

    Unconventional beams are beams that deviate from the standard formats in which the beams were constructed. Such as the slabs may be designed or developed in such a way that the loads from the beams may be transferred directly to the columns or instead of a single beam numerous small beams run through the slab. These kinds of beams were known as unconventional beams.

1. Flat Plate slabs
2. Flat slabs
3. Ribbed slabs
4. Waffle or Grid slabs
5. Hollow Deck slabs
6. Bubble Deck slabs
7. Hardy slabs
8. Dome slabs
9. Pitched roof slabs
10. Post tension slabs
11. Pre tension slabs
12. Prefabricated slabs
13. Arched slabs
14. Cantilever slabs
15. Low roof slabs
16. Sunken slabs
17. Slab on Grade or Grade slabs
18. Composite slabs

Flat Plate Slab

    In Flat plate systems, the loads from the slabs are transferred directly to the columns or lead-bearing walls instead of beams. The Flat-plate system of slabs may be a way or two-way slab system. This is the most commonly used type of slab system in Multi-storey buildings due to its easy preparation of formwork (shuttering) for construction. However, the limitation of this flat plate slab system is that all the slabs need to be in uniform thickness or near uniform thickness with a flat soffit.

Advantages

1. Formwork can be easily done because easy linear slab fabrication 
2. Since there were no or fewer beams the underfloor service will be very easy.
3. The Floor height can be reduced if we are using a Flat plate slab, hence more floors can be constructed within a smaller height than a conventional slab/ building.

Disadvantage

1. Can't be able to take heavy loads like beam-supported slabs.
2. To tackle shear columns need to be larger in dimension or shear reinforcements need to be provided at the columns.
3. Can't be suitable for masonry partitions.

    These slabs were mostly used in Underground parking and Multi-story buildings.

Flat Slabs

     In the Flat slab System, the slabs were directly connected with the columns instead of beams like the flat plate slab but the only difference is that the column will have a drop panel or prop to tackle the shear force. These flat slabs greatly help architects and engineers in constructing unique design buildings due to the absence of beams the building can be easily shaped in the way we want.

    Types - Flat Slab with drop panels, Flat Slab with column head.  

Advantages

1. Rapid Construction
2. Flexibility for designers
3. Flexibility for the proprietor/owner
4. Reduced story height
5. Easy MEP installation

Disadvantages

1. Increased thickness of the slab
2. Restriction of span
3. Larger dimensions of columns

    These types of slabs were mostly used in unsymmetrical buildings and underground parking.

Ribbed Slabs

    These slabs were one-way joist systems in which numerous joists/ridges ran along the length of the slab. The ribs are placed at regular intervals

Advantages

1. The slab is thinner than other beams
2. Minimal Reinforcement is required

Disadvantages

1. Risky when used in sloped topography
2. Floor-to-floor height will be increased than flat slabs

    These slabs were used in Public buildings, Galleries' & Museum 

Waffle or Grid Slab

    Waffle slab which has several grids instead of beams. These grids/joists/ridges were made of reinforced concrete that ran in both directions. But the greatest advantage that it has is that this slab can be used for slabs that are greater than 40m in length. Waffle Slabs are a two-way joist system, which means the joist runs in both directions.

Advantages

1. Bigger Span slabs can be constructed with fewer materials
2. Aesthetically pleasing, you can make a good design out of it, if you plan well
3. If holes are provided between the ribs, MEP can be easily done.

Disadvantages

1. Can't be used in sloped sites, only can be used in flat topography
2. Waffle slabs have greater thickness than flat slabs hence it is not suitable if you are planning for a multi-storey building since you need to provide greater floor height than flat slabs.

    These slabs were used in Public buildings, Museum & Airports

Hollow Deck Slab

    Hollow block slabs is a precast slab also known as voided slabs or hollow core plant, it uses the principle of less concrete in the tension area of the slab. So keep the middle area of the slab hollow with mild support at the middle and more tension reinforcement at the bottom. Hollow deck slabs were the most preferred type of construction for both residential and commercial buildings in nonseismic areas. The Joints between one slab and the other need to be properly grouted to prevent leakages. It is very helpful in the precast concrete structures. 

Advantage

1. Since the middle is hollow considerable quantity of concrete is reduced which reduces the self-weight of the slab.
2. Since less in weight, it is easy to transport
3. It can be produced in factories using manufacturing units that take less labor
4. Can be produced on a large scale in a shorter period.        

Disadvantage

1. Since hollow, the steps on the upper floor can be easily heard on the lower floors and proofing needs to be done with soft floor covering or a screed needs to be provided.

    These were greatly used in the construction of Bridges, Wall Panels, Multi multi-storey prefabricated structures.

Bubble Deck slab

    Bubble Deck slab works on the principle of replacing the concrete in the middle of the slab which doesn't have any structural purpose with hollow plastic spheres. By placing the plastic sphere the dead weight of the slab can be reduced and a large amount of concrete material can be saved. The dead weight is dramatically reduced by using this bubble deck method.

Advantages

1. Economic savings
2. Environment impact - Greater reduction in concrete which leads to less production and hence less pollution
3. They are fireproof

Disadvantages

1. The reinforcement needs to be specially made to fit the bubbles in the bubble deck slab

    These types of slabs were literally used for all types of buildings from apartments to multi-story buildings, etc.,

Hardy Slabs

    These slabs were made by placing hard hollow concrete blocks in an orderly fashion with proper spacing between them which were known as ribs. These ribs were then reinforced with steel and concrete was poured into the ribs until the blocks were completely submerged into the concrete.

    These slabs were literally used in all types of buildings from normal residential buildings, to Hospital, Office buildings and public buildings.

Dome Slabs

    Dome slabs were used from times immemorial from ancient times. The advantage is that it can take up a lot of compressive strength but less tension hence reinforcement needs to be given properly. In ancient times these domes were in greater thickness because they didn't know how to tackle the tension. But after the invention of steel, these became easy. But still, dome structures are more expensive than other flat slabs because it is rarely used you can get a roof terrace with a dome, so most modern residential and commercial building won't have these dome slabs. However, it is extensively used in special buildings for aesthetical purposes.

    These were used in museums, meditation halls, galleries, and religious structures. 

Pitched Roof Slabs

    Pitched Roofs are sloped roofs that are heavily used in high rainfall and snowfall areas. Proper Design and reinforcement need to be provided. Nowadays, these pitched roof becomes an aesthetic slope in non-rainfall and snowfall areas.

    These slopes were heavily used in resorts, public buildings, and restaurants to be unique buildings to attract customers and these roofs were extensively used in heavy rainfall & snowfall areas.

Post-tensioned Slabs

    Post-tension slabs were made in situ prepared slabs where the tendons(Steel wire) were placed in the slab before concreting and they were stressed after the concrete was hardened to increase the tension-carrying capacity of the slabs. 

    These post-tension techniques were extensively used in a flyover to connect pieces of prefabricated slabs to make the full flyover.

Pre-Tensioned Slabs

    These were slabs that were made as same as the post tension the only difference is that the tensioning of the tendons was made before the concrete hardened completely. When the concrete gets hardened completely the load is then transferred to the slabs.

Prefabricated Slabs

    These were the slabs that were made in the factory or they were cast in the site and then placed in the respective places in the structure. The prefabricated slabs can be any one of the following types:

1. Pre-tension slabs
2. Post-tension slabs
3. Hollow core slabs
4. Ribbed Slabs

    This kind of prefabricated slabs was greatly used in high-rise buildings where the speed of construction can be greatly increased due to the mass production possible with prefabricated structures.

Arched Slabs

    Arched Slabs were mostly provided in the tunnels and Bridges. In the Tunnel arched slabs prevent the soil or hard strata above from falling into the tunnel. In Bridges, the arched slab helps the high-speed winds to get deflected. Proper Design and Reinforcements need to be provided for the arched slabs.

    These were greatly used in Bridges and Tunnel.

Cantilever Slab

    Cantilever slabs were the slabs that were supported on one end and freely hung on the other end. Normally the cantilever slabs need not protrude more than 4 feet outside the support. If it needs to get extended then it needs to be properly supported by heavy beams to support the cantilever slabs.

    They were mostly used for residential and aesthetic purposes. 

Low roof slab

    These were the slabs that were provided at the lintel level, they were used in the residential and in some commercial buildings for storage purposes.

Sunken Slabs

    Sunken Slabs were the slabs that were provided in the restroom and washroom of a floor where the slab level is below the normal slab so that the wet area (Restroom) water doesn't enter the Dry area( Normal Room). Another advantage of this type of sunken slab is that it provides a cover for the water & sewage pipelines. However proper waterproofing needs to be done to prevent water leakage. But the sunken slab concept is suitable for similar floor plans for all floors or else a proper false ceiling needs to be provided.

    These types of sunken slabs have become very common in most of the residential and commercial buildings nowadays.

Slab on Grade or Grade Slab

    These slabs were the slabs that were constructed at the grade beam level. Before the slab is laid the ground needs to be properly compacted to prevent differential settlement. These slabs don't need that much steel reinforcement because they mostly take the compression. The thickness of the slabs needs to be slightly higher than that of the normal slabs.  

    These types of slabs were mostly used for car parking and ground floors.

Composite slabs

    When an object is made of two or more materials then it is known as composite material for example alloys are composite materials. In the same manner in composite slabs, the base of the slab is provided with a steel deck to increase the torsional stability during bucking.

    These were extensively used in the steel structures.