Concrete Slab Construction



Slabs are constructed to provide flat surfaces, usually horizontal, in building floors , roofs, bridges, and other types of structures. The horizontal slabs of steel reinforced concrete, typically between 4 and 20 inches (100 and 500 millimeters) thick, are most often used to construct floors and ceilings, while thinner slabs are also used for exterior paving.
The slab may be supported by walls, by reinforced concrete beams usually cast monolithically with the slab, by structural steel beams, by columns, or by the ground. The depth of a slab is usually very small compared to its span.

A concrete slab may be prefabricated or on site. Prefabricated concrete slabs are built in a factory and transported to the site, ready to be lowered into place between steel or concrete beams. They may be pre-stressed (in the factory), post-stressed (on site), or unstressed. It is vital that the wall supporting structure is built to the correct dimensions, or the slabs may not fit. Wikipedia

Also check: Beam structures

Types of slab and their uses

In this article, different types of slab are discussed based on their classification including the one-way slab and two-way slab and you’ll get to know their uses.

1. Flat Slab: This is a reinforced concrete slab supported directly by concrete columns or caps. Flat slab can be describe as beamless slab because they are supported on columns. In this type of slab the load are directly transferred to the columns.
The primary function of a flat slab is to provide plain ceiling surface giving better diffusion of light and they are generally used in parking decks, commercial buildings, hotels or places where beam projections are not desired.

2. Conventional Slab: The type of slab are supported with Beams and columns is known as conventional slab. In this kind of slab the thickness of slab is small whereas depth of beam is large and load is transferred to beams and from beams to columns. It requires more formwork when compared with the flat slab. and there is no need of providing column caps in conventional slab. The thickness of conventional slab is 4″ or 10cm. 5″ to 6″ inches is recommended if the concrete will receive occasional heavy loads, such as motor homes or garbage trucks.

Conventional slab are classified into two types based on their length and breath:

  • One Way Slab: This is a type of conventional slab which is supported by beams on the two opposite sides to carry the load along one direction. The ratio of longer span (l) to shorter span (b) is equal or greater than 2, considered as One-way slab because this slab will bend in one direction i.e in the direction along its shorter span.
  • Two Way Slab: This is a type of conventional slab supported by beams on all the four sides and the loads are carried by the supports along both directions. In two way slab, the ratio of longer span (l) to shorter span (b) is less than 2.The slabs are likely to bend along the two spans in this load is transferred in both the directions to the four supporting edges and hence distribution reinforcement is provided in both the directions.

3. The hollow core ribbed slab: This slab have between four and six longitudinal cores running through them, the primary purpose of the cores being to decrease the weight, and material within the floor, yet maintain maximal strength. It is inserted between the beams by the use of tower cranes and the slab is filled with screed.
Hollow core ribbed slabs have excellent span capabilities, achieving a capacity of 2.5 kN/m over a 16m span. The long-span capability is ideal for offices, retail or car park developments. It primary function is that, it is an excellent fire resistance and sound insulation are another attributes of hollow core slab due to its thickness and it is easy to install and requires less labour.

4. Hardy Slab: This slab is constructed by hardy Bricks. Hardy bricks are hollow bricks and made up of concrete, they are used to fill portions of the slab thickness it saves the amount of concrete and hence the own weight of the slab is reduced.
These kinds of slabs are generally seen in Dubai and China.
Hardy slab is used where temperatures are very high. To resist the temperature from top of the slab thickness is increased. The heat coming from walls are resisted by using special bricks which has thermacol in it. Thermacol is the best insulator of sunlight.
Hardy slabs are further classified into two types:

  • One way Hardy Slab
  • Two way Hardy slab

5. Waffle Slab: this type of slab is a reinforced concrete roof or floor containing square grids with deep sides and it is also called as grid slabs. Waffle slab is majorly used at the entrance of hotels, Malls, Restaurants for good pictorial view and to install artificial lighting and where we find a hollow hole in the slab when the formwork is removed. It primary function is that, it can carry heavier loads and span longer distances than flat slabs as these systems are light in weight and can be used as both ceiling and floor slab.
Waffle slab are classified into two based on the shape of Pods (PVC Trays), they are:

  • Triangular pod system
  • Square pod system

6. Dome Slab: This is the type of slab that is generally constructed in temples, Mosques, palaces etc. It is built on the conventional slab. Thickness of Dome slab is 0.15m, they are semi-circle in shape and the shuttering is done on a conventional slab in a dome shape and concrete is filled in shuttering forming dome shapes. This is a one of the types of concrete slabs.

7. Pitch roof slab: This is an inclined slab , generally constructed on resorts for a natural look. Compared to traditional roofing materials Tile-sheets used in pitch roof slab are extremely lightweight. This weight saving reduces the timber or steel structural requirements resulting in significant cost savings. Tile-sheets are tailor made for each project offering labour cost savings and reduced site wastage. And the thickness of the slab is depends on the tiles we using it may be 2″-8″. This is a one of the types of concrete slabs.

8. Slab with Arches: This is a type of slabs which is generally adopted in the construction of bridges. Bridges are subjected to two loads moving load from the vehicles and Wind load. These slabs are adopted at a place where there is a need of redirecting wind load and if there is a long curve in direction of slab these slabs are adopted. It resists the fall of the bridge due to heavy wind load.
They were originally built of stone or brick but these days are built of reinforced concrete or steel. The introduction of these new materials allow arch bridges to be longer with lower spans. This is a one of the types of concrete slabs.

9. Post tension slab: The slab which is tensioned after constructing slab is called Post tension slab. Reinforcement is provided to resist the compression. In Post tension slab the reinforcement is replaced with cables/ steel tendons.
Post-tensioning provides a means to overcome the natural weakness of concrete in tension and to make better use of its
strength in compression. The principle is easily observed when holding together several books by pressing them laterally. Linder such pressure the whole row gains enough stiffness and strength to ensure its integrity.
In concrete structures, this is achieved by placing high-tensile steel tendons/cables in the element before casting When the concrete reaches the desired strength the tendons are pulled by special hydraulic jacks and held in tension using specially designed anchorages fixed at each end of the tendon. This provides compression at the edge of the structural member that increases the strength of the concrete for resisting tension stresses. If tendons are appropriately curved to a certain profile, they will exert, in addition to compression at the perimeter, a beneficial upward set of forces (load balancing forces) that will counteract applied loads , relieving the structure from a portion of gravity effects. This is a one of the types of concrete slabs.
In this type of slab, cables are tied instead of reinforcement. In Steel reinforcement, the spacing between bars is 4inch to 6inch where as in Post tension slab the spacing is more than 2m.

10. Pre Tension Slab : The slab which is tensioned before placing the slab is called Pre tension slab. The slab has same features of Post tensioning.

11. Cable suspension slab: If the span of the slab is very long, then we go for cable suspension slab which is supported on cable such as London bridge, Howrah bridge etc. Generally, in the construction of houses for every 4m, we provide a column whereas in cable suspension slab for every 500m we provide a column. This kind of slab is provided where the length of the span is more and difficulty in building columns. The slabs are tied with cables and these cables are joined to columns.

12. Low roof slab: This is the type slab which provided above the door for storage purpose is called Low roof slab. This slab is closed at all ends and open at one end and it lies below the actual slab and above the door sill level. These types of concrete slabs are used in houses.

13. Projected slab: This type of slab has one side fixed and the other side is free and it is generally constructed in hotels, Universities, function halls, etc. to use that area for dropping or picking up zone and for loading and unloading area. This is a one of the types of concrete slabs.

14. Grads Slab/ Slab on grade: This type of slab is casted on the surface of the earth is known Ground slab. It is used in the Basement floor.
Grade slabs is of two types:

  •  Usually after casting Plinth beams. Sand is filled at an height of 0.15m and then Sand level is rammed. Then PCC is poured on sand upto a height of Plinth beams. Its an economic way of constructing ground slab which used.
  • In high rise buildings after constructing Plinth beam the termite control is done in between the beams and then polythene sheet is laid to avoid termites inside the slab and then steel mesh is provided and concrete is filled. This costs more when compared with the previous one and requires more concrete than the first one.

15. Miscellaneous Slabs: This type of slab are use for multiple purpose and it can be constructed for room chajja, kitchen slab, lintels, sun shade slab etc.

  1.  Room Chajja or Loft : This kind of chajja (Slab) is provided in drawing rooms and kitchen for storing House material. The usual difference between low roof slab and Room chajja is Low roof slab hides house material and whereas Room Chajja or Loft doesnt hides house material they are open and provided above the door side. This is a one of the types of concrete slabs.
  2. Kitchen Slab: The slab is provided in the kitchen for its platform. For placing stove and other kitchen, material is called Kitchen Slab. It has a breadth of 0.5m and length of wall and thickness is 2″.
  3. Lintels: Lintels are provided Inside building above the doors and windows to re direct the top load. There are two types of lintels.
  • Pre cast Lintels: Lintels which are manufactured in factories is called Pre cast Lintels.
  • Cast in situ: Lintels are casted at the site itself is called Cast in situ lintels.
    The length of the lintel is more than door length and has a width of the wall, thickness of lintel is 0.1m
  • Sun Shade slab: Sun Shade is provided outside building above the Doors and windows are called Sun shade slab. The slab stops rain to come inside the building and direct sunglight. This is a one of the types of concrete slabs.

16. Sunken Slab: Slab which is provided below the washrooms to hide the sewage pipes or sewerage pipes is called Sunken slab. Since the pipes that carry water are concealed below the floor, care has to be taken to avoid leakage problems. After casting sewage pipes in the slab the slab is filled with coal or broken pieces of bricks. There are two types of the sunken slab. The slab which is provided below the normal floor level at a depth of 200mm to 300 mm and filled with broken pieces of bricks is called Sunken slab and the Slab which is provided above the normal floor level at a height of 200mm to 300mm and filled with coal or broken pieces of bricks is also called Sunken slab .

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