Slabs and its varieties used in Construction
Slab and Its Importance
Concrete slab, a very common and important structural element, is constructed to provide flat, useful surfaces. It is a horizontal structural component, with top and bottom surfaces parallel. The depth of a concrete slab floor is very small compared to its span. Slabs may be supported on two sides only or have beams on all four sides.
On the basis of reinforcement provided, beam support, and the ratio of the spans, slabs are generally classified into one-way slab and two way slab. The former is supported on two sides and the ratio of long to short span is greater than two. However, the latter is supported on four sides and the ratio of long to short span is smaller than two.
The importance of slabs is mentioned below:
- Provide a flat surface
- To support load
- Sound, heat and fire insulator
- Act as a divider (privacy) for the occupants
- Upper slab became the ceiling for the storey below
- Space between slab and ceiling can be used to place building facilities
Kinds of Slabs
Keeping in mind of the type of building,aesthetic features, and the architecture slabs must be selected: Here we are going to discuss about different kinds of slabs and its merits and demerits
- Conventional slabs
- Flat slabs
- Flat plates
- Waffle slabs
- Hollow core slabs
- Ribbed slab
- Bubble Deck Slab
- Hardy Slab
- Dome Slab
- Pitch Roof Slab
- Pre Tensioned Slab
- Post Tensioned slab
- Prefabricated Slab
- Arched Slab
- Cantilever Slab
- Low Roof Slab
- Sunken Slab
- Slab On Grade
- Composite Slab
Conventional Slabs are of two types One way slab and two way slab
One way Slab
One way slab is supported by beams on the two opposite sides to carry the load along one direction. The ratio of longer span to shorter span is >= 2. such slabs bends in the direction along its shorter span. Main reinforcement is provided in shorter span and distribution reinforcement is provided in longer span. Distribution bars are cranked to resist the formation of stresses.
Two way Slab
Two-way slabs are supported on all sides. Slabs on beams are suitable for spans between 6-9m, and live loads of 3-6KN/m2 . The reinforcements in both direction increase the stiffness of the slabs, producing relatively low deflection. Additional formwork for the primary and secondary beams is needed.
These type of reinforced slabs are supported directly by columns or caps, without the use of beams. This type of slab is generally easy to construct and requires little formwork. The loads are directly transferred to the columns.
Flat plates can be constructed as either one-way or two-way slabs and are directly supported by columns or walls and are easy to construct and requires simple formworks.
Flat plates are most suitable for spans of 6-8m, and live loads between 3-5 KN/m2. The range of spans for prestressed flat plates is between 8-12m, and it can also be constructed as post tensioned slab.
This type of slab contains square grids with deep sides. Waffle slab construction process includes fixing forms, placement of pods on shuttering, installation of reinforcement between pods, installation of steel mesh on top of pods, and pouring of concrete.
Grid slabs are suitable for spans of 9-15m and live loads of 4-7 KN/m2.
Hollow Core Slab
It is a type of precast slab through which cores are inserted. The slab self weight decreases considerably, and structural efficiency increases, the cores also acts as Service Ducts. It is suitable for cases where fast constructions are desired. There is no restriction on the span of the hollow core slab units, and their standard width is 120mm and depth ranges from 110mm to 400mm.It is suitable for offices, retail or car park developments.
It is constructed using hardy bricks which significantly decline the amount of concrete and hence the slab self-weight. The thickness of slab is commonly greater than conventional slab at around 270mm.
It is economical for spans of length up to 5m, and it reduces the quantity of concrete below neutral axis, and moderate live loads shall be imposed. It is constructed at locations where the temperatures are very high. The application of this type of slab can be seen in Dubai and China.
This kind of slab is generally constructed in temples, Mosques, palaces etc. And Dome slab is built on the conventional slab. The thickness of Dome slab is 0.15m. Domes are in the semi-circle in shape and shuttering is done on a conventional slab in a dome shape and concrete is filled in shuttering forming dome shapes. This is one of the types of concrete slabs.
Pitch Roof Slab
Pitch roof 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.
Pre Tensioned Slab
The slab which is tensioned before placing the slab is called Pre tension slab. To pretension concrete the steel is first tensioned in a frame or between anchorages external to the member.
The concrete is then cast around it. After the concrete has developed sufficient strength the tension is slowly released from the frame or anchorage to transfer the stress to the concrete to which the tendons have by that time become bonded.
The force is transmitted to the concrete over a certain distance from each end of a member known as the transfer length.
Post Tensioned Slab
The slab which is tensioned after constructing a 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 which is in tension and to make better use of its compression strength. The principle is easily observed when holding together several books by pressing them laterally.
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, post stressed or unstressed.
This is a type of slabs which is generally adopted during Bridge Construction. Bridges are subjected to two loads moving loads from the vehicles and Wind load. Slab with arches or (Arch slab) are adopted at a place where there is a need for redirecting the load of the wind 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.
Cantilever Slab are common features in buildings due to the need to have bigger spaces at upper floors. To get this architects normally extend the slab beyond ground floor, building line, thereby forming a cantilever.
Low Roof Slab
The slab which provided above the door for storage purpose is called Low roof slab. This type of slab is closed at all ends and opens at one end. This slab lies below the actual slab and above the door sill level. These types of concrete slabs are used in houses.
The sunken slab is one of the familiar and adaptable ways to maintain architectural design in especially the washroom, as well as laundry areas where pipes are installed below the floor or tiles. This system is accepted to provide an easy way to install sanitary products and built eye-catching views in those areas.
Slab on Grade
Slabs which are directly laid on the earth, to support walls and other structural elements are called as Slab on grade or Grade Slabs.
This type of slab is casted directly on ground level. Grade slab itself acts as a foundation for the building which does not need any footings.
Composite slabs comprise reinforced concrete cast on top of structured steel decking, which acts as formwork during construction and external reinforcement at the final stage. The decking may be either re-entrant or trapezoidal, as shown below.
Trapezoidal decking may be over 200 mm deep, in which case it is known as deep decking. Additional reinforcing bars may be placed in the decking troughs, particularly for deep decking. They are sometimes required in shallow decking when heavy periods of fire resistance are combined with heavy loads.
The floor system of a structure can takes many forms such as in situ solid slab, pre-cast units, ribbed slab etc. Slabs may be mounted on either steel or monolithic concrete beams, walls or even directly over columns. Concrete slab behaves primarily as a flexural member and its design philosophy is similar to that of beams.