A reinforced concrete slab is a crucial structural element and is used to provide flat surfaces(floors and ceilings) in buildings. 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.
Varying conditions and stipulations ask for the selection of appropriate and cost-effective concrete slab, keeping in view, the type of building, architectural layout, aesthetic features, and the span length. Concrete slabs, therefore, are further classified into one-way joist slab, flat slab, flat plate, waffle slab, hollow core slab, precast slab, slabs on grade, hardy slab, and composite slab.
1. One-Way Slabs on Beams
Cast in situ method is used to construct one-way slabs on beams which involves fixing of forms followed with the installation of reinforcements, and finally the pouring of fresh concrete.
One-way slabs on beams are most suitable for spans of 3-6m, and a live load of 3 to 5KN/m2. They can also be used for larger spans with relatively higher cost and higher slab deflection. Additional formwork for the beams is however needed.
2. One-way joist slab (Ribbed slab)
It consists of a floor slab, usually 50 to 100mm thick, supported by reinforced concrete ribs (or joists). The ribs are usually tapered and are uniformly spaced at distances that do not exceed 750mm. The ribs are supported on girders that rest on columns.
A one-way joist concrete slab is suitable for spans of 6-9m and loads of 4-6KN/m2. Because of the deep ribs, the concrete and steel quantities are relatively low, but expensive formwork is needed.
3. Waffle Slab (Grid slab)
It is a type of reinforced concrete slab that 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 4-7KN/m2. Formwork, including the use of pans, is quite expensive.
4. Flat Plates
Flat plates can be constructed as one-way or two-way slabs and it is directly supported by columns or walls. It is easy to construct and requires simple formworks.
Flat plates are most suitable for spans of 6 to 8m, and live loads between 3 and 5KN/m2. Added to that, the range of spans for prestressed flat plates is between 8-12m, and it can also be constructed as post-tensioned slabs.
The advantages of adopting flat plates include low-cost formwork, exposed flat ceilings, and faster construction. Flat plates have low shear capacity and relatively low stiffness, which may cause noticeable deflection.
5. Flat Slabs
This is typically a reinforced slab 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 slabs are most suitable for spans of 6 to 9m, and for live loads of 4-7KN/m2. They need more formwork than flat plates, especially for column capitals. In most cases, only drop panels without column capitals are used. It can be constructed as a post-tensioned flat slab.
6. Two-way Slabs on Beams
The construct of this type of slab is similar to that of one-way slab on beams, but it may need more formworks since two-way slabs are supported on all sides. Slabs on beams are suitable for spans between 6 and and live loads of 3-6KN/m2 . The beams increase the stiffness of the slabs, producing relatively low deflection. Additional formwork for the beams is needed.
7. Hollow core slab
It is a type of precast slab through which cores are run. Not only do these cores decline slab self-weight and increase structural efficiency but also act 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.
The slab units are commonly installed between beams using cranes and the gaps between units are filled with screeds. It has been observed that, hollow core slab can support 2.5 kN/m2 over a 16m span. It is suitable for offices, retail or car park developments.
8. Hardy Slab
It is constructed using hardy bricks which significantly decline the amount of concrete and eventually the slab's self-weight. The thickness of slab is commonly greater than conventional slab and around 270mm.
The construction of hardy slab involves formwork installation, hardy block placement, placement of reinforcement into gaps between blocks, placement of steel mesh on the blocks, and finally pouring of concrete.
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.
9. Bubble Deck Slab
It is constructed by placing plastic bubbles which are prefabricated and the reinforcement is then placed between and over plastic bubbles and finally, fresh concrete is poured. The plastic bubbles replace the ineffective concrete at the center of the slab.
Bubble Deck slabs reduce weight, increase strength, larger spans can be provided, fewer columns needed, no beams or ribs under the ceiling are required. Consequently, not only does it decline the total cost of construction but it is also environmentally friendly since it reduces the amount of concrete.
10. Composite Slab
Commonly, it is constructed from reinforced concrete cast on top of profiled steel decking. The decking acts as formwork and working area during the construction phase, and it also acts as external reinforcement during service life of the slab.
For a steel decking of thickness between 50-60mm, the span of the slab can reach up to 3m. However, if the steel decking thickness is increased up to 80mm, slabs with span of 4.5m can be constructed.
11. Precast Slab
Precast concrete slabs are cast and cured in manufacturing plants, and then delivered to the construction site to be erected. The most outstanding advantage of the preparation of slabs in manufacturing plants is the increase in efficiency and higher quality control which may not be achieved on site.
The most commonly used precast slabs are the channel and double-T types. They can be used for spans up to 15m. The double-T slabs vary in sizes and spans up to 15m have been used.
The tongue-and-groove panel could vary in size based on the design requirement. When they are placed, the tongue of one panel is placed inside the groove of adjacent panel.
With regard to the cost of precast slabs, it is reported that precast concrete slabs are cheaper than cast in situ concrete slab by approximately 24%.
12. Slab on grade
The slab which is cast on the surface of the earth is called a Ground slab. Generally, slab on grade is classified into three types:
1. Slab on ground
It is the simplest type of slab on grade which is a composite of stiffening beams constructed from concrete around perimeter of the slab, and has a slab thickness of 100mm. It is suitable for stable ground which is mostly composed of sand and rock and not influenced by moisture, and soils that undergo slight movement due to moisture.
2. Stiffened raft slab
It is similar to slab on ground apart from stiffening beams which are set in channels through the middle of the slab. Consequently, it creates a kind of supporting grid of concrete on the base of the slab. Soil with moderate, high amount, and severe movement due to moisture.
3. Waffle raft slab
It is constructed entirely above the ground by pouring concrete over a grid of polystyrene blocks known as 'void forms'. Waffle raft slabs are generally suitable for sites with less reactive soil, use about 30% less concrete and 20% less steel than a stiffened raft slab, and are generally cheaper and easier to install than other types. These types of slabs are suitable only for very flat ground.
Frequently asked Questions
The major types of concrete slabs used in construction are one-way joist slab, flat slab, flat plate, waffle slab, hollow core slab, precast slab, slabs on grade, hardy slab, and composite slab.
The slab which is cast on the surface of the earth is called a Ground slab. It can be waffle raft slab, stiffened raft slab or slab on ground type.
The most commonly used precast slabs are the channel and double-T types. They can be used for spans up to 15m. The double-T slabs vary in sizes and spans up to 15m have been used.
Bubble Deck slabs reduce weight, increase strength, larger spans can be provided, fewer columns needed, no beams or ribs under the ceiling are required. Consequently, not only does it decline the total cost of construction, but it is also environmentally friendly since it reduces the amount of concrete.
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