Reinforced concrete slab is crucial
structural element which provide flat surfaces in buildings. slabs are
classified into one-way slab and two-way slab. The former is supported on two
sides only and 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 2.
Different types of reinforced concrete slab for instance one-way solid slab, one-way joist slab, flat slab, flat plate, two-way slabs on beams, waffle slab, hollow core slab, precast slab, slabs on grade, hardy slab, and composite slab can be used for various buildings. The selection of sufficient and cost effective concrete slab is based on type of building, architectural layout, aesthetic features, and the span length.
1. One-Way Slabs on Beams
Cast in situ method is used to construct one-slabs on beams. Initially, forms are fixed, then reinforcements are installed, and finally fresh concrete is poured.
One-way slabs on beams are most suitable for spans of 3-6m, and a live load of 3 to 5KN/m2. They can be used for larger spans with relatively higher cost and higher slab deflection. Additional formwork for the beams is 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 live 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 concrete.
Grid slabs are suitable for spans 9-15m of and live loads of 4-7KN/m2. Formwork, including the use of pans, is quite expensive.
4. Flat Plates
Flat plates could be constructed as
one-way or two-way slabs and it is directly supported by columns or walls. it is
easy to construct and require 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 fast construction. Flat plates have low shear capacity and relatively low stiffness, which may cause noticeable deflection.
5. Flat Slabs
a 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 post-tensioned flat slabs.
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 9m, 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
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.
is no restriction on the span of the hollow core slab units, and their standard
with 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 is reported 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
constructed using hardy Bricks which significantly decline the amount of
concrete and eventually slab self-weight. The thickness of hardy slab is
commonly greater than conventional slab and around 270mm.
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 up to 5m, and it reduces quantity of concrete below neutral axis, and moderate live loads shall be imposed. It is constructed in locations where temperature is 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 then reinforcement are placed between and over plastic bubbles and then fresh concrete are 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 also environmentally friendly since it reduces 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 steel decking thickness of 50-60mm, the span of the slab can reach up to 3m. however, if the steel decking thickness increased to 80mm, slabs with span of 4.5m can be constructed.
11. Precast Slab
slabs are casted and cured in manufacturing plants, and then delivered to the construction
site to be erected. The most outstanding advantages of plants is the increase
in efficiency and higher quality control which may not be achieved on site.
The most commonly
used precast slabs the channel and double-T types. They can be used for spans
up to 15m. The double-T slabs vary in sizes and spans of up to 15m have be
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 casted on the surface of the earth is called a Ground slab. Generally,
slab on grade are classified into three types:
1. Slab on ground
It is simplest type of slab on ground which
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 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 slab are only suitable for very flat ground.