Civil Engineers deal in different types of concrete slabs for roofing, and flooring. Some of these slabs are precast and some are cast in-situ.
Two of the most common concrete slabs are one way and two way slabs depending on the ratio of slabs. You can decide the type of type of slab depending on the use and application of structure.
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What is slab in construction?
But what is a slab in construction?
Well, we can define a concrete slab as a 2d structural element that is generally several inches thick. It takes area loading from structural point of view as load is distributed across the entire area.
The slab (ceiling or flooring) transfer the area load to the supporting beams, columns, walls or the ground. Besides, it provides a flat useful surface for the occupants of the structure or a building.
Anyhow, slabs in construction can either be of reinforced concrete or unreinforced concrete.
The need of reinforcement depends on the design requirements and loading conditions.
Reinforced concrete slabs are further classified depending on the type of supports as well as the ratio of slab spans.
Applications and Uses of Concrete Slabs
Here’re some common uses of concrete slabs:
– Concrete slabs provide a flat and horizontal surface for residential and commercial buildings.
Concrete slabs one above the other, act as a ceiling or roof that provides protection to the occupants from outside elements.
In parking lots or multistory buildings, inclined concrete slabs are provided as a ramp to let vehicles enter or exit.
Lean and thin slabs are used for exterior paving. These slabs are called mud slabs.
Some concrete contractors provide leaner slabs as a patio along the premises of a house.
Medium-sized precast concrete slabs find application as false flooring for modern facilities.
Experts recommend concrete slabs as pool decks. You can have your slab printed, stained or even textured to match the design and appearance of your exterior building.
What are the types of concrete slabs?
Following are the main types of Slabs we discuss in detail ahead:
One Way Slab
Two Way Slab
Ground Bearing Slab
Hollow Core Ribbed Slab
1.One Way Slab
This classification of slab is based on the ratio of spans and the sides of support. A one way slab is supported on two sides only with ratio of long to shorter span as smaller than or equal to two. In this type of slab, the bending is along the shorter span i.e. only in one direction. Main rebar is along the shorter span while distribution rebar is along the longer span.
One way slab can be two types:
One way slab supported on beams at two ends
One way slab with joists (ribs) – called ribbed slab
In most of the buildings, we provide cast in-situ one way slab to support on edge beams. In-situ method includes fixing of formwork, installation of reinforcement, and pouring of concrete. It is an cheap slab for smaller spans of 3 to 6 meter. It is ideal for loads of 3 to 5 KN/m2.
For spans greater than 6 meters, experts recommend one-way joist slab. It is a ribbed slab that have tapered ribs equally spaced at distance lesser than 750 mm. These ribs are supported on girder that further rests on columns. You may need lesser amount of reinforcement for this slab but at the same time, expensive formwork is required.
2.Two Way Slabs
Two way slab is commonly used in framework structure. Unlike one way slab, it has support on all the four sides and it bends in a bowl of shape.
Anyhow, it is ideal for areas where you need span of 6 to 9 meters and live load is 3 to 6 kN/m2. Because of more support on sides, it has lesser deflection and has more stiffness.
But you need to provide more reinforcement and the formwork required would be more.
For a two way slab, the ratio of longer-span to shorter-span is less than 2. The load is carried in both directions.
3.Ground Bearing Slab
As the name suggests, it is the simplest type of concrete Slab that is mainly touching the ground. We also name it as “on-ground” and “slab-on-grade”. It typically has a square mesh of reinforcement at the bottom of the slab with 50 mm cover. But reinforcement is not always necessary. For areas with good subgrade support, reinforcement can be omitted.
Ground bearing slab is used in areas
Where the ground is flat and little slope exists
The subgrade is stable and fire.
Subgrade soil is gravely and non-clayey or non-expansive.
The load is static and more specifically uniform.
Ground bearing slab is cheap and useful for ground floors of residential and commercial buildings. The thickness of the slab is more than 100mm. But the thickness of slab depends on the type of loading and the soil bearing capacity. To avoid shrinkage cracks, we provide construction and contraction joints at 6 meter intervals least.
If you opt to use unreinforced slab, be sure to provide apt curing after casting. The curing would allow good hydration reaction and avoid shrinkage cracks during the process.
Suspended slab is opposite of what we know in ground bearing slabs.
Suspended concrete slabs are:
Not in contact with the ground or earth
are above-ground level
Are like floors or roofs of a structure.
Span a space of 3 to 6 meters
Suspended slabs act like a cantilever. They span a distance and provide unobstructed passage like a roof. These slabs “MUST” have reinforcement for safety. But despite that, you need to ensure thickness of 100 to 500 mm. But, the thickness depends on the type of loading, number of supports, and the span length.
One of the most crucial aspects in construction of suspended slab is formwork and scaffolding.
Two of the most common type of suspended slabs are:
one way slab
two way slab
As the name suggests, unreinforced slab doesn’t have reinforcement against bending. That’s why it is the cheapest of all other types of concrete slabs. In some cases only shrinkage or temperature reinforcement is provided. They are mostly at-grade slabs.
We found applications of these slabs in:
Yards and decks
Concrete pavements and highways
Screeding for floors
Reinforced concrete slabs gives savvy answers for a large number of circumstances. Anyhow, you need to make sure the service loading (static or dynamic) stats well within the limits of concrete’s flexural strength.
Mud Slab is a low-quality thin layer of concrete. In most common cases, it is only 2 to 6 inches thick, poured underneath the primary Slab yet over a layer of wet or sloppy soil. It can fill in as the surface for the waterproofing layer is applied.
Mud slabs has uses in subfloors, crawlspaces, pathways, paving, and leveling surfaces. We mostly use them when we need clean and flat surface. Mud slabs are like blinding or lean concrete where you need a flat surface to fix reinforcement.
Recently, some manufacturers are providing, precast concrete slab panels or bricks. They are placed on a yard above a well-compacted ground. These slabs prevent muddy conditions in your courtyards.
7.Flat Slab and flat plate slab
A reinforced concrete slab that rests directly over columns is known as a flat slab. You won’t need a beam or extra support. In most of the cases, only extra reinforcement is provided at the zones near columns to avoid punching shear. Another type of similar slab is a flat plate slab.
In flat plate slab, there’s no column capital or drop panel. A column capital is flaring up of the columns at their joint with the slab. This provides extra supporting area at the periphery of the column. In case of drop panel, a thickened part of slab is provided at few meters along the periphery of the column.
If you have a span of less than 8 meters and live load is less than 5 kN/m2, you can go with a flat slab without any column capital or drop panel. But in other cases, you may have to use flat slab with column capital or drop panel.
A flat slab is cheap and fast to construct. But it has low stiffness, and you can see deflection even with naked eyes.
8.Hollow Core Slab
If you need fast construction and are looking for a cheap and structurally-efficient slab, hollow core slab has a lot to offer.
Hollow core slab is a precast type of slabs that has hollow cores in their depth. They are light in weight and are can take depth of 110 to 400 mm. In case of precast units, one unit is 1.2 meters wide that are joined together with keys.
The slab has an enhanced qualities of fire-resistance, sound-proofing, and heat insulation. It is a cost-effective solution for modular construction. You can use hollow core slabs for parking garage decks and roof structures. Other uses are floor structures, warehouse, office building, schools, and airport terminals.
Hollow core slabs sometimes are pre-stressed to enhance their efficiency during serviceability.
Hard slabs are generally used in areas of high temperature like in Dubai, UAE, and other gulf countries. The name derives from the use of hardy bricks in the core of the slab.
During construction, we place hardy hollow bricks in the concrete slab below neutral axis. We provide reinforcement between the hardy core blocks. This considerably reduces the quantity of concrete required for the slab.
Though you need to increase the thickness of the slab, but cost-efficiency is at higher-end. Hardy slab roofs tend to be lighter and are very efficient for insulation. The hollow portions of the hardy bricks do the trick of providing air between two layers of concrete slab.
It is one of the most useful slabs to achieve long and unobstructed spans. We found its application in auditoriums and cinema halls. To avoid supports or columns, we provide thick supporting beams at uniform distance in both the directions like a grid.
This grid of supporting beams forms a hollow part between. You can provide artificial lighting for enhanced view like in restaurants, hotels, and shopping malls.
Here I’m, Saad Iqbal – a civil engineering geek and a passionate blogger – with handy graphics designing skills. I’m in the online civil engineering community from over a decade now and have many fans, followers and friends throughout the world.