Traditionally the concrete floor system is one of the best methods for in situ concrete construction. Concrete floor systems are made of bars, fabric and some strands which are highly stressed. The prestressing with cables in concrete helps it to withstand the deflection. With concrete floor systems, the need to camber formwork can be minimised.
Flooring systems come in all sizes and forms. There are different variables which act as a catalyst in the success of the actual floor system, namely different spans, cantilevers, the extent of continuity and the effects of beams, walls and columns on the concrete slab system. With the advent of technology, we are availed with so many packages which can design the pieces with appropriate loading and with many different combinations of loadings while performing trials.
Building’s nature is as per the design considerations, structural competency, and usage. The amount of usage, type of usage, etc. are important metrics to consider when designing the concrete flooring system. Steel is a major solution which caters to different usage, which also offers flexibility along with adaptability. These solutions are sustained throughout the lifecycle of the building.
Due to its efficiency the in situ concrete slab is used worldwide for building construction. In situ concrete floor, construction is labour intensive, and developing countries take advantage of it being so. The designers also have a free hand when it comes to casting in situ concrete primarily because the vast range of designs which can be executed and cast on site with the appropriate usage of formwork. There are many renowned designers such as Le Corbusier, Louis Kahn, Carlo Scarpa and Tadao Ando. All these artists work in close collaboration with the in situ concrete flooring method.
Here are some of the gratification of in situ concrete flooring:
– Flexibility of shape and size
– Adaptability to any design with different texture and colour
– Casting in “Sandwich” to incorporate polystyrene sheet insulation.
– No maintenance
– Universal Availability
Evolution of in situ concrete floor construction
Until the 20th-century cast, in-situ concrete was the primary method which was used for construction. Now it is widely used in many other countries due to the sheer ease of use and efficiency. Still, there is much more needed on the cost and budgeting front. The structural qualities of the cast in situ concrete are always leveraged entirely by the designers whenever and wherever needed.
Classification of in situ concrete floor
The type of concrete floor depends on upon the way the slabs that are resting on the columns. It is also determined by the kind of support the columns are provided to the slabs. The type of concrete slab usage depends on the usage as well. So where there are a lot of design considerations, different kinds are gauged to choose the best one from them. Here, we analyse various types of concrete floor slab which are used in standard construction practices.
Assortment guides for concrete floors
Selecting the type of concrete floor slab for a particular kind of construction is a tough job. The main aspect which is considered while deciding it is the span of the slab. So for the flat plate flooring system, with a single or double span, reinforced or pre-stressed, the span length lies from 4-12 meters.
The flat slab needs a span length of 6 to 12 meters for multi-span, reinforced or pre-stressed concrete slab.
The ribbed slab which is preferred for high design consideration & needs a span length of 6-9 meters for construction. Band beam and slab can be constructed with a single span of 6 to 14 meters. This is just a brief guide for a comparative span of different floor systems and can be used indicatively.
A flat slab
A one-way or a two-way slab with a thick slab at the column is a flat slab. This thickening is called as the drop panel. These drop panels act as T-beams which fortifies the slab. The drop panels increase the shear capacity and bring more stiffness on the floor so that it deals with vertical loads appropriately.
1. Formwork which is straightforward and easy to work on
2. No beams, the under floor services are simplified outside the drops
3. Less structural depth
4. No shear reinforcement at the column
1. Like flat plate slab, this slab also needs medium span
2. Not suitable for bricks
3. If there is a mechanical ducting, then the drop panel may interfere
4. While doing vertical penetrations area under the drop panel needs to be avoided.
5. The deflection at the middle strip is a critical factor.
A flat plate slab
It is also one or two-way concrete slab system which is supported directly on the columns or walls which are load bearing. These are used most commonly in constructing buildings. The most important features of flat plate floor slab are its near perfect uniform thickness with a plain soffit which needs a basic framework and is the easiest to construct. There is a great flexibility while installing services horizontally.
Advantages of flat plate slab
1. Easy and straightforward construction due to simple formwork with direct fix or ceiling which is sprayed
2. There are no beams
3. Minimum depth and reduced floor height
Disadvantages of flat plate slab
1. Spans are of medium length
2. The lateral loading capacity is limited
3. Needs shear heads or shear reinforcement for a column or at least massive columns
4. Deflection is a controlling aspect
5. Not suitable for supporting brick partition.
6. Inappropriate for heavy loads.
Beam and slab
This is the most common system which is used in the construction of medium level buildings. This consists of frames of beam and column with a span of supporting slab. It is the oldest method which has been used for decades. The depth of the beam provides a floor which is stiff and is capable of holding long spans comfortably. It is also able to resist the lateral loads. However, the coordination of services is a tedious task which has also resulted in less usage of this system.
1. Traditionally tried and tested solution
2. Useful for long spans
1. Large ducts may have problem penetrating through the beams
2. Floor depth
3. The height of the floor
Ribbed floor or waffle floor
This is the most interesting concept of in situ concrete flooring. These are the equally placed ribs which are supported by the column. The one-way slab is called the ‘ribbed slab’, and the two-way slab is called the ‘waffle slab’. This style of flooring, however, is not too common because of its exorbitant costs and a lower fire rating. Heavy loads can be taken by the ribbed slab and has reasonably big span to support. It provides the necessary stiffness to the slab and is suitable when the soffit needs tobe revealed.
Advantages of ribbed or waffle floor
1. Weights and material may be saved
2. Can be used for exorbitantly long spans
3. Attractive appearance
4. Reusable formwork makes it economical
5. Easy for vertical penetrations across the ribs
Disadvantages of ribbed or waffle floor
1. Depth of the slab may be a controlling factor for fire rating
2. Needs specialised formwork and skilled labour
3. High floor to floor height ratio
4. Hard to handle larger vertical penetrations.
Band beams and slab
When there is a line a parallel, shallow beams which are wide, and the floor slab spans over these beams transversely, it is a band beam and slab system. This is a continuous slab, and the beams carry all the load of the slab. Band beams which are also called as slab bands which are thickened are two-way slabs. They are treated as slabs and not beams except for the consideration of shear resistance.
Slab and joist
This is a system which has slabs spanning in between beams and is treated as one directional intermediate joist. The requirement of fire resistance determines the slab thickness.
This is also one of the most used methods for concrete floor construction.
Some of the major types of the concrete slab which can be cast in situ are analysed in this article. The analysis also makes it clear that the usage of the building, time and cost primarily determines the type of slab which needs to be used for that particular type of construction.
Based on the above information it can be concluded that based on the usage once the type of floor system is decided, there should compulsorily be some trial runs with different loading, thickness and other integrated parts of the design. This shouldbe considered as the preliminary design process.
Every project needs to be completed on time and should be as per the budgetary considerations. Usage of different types of slabs spells out the need to identify the one which is synonymous with the available time frame and appropriate budget. Otherwise, it may lead to severe delay and some serious budgetary implications. Foresight is paramount in the business of construction. All designers while considering the designing and aesthetic aspect should also think about the time and budget factors to give way to a beautiful structure, whose beauty and elegance is not limited by worldly constraints.