Types & Construction of RCC Slabs

1. RCC Floor
Hrishi Desai
0813

2. Introduction
A Reinforced Concrete Slab is the one of the most important component in a
building. It is a structural element of modern buildings. Slabs are supported on
Columns and Beams.
RCC Slabs whose thickness ranges from 10 to 50 centimetres are most often used
for the construction of floors and ceilings
Thin concrete slabs are also used for exterior paving purpose.
In many domestic and industrial buildings a thick concrete slab, supported on
foundations or directly on the sub soil, is used to construct the ground floor of a
building.
In high rises buildings and skyscrapers, thinner, pre-cast concrete slabs are slung
between the steel frames to form the floors and ceilings on each level.
While making structural drawings of the reinforced concrete slab, the slabs are
abbreviated to “r.c.slab” or simply “r.c.”.
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3. Types of Design
For a suspended slab, there are a number of designs to improve the strength-
to-weight ratio. In all cases the top surface remains flat, and the underside is
modulated:
Corrugated Slab, usually where the concrete is poured into a corrugated
steel tray. This improves strength and prevents the slab bending under its own
weight. The corrugations run across the short dimension, from side to side.
Ribbed Slab, giving considerable extra strength on one direction.
Waffle Slab, giving added strength in both directions.

4. Waffle Slab Corrugated Slab
Single Ribbed Slab

5. Construction
A concrete slab can be cast in two ways: It could either be prefabricated or
cast in situ.
Prefabricated concrete slabs are cast in a factory and then transported to
the site ready to be lowered into place between steel or concrete beams.
They may be pre-stressed (in the factory), post-stressed (on site), or
unstressed. Care should be taken to see that the supporting structure is built to
the correct dimensions to avoid trouble with the fitting of slabs over the
supporting structure.
In situ (locally on site) concrete slabs are built on the building site using
formwork. Formwork is a box-like setup in which concrete is poured for the
construction of slabs.

6. For reinforced concrete slabs, reinforcing steel bars are placed
within the formwork and then the concrete is poured.
Plastic tipped metal, or plastic bar chairs are used to hold the
reinforcing steel bars away from the bottom and sides of the form-
work, so that when the concrete sets it completely envelops the
reinforcement.
Formwork differs with the kind of slab. For a ground slab, the
form-work may consist only of sidewalls pushed into the ground
whereas for a suspended slab, the form-work is shaped like a
tray, often supported by a temporary scaffold until the concrete
sets.

7. Materials used for the formwork
The formwork is commonly built from wooden planks and boards,
plastic, or steel. On commercial building sites today, plastic and
steel are more common as they save labour.
On low-budget sites, for instance when laying a concrete garden
path, wooden planks are very common. After the concrete has set
the wood may be removed, or left there permanently.
In some cases formwork is not necessary – for instance, a ground
slab surrounded by brick or block foundation walls, where the
walls act as the sides of the tray and hardcore acts as the base.

8. Span – Effective Depth ratios
Excessive deflections of slabs will cause damage to the ceiling, floor finishes
and other architectural details. To avoid this, limits are set on the span-
depth ratios.
These limits are exactly the same as those for beams. As a slab is usually a
slender member the restriction on the span-depth ratio becomes more
important and this can often control the depth of slab required in terms of
the span – effective depth ratio is given by,
Minimum effective depth = span/(basic ratio x modification factor)
The modification factor is based on the area of tension steel in the shorter
span when a slab is singly reinforced at midspan, the modification factors
for the areas of tensions and compression steel are as given in the figure 2
and 4 of the code.

9. Process
Shuttering
The process of constructing the
RCC slab commences by erecting
the centring and shuttering.
Wooden shuttering is usually
provided for the purpose although
steel shuttering is recommended
for getting a good under surface
of the slab. Once the shuttering
has been laid in a level, it has to
be cleaned properly.

10. Reinforcement - The reinforcement
bars are then laid as per design in both
directions and tied properly with
binding wire.
Mixing concrete - All raw materials,
cement, sand, coarse aggregates and
water should be measured as per
requirement of mix design
The raw materials are then mixed
uniformly, while ensuring that a proper
water-cement ratio is maintained which
is just enough to impart sufficient
workability needed to pour concrete.
Ideally, the mixing should be done in a
mechanical mixer. The concrete mixer
should be allowed to rotate for at least
2 minutes.

11. Pouring Concrete
The concrete is then laid
continuously to the required
thickness and compacted using
a needle vibrator to ensure that
the cement slurry adequately fills
the gaps between aggregates.
Vibrated effectively such that no
air gaps are left in the concrete
After the concrete has been
poured, it should be finished with
a steel float to ensure that the
top surface is finished smooth
and to make fine adjustments for
uniform slab thickness.

12. Curing
Curing is critical for a good quality
concrete slab and is is commonly
done by flooding the slab with
water after dividing the slab into
smaller portions with cement
mortar partitions. The curing of
slab should begin the next day of
casting and continue for at least
14 days for adequate concrete
strength. Inadequate curing will
result in poor quality concrete
and can undo the benefit of an
appropriate mix and good quality
mixing and compaction

13. De-shuttering
The last, but very important step.
The side shuttering of the slab can be removed after 48 hours.
The bottom shuttering should not be removed before 14 days.
The under surface of the concrete should be cleaned with steel
brushes such that no impurities are left on the surface.
Although the slab should be constructed under the supervision of a
professional, the owner should oversee that all the precautions are
being taken during construction

15. Advantages
Easy to construct, the raw materials are
easily available in almost all places.
Fire resistant
Provides permanent durable roofing,
capable of multi-storied construction
Very versatile and can be cast in any form.
Being a conventional option, the skill level
for casting RCC slabs is commonly available
Requires very low maintenance.
Limitations
A typical 120 mm slab weighs 300kg/m2.
This has to be designed for a live load of at
least 150kg/m2. This makes the self weight
of the slab as 50% of the design load
resulting in 50% structural efficiency.
The concrete in the tension zone does not
take any stress and thus it is expensive to
provide the same in the tension zone.
The embodied energy of a conventional RCC
slab is high because of its two main energy
intensive constituents cement and steel.

16. Hrishi Desai
0813
Thank You!