Journey of slabs: From Conventional slab to Waffle slab

Ramendra Kumar Mishra, Research Scholar, Dr CV Raman University, Bilaspur (C.G.)

Prof. (Dr.) M.K. Tiwari, Professor and Head, Deptt. of Civil Engineering, Dr CV Raman
University, Bilaspur (C.G.)

Abstract: In civil engineering the construction of structures like high-rise buildings is

an integral part of work. During these constructions, the use of concrete slabs is
expedient to provide better strength as well as stability. In the modern era, emphasis is
being laid on the construction of sophisticated and modern buildings which uses modern
and special slab. There are many types of slabs that have been developed over the course
of time. The present study reveals the chronological advancement in the construction
and application of slabs. The waffle slab is the recent type whereas normal RCC can be
considered as a conventional slab.
Keywords: Concrete, slabs, RCC, flat slab, waffle slab.
Introduction:
Historical Background – Concrete is a kind of biphasic composite material that
comprises a matrix (binder paste) that may include a filler (granular skeleton). Cement
is often included in the binder matrix, however, it is not necessary for the composition
of the concrete. Without using cement, concrete may be created by using other accessible
binders. Later, the development of Portland cement by Louis Vicat in 1817, reinforced
concrete by Monier and Lambot, and the involvement of architect Auguste Perret were
the turning points for the broad usage of concrete as a building material, which was in
turn accountable for a new method to construction in the 20th century.
Concrete slabs are often used in modern constructions to provide flat, horizontal surfaces with
structural support. Steel-reinforced slabs, which are typically between 100 and 500mm thick,
are most often utilized to create ceilings and floors, however, smaller mud slabs may be utilised
for outside pavement.
Many homes and businesses have a big concrete slab for their ground floor, which is either
anchored to the ground by foundations or lies on the earth without any further support. It is
common practice to classify these slabs as either suspended or ground-bearing. A slab is
considered ground-bearing if it is directly attached to the foundation, and suspended if it is not.
Several common types of multi-story building slabs include:
Applications for block and beam, also known as block and rib, are mostly found in residential
and commercial settings. Pre-stressed beams and hollow blocks make up this sort of slab, which
is temporarily supported until it sets, usually after 21 days.
a precast hollow core slab that is crane-installed on location.
Thinner, pre-cast concrete slabs are strung between the steel frames of high-rise structures and
skyscrapers to create the floors and ceilings on each level. Along with homes, high-rise
structures and sizable retail centres also utilize cast-in-situ slabs. Shutters and reinforced steel
are used to cast these in-situ slabs on the spot. (Mr Kiran S)
Conventional Slab: The term ‘conventional slab’ is referred to a slab that is supported by
columns and beams. As a result of the contrast between the thin slab and the heavy beam, the
load is transferred to the beams and then the columns. In contrast to a flat slab, it needs
additional formwork. Column caps are not required in traditional slab designs. Conventional
slabs are 4′′ (10 cm) thick. If the concrete may sometimes be subjected to large loads like trash
trucks or motor homes, 5′′ to 6′′ inches is recommended. Buildings with reinforced concrete
frames are often utilised in construction.
Conventional slab or regular slab is supported by beams and columns. In these designs, the
beam is transmitted to the load-bearing beams and subsequently to the columns while the slab's
thickness is quite minimal. When compared to a flat slab, this needs more formwork. On the
normal type slab, column caps are not required.
Standard concrete slabs are 4 m long and square in form. A typical slab of concrete is reinforced
with main (primary) reinforcing bars that are horizontally organised and distribution bars that
are vertically distributed. (N Krishna Raju)

Fig. 1 Conventional Slab

Depending on their length and width, these types of slabs are divided into 2 categories:
i. 1-Way Slab
ii. 2-Way Slab
Advantages:
i. High break time since the beams are quite deep.
ii. Beams provide lateral load resistance.
iii. slab's thinness makes it lightweight.
Disadvantages:
i. The height of the structure will rise from floor to floor.
ii. The building's lighting and ventilation are hindered by beams.
iii. The formwork is complex.
iv. Reinforcement is complex.
v. It has a long construction span.

Flat slab
These slabs are beam-less structures that are very useful today. In flat slab structures, we only
add panels to the tops of the columns and thicken the slab. Flat slab structures are more practical
because their floor-to-floor heights are reduced, they require less material to build, and they
meet various architectural requirements.
Flat slab characteristics
• Layout flexibility in the room
• Saving in building height
• More rapid construction
• M&E service installation is simple.
• Buildable score
• Usage of prefabricated welded mesh
Types of flat slab: Flat slabs come in a variety of varieties that are utilised in building. These
consist of:

Simple flat slab

These are typically crafted from concrete and are held up without the need for beams by the
use of caps and columns. The flat slab requires nothing in the way of formwork and may be
constructed quickly and easily.
Loads of the building are transmitted to the columns so that the structure may remain
effectively balanced. These basic flat slabs can support spans between 6 and 9 metres in length.
The "span" refers to the horizontal space between the building's two separate pillars. When it
comes to dynamic loads, flat slabs excel up to 7 kN/m2.
Flat slab featuring drop panels
"Flat slabs with drop panels" is referred to the slabs that have column capitals and drop panels.
The drop panel is referred to the slab's greater thickness at its highest column. These drop
panels may significantly increase the flat slab's shear resistance.
The drop panels reinforce the flat slab, making the structure more resilient to natural disasters.
Negative moment capacity refers to the bending moment produced by the combined action of
compressive stress at the beam's base and tensile stress at its apex. The total stiffness of the
slab is increased while the deflection is substantially decreased due to these flat plates.

Fig. 2 Flat slab with drop panels

Flat slab featuring column heads
A column head is an extension of reinforced concrete that sits on top of a column and
functions as an essential component of the overall structure. For architectural purposes, the
angles of such column heads may be customised to be anything the architect wants. However,
if you want to imprint designs into the concrete, you should pour it at a 45-degree angle on
both sides of the column.

Fig. 3 Flat slab with a column head

Flat slab: Problems
Punching shear failure during flat slab design is a significant problem. Extreme localised stress
is what causes the flat slab to break. This usually happens towards the base of the column or
where the column joins the slab. (Sarita R)
When a column's focused support response pushes against the slabs, cracks develop all across
the loaded regions of the slab, resulting in a truncated collapse of the slab. This is known as a
punched shear failure. To overcome this problem, the following strategies are used:
Increase the slab's overall thickness, taking into account drop panels and column heads.
By making the columns larger, the shear perimeter may be hidden.
Give the necessary shear reinforcement. (Ahmed Ibrahim)
Advantages
Because of its advantages over alternative reinforced concrete floor systems, flat slabs are
frequently used by engineers in construction. Some of the most salient benefits of flat slabs are
as follows:
• Flexibility in room layout.
• Partition walls can be placed anywhere.
• False ceilings can be omitted.
• Owners may choose room layouts.
• Flat slab reinforcement is simpler to install.
• Reinforcement placement is easier.
• Flat slab uses a big table structure.
• Ease of Framework Installation.
• No beams minimise floor height and building height.
• Foundation load will also reduce.
• Building height can be reduced.
• The vertical member might be saved by 10%.
• Big table structure speeds building.
• Less construction time.
• Standard sizes
• Prefabricated welded mesh.
• Better quality control.
• Less installation times
Disadvantages
The main drawbacks are listed below.
• Span length is medium.
• Large spans cannot be obtained using a flat plate method.
• Not advised for delicate (masonry) partitions.
• Critical middle strip deflection.
• Drop panels may block larger mechanical ducting.
• The center strip deflection may be crucial in flat slabs.
• Higher slab thickness. In comparison to the typical reinforced concrete 2-way slab
design, flat plate slabs have a greater thickness.

Flat Slab: Uses

The columns and beams in a conventional structure serve the function of providing support for
the slab. Because beams aren't necessary when using the flat slab method, which eliminates the
need for them, the slab itself is directly supported by the columns. A flat slab is a common
strategy for creating asymmetrical column layouts due to its practicality, adaptability in interior
design, and simplicity of construction.
Flat slabs are often used in construction because they allow for greater aesthetic variety and
light dispersion, and they also reduce the amount of formwork that is necessary for the building.
It is common practice to make use of flat slabs in the construction of irregular column layouts,
such as storeys with bending shapes, ramps, and other similar elements, in public venues such
as parking garages, skyscrapers, theatres, and other similar elements.
Using flat slabs provides a solution for the problem of depth, as well as favourable conditions
for the building of flat soffits and increased design arrangement versatility. Flat slabs provide
a high degree of flexibility and may be employed in situations where there is a possibility that
the interior layouts may need to be changed in the future.
Waffle Slabs can be defined as “A reinforced concrete slab with equally spaced ribs parallel
to the sides, having a waffle appearance from below. A Waffle Slab is a type of building
material that has two-directional reinforcement on the outside of the material, giving it the
shape of the pockets on a waffle”. (Anjali Mishra)
The bottom of a waffle slab is supported by concrete joists that span in opposite directions and
are built of reinforced concrete. The R.C. ribs are known as waffles because of the grid pattern
they produce. Another name for it is a 2-way joist slab. It is often utilized when the span
exceeds 12 metres. It is more durable than other slab types. 2 sections make up the slab. The
two parts—the top section, which has a flat surface, and the bottom half, which is made up of
joists—combine to form a grid-like structure. When the grid has had any mould removed from
it, it appears. Additionally, it is used when a building is being loaded heavily. When a structure
has to be stiff yet still have minimum vibration, like a laboratory or industrial facility, this kind
of slab is utilized. (Ulfat)
A waffle slab considerably increases structural stability without adding a lot of extra material
to a substance. A waffle slab is therefore ideal for broad, flat regions like floors or foundations.
Construction of reinforced concrete roofs and floors using a grid of square, deep ribs with
coffers between them. (Anupoju)

Fig. 4 Waffle slab

Waffle Slabs Characteristics
• In general, flat areas benefit from using waffle slabs.
• Comparatively speaking, relatively little concrete is utilised.
• Reinforcement for waffle slabs may be found in the form of either mesh or individual
bars.
• There is no need for a separate excavation to be carried out for the beams in the case of
a waffle slab.
• The bottom of the slab has a waffle-like appearance that was created with cardboard
panels, pods, etc.
• Waffle slabs should be between 85 and 100 mm thick, with a maximum slab depth of
between 300 and 600 mm.
 • Waffle slab beams or ribs typically range in width from 110 to 200 mm.
• Rib spacing should be between 600 and 1500 mm.
• Up to 16 metres, reinforced waffle slabs may be built, but for spans longer than that,
prefabricated waffle slabs are preferred.
• Waffle slab is less expensive than strengthened rafts and footing slabs and is effective
against shrinking.
• Only 70% of the concrete and 80% of the steel used for the stiffening raft are needed
for the waffle slab. (Anurag Sharma)
Advantages
• In general, flat areas benefit from using waffle slabs.
• Comparatively speaking, relatively little concrete is utilised.
• Reinforcement for waffle slabs may be found in the form of either mesh or individual
bars.
• In the case of a waffle slab, a separate excavation for beams is not necessary.
• The bottom of the slab has a waffle-like appearance that was created with cardboard
panels, pods, etc.
• Waffle slabs should be between 85 and 100 mm thick, with a maximum slab depth of
between 300 and 600 mm.
• Waffle slab beams or ribs typically range in width from 110 to 200 mm.
• Rib spacing should be between 600 and 1500 mm.
• Up to 16 metres, reinforced waffle slabs may be built, but for spans longer than that,
prefabricated waffle slabs are preferred.
• Waffle slab is less expensive than reinforced rafts and footing slabs and is effective
against shrinking.
• Only 70% of the concrete and 80% of the steel used for the stiffening raft are needed
for the waffle slab.
Disadvantages
• Not ideal for locations with substantial live loads, such as large industrial facilities and
warehouses.
• The thickness of the slab is controlled to ensure that it satisfies the requirements for fire
resistance.
• Formwork cost is expensive.
• Installation of electrical apparatus might provide a number of challenges at times.

Conclusions and Comments:

It has been determined that the waffle slab is preferable to the flat slab and conventional slab
based on the results of numerous investigations and discussions. The chronological
development of slab systems has played a tremendous role in concrete technology and high–
rise building design in the whole world. The advancement in building construction has opened
a market for researchers and developers to use eco–friendly and cost–effective construction of
slabs. Besides these, the requirements of longer span, stability against seismic activities etc.
also put emphasis on the design of modern types of slabs. In this way, we see the development
of flat and waffle slabs for building construction.
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