Reinforced Concrete Structure and Detailing Module

Civil Engineering Department Saad.bilbas@epu.edu.iq 155

 Reinforced Concrete Slab
Slabs are constructed to provide flat surfaces, usually horizontal in building floors,
roofs, bridges, and other types of structures. The slab may be supported by walls
or by reinforced concrete beams usually cast monolithically with the slab or by
structural steel beams or by columns, or by the ground. Slabs are classified into 16
types.
Different Types of concrete slabs in construction:-
There are 16 different types of Slabs in Construction. Some of them are outdated
and many of them are frequently used everywhere. Thus it will be given a detailed
explanation of each slab where to use particular slab.

1. Flat Slab:-
The flat slab is a reinforced concrete slab supported directly by concrete columns
or caps. Flat slab doesn’t have beams so it is also called as beam-less slab. They
are supported on columns itself. Loads are directly transferred to columns. In this
type of construction, a plain ceiling is obtained thus giving attractive appearance
from an architectural point of view. The plain ceiling diffuses the light better and is
considered less vulnerable in the case of fire than the traditional beam slab
construction. The flat slab is easier to construct and requires less formwork. This is
one of the types of concrete slabs. The thickness of the flat slab is minimum 8″ or
0.2m.

Flat Slabs are used at:
• To provide plain ceiling surface giving better diffusion of light
• Easy constructability with the economy in the formwork
• Larger headroom or shorter store height & pleasing appearance.
• This kind of slabs are provided in parking
• Flat slabs are generally used in parking decks, commercial buildings, hotels or
places where beam projections are not desired.

Advantages of Flat Slab:
• It minimizes floor-to-floor heights when there is no requirement for a deep
false ceiling. Building height can be reduced
• Auto sprinkler is easier
• Less construction time
• It increases the shear strength of the slab
• Reduce the moment in the slab by reducing the clear or effective span
Disadvantages of Flat slab:
• In flat plate system, it is not possible to have large span
• Not suitable for supporting brittle (masonry) partitions
• Higher slab thickness
There are four different types of concrete Flat Slabs:-
1. Slab without drop and column without column head(capital)

2. Slab with drop and column without column head
3. Slab without drop and column with column head
4. Slab with drop and column with column head

2. Conventional Slab:-
The slab which is supported on Beams and columns is called conventional slab. In
this kind, the thickness of slab is small whereas depth of the beam is large and load
is transferred to beams and then to columns. It requires more formwork when
compared with the flat slab. In conventional type of slab there is no need of
providing column caps. The thickness of conventional slab is 4″ or 10cm. 5″ to
6″ inches is recommended if the concrete will receive occasional heavy loads,
such as motor homes or garbage trucks.
Also, Conventional concrete slabs are square in shape. Reinforcement is provided
in conventional slab and the bars which are set in horizontal are called Main
Reinforcement bars and bars which are set in vertical are called Distribution bars.

Based on length and breadth of Conventional Slab is classified into two types:
 One-Way Slab
One way slab is supported by beams on the two opposite sides to carry the load
along one direction. The ratio of longer span (l) to shorter span (b) is equal or greater
than 2, considered as One-way slab. In this type slab will bend in one direction i.e in
the direction along its shorter span. However minimum reinforcement known as
distribution steel is provided along the longer span, the main reinforcement to
distribute the load uniformly and to resist temperature and shrinkage stresses.
In general length of the slab is 4m. But another side length is more than 4m. So it
satisfies the above equation. Main reinforcement is provided in shorter span and
distribution reinforcement is provided in longer span. Distribution bars are cranked
to resist the formation of stresses.

 Two-Way Slab
Two way slab is supported by beams on all the four sides and the loads are carried by
the supports along both directions, it is known as two way slab. In two way slab, the
ratio of longer span (l) to shorter span (b) is less than 2. The slabs are likely to bend
along both the directions to the four supporting edges and hence distribution
reinforcement is provided in both the directions.
𝑳𝒐𝒏𝒈 𝑺𝒑𝒂𝒏
𝑺𝒉𝒐𝒓𝒕 𝑺𝒑𝒂𝒏
𝟏
𝒃
In this kind of slab, the length and breadth of the slab are more than 4m. To resist the
formation of stresses distribution bars are provided at both the ends in two way slab.

3. Hollow core ribbed Slab or Hollow core slab:-
Hollowcore ribbed slabs derive their name from the voids or cores which run
through the units. The cores can function as service ducts and significantly reduce
the self-weight of the slabs, maximizing structural efficiency. The cores also have
a benefit in sustainability terms in reducing the volume of concrete used. Units are
generally available in standard 1200 mm widths and in depths from 110mm to 400
mm. There is total freedom in length of units. These type of slabs are Pre-casted
and it is used where the construction has to be done fast.
Moreover, the primary purpose of the cores being to decrease the weight, and
material within the floor, yet maintain maximal strength. To further increase the
strength, the slabs are reinforced with 12mm diameter steel strand, running
longitudinally. These are types of concrete slabs.

Hollow core slab installation:-
By using tower cranes Hollow slabs are inserted between the beams. The gaps
between the slab is filled with screed.
Screed is a concrete material generally are used 20mm aggregate in concrete whereas
in screed also are used baby chips(small broken stones) as aggregates.
Hollow core ribbed slabs have excellent span capabilities, achieving a capacity of 2.5
kN/m2 over a 16m span. The long-span capability is ideal for offices, retail or car
park developments. Units are installed with or without a structural screed, depending
on requirements. Slabs arrive on-site with a smooth Pre-finished soffit. In car parks
and other open structures, Pre-finished soffits offer a maintenance free solution.

Hollow core slab Advantages
1. Hollow core ribbed slab not only reduces building costs it also reduce the overall
weight of the structure
2. Excellent fire resistance and sound insulation are another attributes of hollow core
slab due to its thickness
3. It eliminates the need to drill in slabs for electrical and plumbing units
4. Easy to install and requires less labor
5. Fast in construction
6. No additional formwork or any special construction machinery is required for
reinforcing the hollow block masonry

Hollowcore slab Disadvantages:
1. If not properly handled, the hollow core ribbed slab units may be damaged during
transport.
2. It becomes difficult to produce satisfactory connections between the precast
members.
3. It is necessary to arrange for special equipment for lifting and moving of the
precast units.
4. Not economic for small spans.
5. Difficult to repair and strengthen
4. Hardy Slab:-
Hardy slabs are generally seen in Dubai and China. Hardy slab is constructed by
hardy Bricks. Hardy bricks are hollow bricks and made up of concrete Hollow
blocks. These blocks are used to fill portions of the slab. Hardy slabs saves the
amount of concrete and hence the own weight of the slab is reduced. This kind of slab
has a more thickness 0.27m when compared with the conventional one. The method
of installing Hardy slab is different from normal and it is clearly explained below:

The process of hardy blocks execution is
as follows:
Step 1 : Formwork is arranged and then
shutters are fixed on the formwork.
Step 2 : Hardy blocks are placed on the
shutter with one brick gap on the entire
shutter.
Step 3 : The gaps between the bricks are
called a rib. Reinforcement is provided in
a form of the beam within the gap.
Step 4 : After placing the rib, the plane
steel mesh is placed on entire slab area
resting on ribs.
Step 5 : Now pouring of concrete is done
on a slab.

Hardy slab can be used where temperatures are very high. To resist the temperature
from top of the slab thickness is increased. The heat coming from walls are resisted
by using special bricks which has thermacol in it. Thermacol is the best insulator of
sunlight.
Advantages of Hardy Slab:
1. Reducing slab weight by reducing the amount of concrete below neutral axis.
2. Ease of construction, especially when all beams are hidden beams.
3. Economic for spans > 5m with moderate live load: hospitals, office and
residential buildings.
4. Improved insulation for sound and heat.
Disadvantages of Hardy Slab:
1. If not properly handled, the hollow core ribbed brick units may be damaged
during transport.
2. Not economic for small spans.
3. Difficult to repair and strengthen
Hardy slabs are further classified into two types:
• One way Hardy Slab
• Two way Hardy slab

Waffle Slab:-
Waffle slab is a reinforced concrete roof or floor containing square grids with deep
sides and it is also called as grid slabs. This kind of slab is majorly used at the
entrance of hotels, Malls, Restaurants for good pictorial view and to install artificial
lighting. This a type of slab where we find a hollow hole in the slab when the
formwork is removed. Firstly PVC trays (pods) are placed on shuttering then
reinforcement is provided between the pods and steel mesh is provided at top of the
pods and then concrete is filled. After concrete sets, the formwork is removed and
PVC pods are not removed. The concrete waffle slab is often used for industrial
and commercial buildings while wood and metal waffle slabs are used in many
other construction sites. This is a one of the types of concrete slabs.

Where to use Waffle Slab & Waffle slab details:
A waffle slab has a holes underneath, giving an appearance of waffles. It is usually
used where large spans are required (e.g auditorium, cinema halls) to avoid many
columns interfering with space. Hence thick slabs spanning between wide beams (to
avoid the beams protruding below for aesthetic reasons) are required. The main
purpose of employing this technology is for its strong foundation characteristics of
crack and sagging resistance. Waffle slab also holds a greater amount of load
compared with conventional concrete slabs.

Types of Waffle slabs:
Based on the shape of Pods (PVC Trays) They are classified into different types some
of them are:
1.Triangular pod system
2.Square pod system

Advantages of Waffle slabs:
1. Waffle slabs are able to carry heavier loads and span longer distances than flat
slabs as these systems are light in weight.
2. Waffle slab can be used as both ceiling and floor slab.
3. Suitable for spans of 7m-16m ;longer spans may be possible with
posttensioning.
4. These systems are light in weight and hence considerable saving is ensured in
the framework as the light framework is required
Disadvantages of Waffle slabs:
1. Waffle slab is not used in typical construction projects.
2. The casting forms or moulds required for precast units are very costly and
hence only economical when large scale production of similar units are desired.
3. Construction requires strict supervision and skilled labour.

Dome Slab:-
These kind of slab is generally constructed in temples, Mosques, palaces etc. And
Dome slab is built on the conventional slab. Thickness of Dome slab is 0.15m.
Domes are in the semi-circle in shape and shuttering is done on a conventional
slab in a dome shape and concrete is filled in shuttering forming dome shapes.
This is a one of the types of concrete slabs.
https://www.youtube.com/watch?v=djE4d3nZUMM

Pitch roof slab:
Pitch roof is an inclined slab, generally constructed on resorts for a natural
look. Compared to traditional roofing materials Tile-sheets used in pitch roof slab are
extremely lightweight. This weight saving reduces the timber or steel structural
requirements resulting in significant cost savings. Tile-sheets are tailor made for each
project offering labour cost savings and reduced site wastage. And the thickness of
the slab is depends on the tiles we using it may be 2″-8″. This is a one of the types of
concrete slabs.

Advantages of Pitched roof type of Slab:
o Pitched roof sheds off rain water better.
o This slab gives you internal storage or room space.
o It is less likely to leak.
o Roof coverings are cheaper.
o If it is a standard pitch, building materials are more cost-effective
Disadvantages of Pitched roof type of Slab:
o This type of slabs are not suggested for long spans.
o Repairs in slabs such as plumbing repair or electric wiring on slabs is difficult.

Slab with Arches:
This is a type of slabs which is generally adopted in the construction of bridges.
Bridges are subjected to two loads moving load from the vehicles and Wind load.
These slabs are adopted at a place where there is a need of redirecting wind load and
if there is a long curve in direction of slab these slabs are adopted. It resists the fall of
the bridge due to heavy wind load.
Furthermore, they were originally built by stone or brick but these days these are built
by reinforced concrete or steel. The introduction of these new materials allow arch
bridges to be longer with lower spans. This is a one of the types of concrete slabs.

Post tension slab:
The slab which is tensioned after constructing slab is called Post tension slab.
Reinforcement is provided to resist the compression. In Post tension slab, the
reinforcement is replaced with cables/ steel tendons.
Also, Post-Tensioning provides a means to overcome the natural weakness of
concrete in tension and to make better use of its strength in compression. The
principle is easily observed when holding together several books by pressing them
laterally.

Procedure in concrete structures, this is achieved by placing high-tensile steel
tendons/cables in the element before casting. When concrete reaches the desired
strength the tendons are pulled by special hydraulic jacks and held in tension using
specially designed anchorages fixed at each end of the tendon. This provides
compression at the edge of the structural member that increases the strength of the
concrete for resisting tension stresses. If tendons are appropriately curved to a
certain profile, they will exert in addition to compression at the perimeter, a
beneficial upward set of forces (load balancing forces) that will counteract applied
loads, relieving the structure from a portion of gravity effects. This is a one of the
types of concrete slabs.

Advantages of Post tension slab:
1. It allows slabs and other structural members to be thinner.
2. It allows to build slabs on expansive or soft soils.
3. Cracks that do form are held tightly together.
4. Post tension slabs are excellent ways to construct stronger structures at an
affordable price.
5. It reduces or eliminates shrinkage cracking-therefore no joints, or fewer joints, are
needed
6. It lets us design longer spans in elevated members, like floors or beams.

Disadvantages of Post tension slab:
1. The post tension slab can be made only by skillful professionals.
2. The main problem with using post tension slab is that if care is not taken while
making it, it can lead to future mishaps. Many a times, ignorant workers do not fill
the gaps of the tendons and wiring completely. These gaps cause corrosion of the
wires which may break untimely, leading to some failures unexpectedly.

Pre Tension Slab :
The slab which is tensioned before placing the concrete is called Pre tension slab.
The slab has same features of Post tensioning.

Cable suspension slab:
If the span of the slab is very long, then we go for cable suspension slab which is
supported on cable such as London bridge, Howrah bridge etc. Generally, in the
construction of houses for every 4m, we provide a column whereas in cable
suspension slab for every 500m we provide a column. This kind of slab is provided
where the length of the span is more and difficulty in building columns. The slabs
are tied with cables and these cables are joined to columns.

Low roof slab:
The slab which provided above the door for storage purpose is called Low roof
slab. Slab is closed at all ends and open at one end. This slab lies below the actual
slab and above the door sill level. These types of concrete slabs are almost used in
houses.

Projected slab:
The slab which has one side fixed and the other side is free is called as Projected
Slab or Cantilever slab. These type of slabs are generally constructed in hotels,
Universities, function halls, etc. to use that area for dropping or picking up zone
and for loading and unloading area. This is a one of the types of concrete slabs.

Grads Slab/ Slab on grade:
The slab which is casted on the surface of the earth is called a Ground slab. This
type of slab is used in the Basement floor.
Usually after casting Plinth beams. Sand is filled at an height of 0.15m and then
Sand level is rammed. Then PCC is poured (Precipitated Calcium Carbonate is
used as a mineral filler improving the rheology and mechanical resistance of the
plastic) on sand upto a height of Plinth beams. Its an economic way of constructing
ground slab which use majorly in India.

Sunken Slab:
Slab which is provided below the washrooms to hide the sewage pipes or sewerage
pipes is called Sunken slab. In this type, the pipes that carry water are concealed
below the floor. Special care has to be taken to avoid leakage problems. After
casting sewage pipes in the slab, also the slab is filled with coal or broken pieces of
bricks. There are two types of the sunken slab.
• The slab which is provided below the normal floor level at a depth of 200mm to
300 mm and filled with broken pieces of bricks is called Sunken slab.
• The Slab which is provided above the normal floor level at a height of 200mm
to 300mm and filled with coal or broken pieces of bricks called Sunken slab.

Miscellaneous Slabs:
1. Room Chajja or Loft :
This kind of chajja (Slab) is provided in drawing rooms and kitchen for storing
house material. The usual difference between low roof slab and Room chajja is
Low roof slab hides house material and whereas Room Chajja or Loft doesn’t hides
house material they are open, and provided above the door side. This is a one of the
types of concrete

2. Kitchen Slab:
The slab is provided in the kitchen for its platform. For placing stove and other
kitchen material is called Kitchen Slab. It has a breadth of 0.5m and length of wall
and thickness is 2″.

3. Lintels:
 Lintels are provided Inside building above the doors and windows to re-direct
the top load. There are two types of lintels.
 Pre cast Lintels: Lintels which are manufactured in factories is called Pre cast
Lintels.
 Cast in situ: Lintels are casted at the site itself is called Cast in situ lintels. The
length of the lintel is more than door length and has a width of the wall,
thickness of lintel is variable.

Sun Shade slab:
Sun Shade is provided outside building above the Doors and Windows are called
Sun shade slab. The slab stops rain to come inside the building and direct
sunglight. This is a one of the types of concrete slabs.

Reinforcement in Concrete Structure:-
Reinforcing steel for concrete is used in the main to provide resistance to tensile stresses in
structural members. The steel reinforcing bars must therefore be placed and secured in the
positions inside form-work where they will be most effective in reinforcing concrete that
will be poured and compacted inside the form work, around the reinforcement. It is of
importance, therefore, that the reinforcement is rigidly fixed in position so that it is not
displaced when wet concrete is placed and compacted. There are different types of concrete
member in a structure which steel reinforcement is being used:-

• Reinforcing of Concrete Foundation
• Reinforcing of Concrete Column

• Reinforcing of Concrete Beam
• Reinforcing of Concrete Slab

• Reinforcing of Concrete Wall
• Reinforcing of Concrete Stair

In addition; reinforcement for structural beams and columns is usually assembled in the
form of a cage with the main and secondary reinforcement being fixed to links or stirrups
that hold it in position. The principal purpose of these links is to secure the longitudinal
reinforcing bars in position when concrete is being placed and compacted. They also serve
to some extent in anchoring reinforcement in concrete and in addition provide some
resistance to shear.
Thus Steel, which has high tensile strength, is used with concrete in order to counteract the
concrete's low tensile strength and ductility. The main purpose of inclusion of steel is resist
tensile stress in particular regions of the concrete that may cause structural failure or
cracking.

ACI (American Concrete Institute) standard hooks for primary reinforcement
180-degree bend plus 4db extension, but not less than 65mm at free end of bar
90-degree bend plus 12db extension at free end of bar

Why rebar have been hooked such as Hooked Anchors?
180-degree bend 4db plus 4db extension, but not less than 65 mm at free end of bar. In order
90-degree of bend 4db plus 12db extension at free end of bar
• The function of hooked anchors is the provision of additional anchorage when there is
inadequate straight length available to develop a bar.
• Commonly, standard hooks described in ACI Code Section 7.1 utilized apart from the case
where it is specifically specified.

Types of Reinforcement in a Concrete Beam by (Cut Bar System)
1. Main Continuous Bottom Bars (bottom steels)- deal with tension force
2. Main Continuous Top Bars (anchor bars)-hold by stirrups in their position
3. Bottom Cut off Bars- deal with tension forces
4. Top Cut off Bars- deal with tension forces (anchor in most times)
5. Stirrups with different spacing-tackle vertical and diagonal shear.

Steel Reinforced Stirrups
• Stirrups with different spacing-tackle vertical and diagonal shear.

Steel Reinforced Stirrups
In beam detailing, the detailer or designer shall provide sizes, spacing, location, and types of
all stirrups which include open and closed stirrups. stirrups may be vertical or inclined.
Moreover, where the design requires closed stirrup for shear, the closure may consist of
overlapped, standard 90o end hooks of one or two-piece stirrups, or properly spliced pairs of
U-stirrups

ACI standard hooks for stirrups
16mm bar and smaller, 90-degree bend 4db plus 6db extension at free end of bar.
19mm, 22, and 25 bar, 90-degree bend 4db plus 12db extension at free end of bar
25mm bar and smaller, 135-degree bend 4db plus 6db or 75mm extension at free end of
bar.

Spacers for Reinforcement:-

To ensure that there is the correct cover of concrete around reinforcement to protect the steel
from corrosion and to provide adequate fire protection, it is necessary to fix spacers to
reinforcing bars between the bars and the formwork. These spacers must be securely fixed so
that they are not displaced during placing and compacting of concrete and strong enough to
maintain the required cover of concrete
Spacers for Reinforcement:-

Cover to reinforcement in columns, beams, foundations, etc. is required for the following
reasons:
• To protect the steel against corrosion.
• To provide sufficient bond or adhesion between steel and concrete.
• To ensure sufficient protection of the steel in a fire.
.
Also, if the cover is insufficient, concrete will split away from the steel. Minimum cover is
never less than the maximum size of aggregate in the concrete, or the largest reinforcement bar
size (take greater value).
 Used RC Slab Cover 20mm
 Used RC Beam Cover 40mm
 Used RC Column Cover 40mm
 Used RC Foundation laid on soil 100mm
 Used RC Foundation laid on lean Concrete 75mm

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