The concept of precast structures also known as

prefabricated/ modular structures.

The structural components are standardized and produced in
plants in a location away from the building site.
Then transported to the site for assembly.
The components are manufactured by industrial methods
based on mass production in order to build a large number of
buildings in a short time at low cost.
PREFABRICATION
Prefabrication is the practice of assembling components of a
structure in a factory or other manufacturing site, and
transporting complete assemblies to the construction site where
the structure is to be located’’
Sections of walls, floors & roof are prefabricated including
window & door frames.
The elements are placed by crane and bolted.
The division and specialization of the human workforce.
The use of tools, machinery, and other equipment, usually
automated, in the production of standard, interchangeable
parts and products.
Compared to site-cast concrete, precast concrete erection is
faster and less affected by adverse weather conditions.
Plant casting allows increased efficiency, high quality control
and greater control on finishes.
The design concept of the precast buildings is based on
build ability.
economy
standardization of precast components.

Precast prestressed
slabs spanning
between walls with
composite in-situ
topping for 1st storey
Manufacturing of Prefabrication will be done in two stages
1) Factory Prefabrication. 2)Site Prefabrication.
FACTORY PREFABRICATION
Factory prefabrication is restored in a centrally located
plant for manufacture of standardized components on a
long form basis. Production of components throughout the
year
SITE PREFABRICATION
Components are manufactured at site near the site of work
as possible. The work is normally carried out in open
space with locally available labour. The equipment
machinery and moulds are of mobile nature.
CASTING
▪ Precast components are casted with controlled cement
concrete in moulds of required shape and sizes.
▪ Vibrator is used to vibrate concrete and this removes any
honeycombing inside the components
▪ After complete curing is done the components are transported
to the site with heavy trucks and erection will be done using
cranes with skilled labours.
TRANSPORTATION
▪ Transport of prefab elements inside the factory depends on the
method of production selected for the manufacture.
▪ Availability of suitable transport equipment, such as tractor
,truck etc. to suits the load and dimension of the member
▪ Care should be taken in negotiating sharp beds uneven of
slushy roads to avoid undesirable stresses in elements and in
transport vehicles.
▪ Care should be taken at base packing for supporting the
elements.
ERECTION OR ASSEMBLING
▪ Set reference line and determine the position of precast elements
to be installed
▪ Level pads should be provided for setting the level of the element.
▪ Check alignment and verticality of the element
▪ For corrugated pipe sleeve or splice sleeve connection, proper
and pour non shrink grout into the pipe inlets provided.
▪ Keep the installation panels undisturbed for at least 24 hours.
▪ Joints casting & sealing.

TRANSPORTATION ERECTION
COMPONENTS
• Flooring • Lintels
• Roofing • Sunshade/Chajja projections
• Beams and girders • Staircase
• Columns
• Walls
MATERIAL USED
•Concrete
•Steel
•Treated Wood
•Aluminium
•Light weight elements
•Ceramic product
TYPES OF PRECAST SYSTEMS
The system of prefabricated construction depends on the extent of the use
of prefab components, their materials sizes and the technique adopted for
their manufacture and use in building.
 OPEN PREFAB SYSTEM
This system is based on the use of the basic structural
elements to form whole or part of a building.
The standard prefab concrete components which can be used
are:-
Reinforced concrete channel units,
Hollow core slabs,
Hollow blocks and battens,
Precast planks and battens,
Precast joists and tiles,
Cellular concrete slabs,
Prestressed/reinforced concrete
slabs,columns , beams
Precast lintels and chajjas,
Reinforced concrete waffle slabs/shells,
Room size reinforced/prestressed
concrete panels,
Reinforced/prestressed concrete walling
elements, and Reinforced/prestressed
concrete trusses
•There are two categories of open prefab systems
depending on the extent of prefabrication used in the
construction as given below:
•Partial prefab open system
•Full prefab open system
PARTIAL PREFAB OPEN SYSTEM
This system basically emphasizes the use of precast
roofing and flooring components and other minor
elements like lintels, chajjas, kitchen sills in conventional
building construction.The structural system could be in
the form of in-situ framework or load bearing walls.
FULL PREFAB OPEN SYSTEM
In this system almost all the structural components are
Prefabricated.
The filler walls may be of bricks or any other local
material.
LARGE PANEL PREFAB SYSTEM
This system is based on the use of large
prefab components.
The components such as Precast concrete
large panels for walls , floors, roofs, balconies,
staircase, etc.
The casting of the components could be at the
site or off the site.
Wall Systems
CrossWall System - In this scheme, the cross
wall are load bearing walls whereas the
facade Walls are non load bearing ,this
system is suitable for high rise buildings.
Longitudinal Wall System - In this scheme
the cross walls are non-load bearing whereas
Longitudinal walls are load bearing walls.This
system is suitable for low rise buildings.
PRECAST FLOORS
precast flooring units could be homogeneous or non homogeneous.
Homogeneous floors :- could be solid slabs, cored slabs, ribbed or
waffle slabs.
Non-homogeneous floors :- could be multi-layered ones with
combinations of light weight Concrete or reinforced/prestressed
concrete.
STAIRCASE SYSTEMS
Staircase system could consist of single flights with in-built risers and
treads in the element only.
The flights are normally unidirectional transferring the loads to
supporting landing slabs or load bearing walls.
BOX TYPE CONSTRUCTION
In this system, room size units
are pre fabricated and erected
at site
Toilets and kitchen blocks also
be similarly prefabricated and
erected at site.
This system derives its stability
and stiffness from the box unit
which are formed by the four
adjacent walls.
Walls are jointed to make rigid
connections among
themselves.
The box unit rests on the plinth
foundation which may be of
conventional or precast type.
JOINT TECHNIQUES/MATERIALS NORMALLY
EMPLOYED ARE:
Welding if cleats or projecting steel.
Overlapping reinforcement, loops and linking steel grouted
by concrete.
Reinforced concrete ties all round a slab.
Prestressing.
Epoxy grouting
Bolts and nuts connection, and
A combination of the above
Two kinds of joints can be distinguished
a) Dry Joints: Joint accomplished by simple placing of two
members by means of welding or fastening,
b) Wet Joints: Joint requires not only casting with cement but also
concreting and grouting material.

dry joint wet joint

ADVANTAGES
• Prefabrication causes lesser noise and dust.
• Offers less energy consumption
• Creates opportunities for good architecture
• Erection can be continued even at -20 degree Celsius
• Carries out high capacity work , Mass production is easier
and quick
• Takes less than half construction time than the conventional
cast-in- situ concrete.
• High quality product
• Overall efficiency is greatly increased
ADVANTAGES
• Provides complex thermal insulation
• Improved quality over on-site construction
• Saves time where weather conditions allow brief periods
of construction
• It helps in safety of workers, environmental impact,
constructability
• Do not disturb traffic during a bridge building as much as
on-site construction
•Potential for lower production costs and other cost
savings.(Labor related savings )
DISADVANTAGES
• Leaks in joints
• Transportation cost is high for voluminous structures
• Large construction requires heavy duty cranes.
• Accuracy is required in measurement & handling to place
in position.
• Larger groups of building from same type of prefabricated
elements tend to look monotonous.
• Need skilled labour and experience.
Sydney Opera House :The distinctive peaked and curved roof is made up of a series of
precast concrete panels that are covered in glossy white- and matte-cream tiles. 2,194
precast concrete sections used.
Diego Portales University :Exposed concrete slab walls add to the natural feel of these
hulking structures so to complement its natural surroundings.
The Pierr :As a stunning example of how precast concrete embraces nature, The Pierre is
a private residence that was built atop a natural stone deposit on the owner’s property in
Washington State. Parts of the stone were cut away to make way for the home. The cut-
away stone was crushed and used in making the cement .

Nestlé R&D Centre, Manesar, India:The 15,000 m2 building was completed in the
exceptional time of one year.. The structural system is comprised of multi-story precast
columns, prestressed beams, precast basement walls, long span prestressed precast slabs
with structural topping and shearwalls.

sydney opera Diego Portales University The Pierr

DIFFERENCE BETWEEN PREFAB CONSTRUCTION
AND CAST IN SITU
PREFABRICATED RCC CAST IN SITU CONSTRUCTION
CONSTRUCTION
prefabricated concrete is cast into a specific cast-in-situ concrete is cast into forms on the
shape at a location other than building site like building site.
factories.

In the precast concrete, elements are In the cast-in-situ concrete, column, slab etc.
manufactured in a controlled casting elements are casted on site in the open
environment and hence it is easier to control environment and hence it is difficult to control
mix, placement and curing. mix, placement and curing.

Weather condition has no effect on casting Weather condition can delay the casting work.
work.

Quality can be controlled and maintained Quality control and maintenance is difficult.
easily.

Precast concrete is cheaper form of In situ concrete is cheaper form of

construction if large structures are to be construction for small structures.
constructed.

Maintenance cost of precast concrete Maintenance cost of cast-in-situ concrete

structure is higher structure is less compared to the precast
concrete structure
Less labours are required. More labours are required

Precast concrete construction is quick as it In situ concrete construction is slow as gaining

can be installed immediately and there is no of strength requires time.
waiting for it to gain strength.

Large number of joints in structural system. Less number of joints present in structural
system.

Less resistant to Earthquake and wind forces More resistant to earthquake and wind forces.
not recommended where seismic loads are
dominant.
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