What is prefabrication? Prefabrication is the practice of
assembling components of a structure in a factory or other
manufacturing site, and transporting complete assemblies or
sub-assemblies to the construction site where the structure is to
be located. It is combination of good design with modern high
performance components and quality controlled manufacturing
procedures. This work is carried out in two stages, manufacturing
of components in a place other than final location and their
erection in position. 4
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Prefabricated sections are produced in large quantities in a
factory and then shipped to various construction sites. This
procedure may allow work to continue despite poor weather
conditions and should reduce any waste in time and material at the
site. Precast concrete units are cast and hardened before being
used for construction Sometimes builders cast components at the
building site and hoist them into place after they harden. This
technique permits the speedy erection of structures. 5
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The Prefabrication as defined will be done in two stages
Manufacturing at factory condition and erection of components at
the required location. This requires certain stages of preparation.
They are Casting. Curing. Transportation and erection. Casting: -
Precast components are casted with controlled cement concrete in
moulds of required shape and sizes. The vibrator is used to vibrate
concrete and this removes any honeycombing inside the components.
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Curing: - After 24 hours of casting, the casted components are
released from the mould and transported to curing tanks. Certain
special components like railway sleepers where high strength is
required are steam cured. 8
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Transportation and erection: - After complete curing is done
the components are transported to the site with heavy trucks and
erection will be done using cranes with skilled labour force.
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For Indian conditions partial prefabrication is well suited.
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Some of the prefabricated components are as explained below
Columns: - A column is a vertical member carrying the beam and
floor loadings to the foundation. It is a compression member and
therefore the column connection is required to be proper. The main
principle involved in making column connections is to ensure
continuity and this can be achieved by a variety of methods.
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15 Column to Column Connection
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Beams: - Beams can vary in their complexity of design and
reinforcement from the very simple beam formed over an isolated
opening to the more common encountered in frames where the beams
transfer their loadings to the column. Methods of connecting beams
and columns are A precasting concrete haunch is cast on to the
column with a locating dowel or stud bolt to fix the beam. A
projecting metal corbel is fixed to the column and the beam is
bolted to the corbel. Column and beam reinforcement, generally in
the form of hooks, are left exposed. The two members are hooked
together and covered with insitu concrete to complete the joint.
This is as shown in the figure. 16
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Waffle unit for flooring / roofing: - These are suitable for
roofs / floors spanning in two directions. They are laid in a grid
pattern. These units are cast in moulds. The saving achieved is not
much. Also Shuttering are complicated and costly. Time consumption
for construction is less 18
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20 SLAB TO BEAM CONNECTION
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21 GLANCE AT BUILDING PROCESS
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Advantages: Saving in cost, material, time & manpower.
Shuttering and scaffolding is not necessary. Installation of
building services and finishes can be done immediately. Independent
of weather condition. Components produced at close supervision.so
quality is good 23
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Advantages: Clean and dry work at site. Possibility of
alterations and reuse Correct shape and dimensions and sharp edges
are maintained. Very thin sections can be entirely precast with
precision. 24
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Disadvantages: Handling and transportation may cause breakages
of members during the transit and extra provision is to be made.
Difficulty in connecting precast units so as to produce same effect
as monolithic. This leads to non-monolithic construction. They are
to be exactly placed in position, otherwise the loads coming on
them are likely to get changed and the member may be affected.
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Disadvantages: High transport cost Need of erection equipment
Skilled labour and supervision is required. 26
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The prefab components and prefab structures eliminate space and
time over conventional constructions. Although prefabrication is
employed to a large extent in a wide variety of countries, in
India, construction industry, in spite of its expansion continues
to adopt same conventional methods. The root of the economic
problem is the fact that the majority of the households do not have
the confidence needed to construct prefab houses. These structures
are easy to erect as it is light material. This type of prefab
buildings were constructed in earthquake prone areas of Lature and
Gujarat. 27