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Build A
Roof:
The Pre Fabricated Way
Today,
as India is witnessing an unprecedented growth, the creation of built
infrastructure is both a pressing need and a pre-requisite for future
growth. Housing continues to be an area where demand exceeds supply by
more than a factor of 2. The challenges posed by this demand vary
significantly in urban and rural areas. Whereas cities need to address
the environmental impact of the resources consumed by large-scale
construction, rural housing needs to focus more on cost reduction and
durability of the buildings. As per the National Housing Bank, there was
a shortage of 80 million units to be met in 2007, of which more than 70%
shortage was in the rural, low-income population segment. In order to
successfully meet this overwhelming demand, solutions need to come from
outside the ‘bricks and mortar’ technological perspective. More than
anywhere else, it is this particular segment that needs to benefit from rationalised
design and optimum use of resources in construction technology.
Pre-cast and Obvious
Benefits
Pre-cast or pre-fabricated building components have long been
established in the developed world as a means of achieving efficiency,
economy and quality in the construction sector. In India, the ‘pre-cast’
approach is yet to achieve adequate maturity in the housing sector, and
needs to be pursued at a large scale. The reasons range from difficulty
in transportation and assembling to availability of skilled labour. In
urban areas, the concentration of construction activity often does not
justify pre-fabrication. If adopted, it can be of particular benefit to
the rural building segment in the following ways.
• Quality: Pre-cast elements are normally constructed under ‘factory’ or
controlled conditions where the quality is much easier to control rather
than on unregulated construction sites.
• Social Benefit: The controlled environment needed for pre-casting can
take the form of a local enterprise which can enhance the income of a
group of people involved in production. It will also increase the skill
levels of construction workers who can then demand premium rates.
• Productivity: Especially observed in developing countries, the
productivity of labour-intensive construction tends to be on the lower
side. In the case of pre-casting, due to the ‘learning curve’ effect,
the productivity tends to increase with repetitive work, resulting in
financial benefit.
• Material Use: Pre-cast is normally synonymous with ‘pre-cast
concrete’, which uses cement and steel as the basic material. The
pre-casting approach allows the use of building elements which derive
strength from the shape and profile instead of relying on inordinate
quantities of cement and steel. This measure cuts down the requirement
of two highly energy intensive and expensive construction materials.
• Speed: Considerable time can be saved when the building elements are
cast beforehand - especially during foundation work - and then installed
immediately afterwards. Since the use of shuttering props is minimised,
work can continue at the lower level.
• Cost: Due to these above reasons, pre-casting has the inherent
potential to reduce construction costs by at least 15 to 20% as compared
to the conventional systems (burnt brick walls and RCC roof).
Pre-cast Roofing: Focus
Areas
In a typical low-rise structure serving as a rural house, conventional
foundations using in situ techniques and local materials are found to be
economical and practical. In walling, although the bricks continue to be
the most common practice, increasing the size of wall blocks proves to
be economical due to greater speed and less mortar consumption. Roof
construction in a building benefits most significantly from pre-cast
technology. It accounts for almost 30% of the building cost and presents
a structural situation that can be interpreted in a variety of ways
which lend themselves to pre-casting techniques.
Most techniques allow for construction of a flat roof serving as an
intermediate floor for vertical expansion. The roofing system is a
combination of pre-cast beams and smaller roofing elements to span
between the beams. Screed concrete is then laid in situ over the
components, to achieve a finished flat surface. The components and
systems so designed can be manufactured and built by manual means in
villages or semi-mechanical techniques in towns. Extra reinforcement is
required to take care of handling and assembling stresses. Furthermore,
to minimise the chances of leakage/ seepage through joints between
pre-cast components, extra care is needed to ensure that joint details
are not complicated and that in situ screed concrete is of the best
quality possible.
Roofing Technologies
• Brick arch panel/ Brick
panel roofing
A brick panel consists of bricks laid in a certain manner with 6 mm MS
bars running through their joints grouted in 1:3 cement mortar. These
panels are placed next to each other and supported on pre-cast beams, so
as to construct a flat roof. An arch can resist great loads by utilising
the natural compressive strength of materials laid in a circular or
segmental manner. If placed together in the profile of an arch, the
bricks or brick tiles can be used to make pre-cast panels that span from
3 to 3.5 feet. This is the principle behind making an Arch Panel Roof
designed by the Indian Institute of Science, Bangalore. The same panel
can also be made with bricks laid on a flat level which can be used to
span 3 to 4 feet. The quality of brick is important in this system and
should not be less than 40 kg/ cm2. The panels are then overlaid with
screed concrete of M15 mix nominally reinforced with 8mm bars.
• Plank and joist roofing
Planks and Joists roofing is a building system constructed by placing
pre-cast RCC planks on pre-cast RCC joists/ beams, filling the joints
with plain cement concrete. The planks are partially cast slabs of 1
feet width and 4 to 6 feet length, depending upon the structural
requirement. The planks are made partly 3 cm thick and partly 6 cm thick
with a 10-cm-wide tapered concrete filling provided for strengthening
the haunch portion during handling and erection. They are made with M15
concrete mix with the reinforcement consisting of only 1.4 kg 6 mm mild
steel laid in a grid.
• Pre-cast joists/ beams
Typically, beams in roofing systems are partially pre-cast and cast
completely with in situ concrete after the roofing elements have been
placed along their length. In this way, the joists, along with the in
situ concrete between roofing elements on either side, act like a
composite T-beam which ensures efficient load distribution of the roof.
Up to a span of 4m, the beams can be cast in a size of 6" x 6" and
reinforced with 8 mm and 10 mm reinforcement. Depending on the section
of beam, 2.5 to 4 kg reinforcement is needed per metre length of beam.
The reinforcement is placed in a triangular manner with the stirrups
projecting out to be grouted with in situ concrete.
Conclusion
From the range of options available in pre-cast roofing technologies,
the systems featured above have been successfully tried and tested in
various buildings: residential, institutional, Economically Weaker
Sections (EWS) housing, etc. The benefits of pre-cast systems are
further maximised if the economies of scale are reached, for instance in
mass housing. Also, it is critical to combine these alternative systems
with efficient planning and execution. It is high time that issue of
housing demand be addressed by making available more ‘intermediate’
technology options such as ones offered by pre-cast systems. The author
can be contacted for more information.
q
Pankaj Khanna
pkhanna@devalt.org
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