Er.Arun
N Purandare is a Graduate in Civil Engineering from
VJTI Mumbai and a Post Graduate from Imeprial College Londan. He has been in
practice from 1965 and has more than 1500 large projects to his credit
including the D.Y.Patil Cricket Stadium Navi Mumbai and MCA Stadium, at
Gahunje,Pune. He is also an Environmental Consultant dealing with the issue of
solid waste management and disposal. He is a member of several professional
organisations. He has read and writted papers nationally and internationally on
Earthquake engineering and Landfill Design. He has designed and patented
Ferrocast Precasting system and is carrying our research in this field.
Building industry is passing through a very
difficult time. The quantum of construction is increasing very fast specially
in residential and commercial projects. This increase is not matched by equal
increase in the availability of trained labour force as well as supervisory
personal. This has resulted in totally untrained and persons with little skills
working in all trades. The obvious outcome is the serious drop in quality of
executed work. One such trade is carpenters for shuttering. There is a serious
shortage of carpenters and those doing this work are of very doubtful quality,
leading to bad concreting, bad line and level and no dimensional accuracy.
Similar problem is experienced with respect to masons for brick work and
plastering. The only other remedy to improve quality is to precast the
structural elements in specially set up casting yards and erect them at site.
Precasting industry, though in great need ,
has not taken root due to its heavy capital costs in plant and machinery. A
system had to be devised which can be started with substantially less capital
cost. All this is possible with Ferrocast Structural elements. The dead load of
individual ferrocast member is about 1/3 of the full section R.C. precast
member making it easy to handle in the casting yard. The subsequent
transportation and erection becomes easier due to lighter dead loads. In case
of residential buildings, the individual elements will not weigh more than
600/700 Kg, thus making use of 2/3 T.
Hydra – mobile crane more than adequate to carry out all lifting and
shifting jobs in the casting yard. This reduced capital cost makes it very
attractive even for “B” and “C” class towns well as on site casting yard.
The present system of making ferrocement
element consists of hand plastering stiff mortar into weld mesh / chicken mesh
layers fixed on welded reinforcement forming a required shape. Hand applied
plaster on weld mesh forms has several inadequacies. The complete section
formed with hand plaster will not have expected strength throughout the cross
section and length. This is a major problem leading to Codes not offering a
value for tensile as well as compressive capacity. The plastered section does
not have dimensional accuracy. The final sectional thickness also cannot be ensured.
This is a detriment to proper structural design of the member. To get over this
difficulty, members in this system cast in moulds, with self compacting mortar.
This ensures correctly cast section with correct section dimensions. This
method has been developed for column, beam and slab elements.
As ACI does not give any design values for
tensile, shear or compression capacity for ferrocement, this material was not
used for the normal structural elements used in building construction. The
present method of ferrocement construction, with variable and doubtful quality,
must have led to this reluctance on the part of the persons drafting the
recommendations. The method suggested was like R.C. design without taking into
consideration the high tensile qualities of ferrocement. This quality problem
is fully eliminated with the use of ferrocasting the structural members.
In order to get correct design stresses,
research was started. Concrete was encased in a ferrocast ring with variable
layers of weld mesh. The confinement provided by the ferrocast ring enhanced
the crushing strength of M-30 concrete beyond. 60 MPa. This was double the
concrete mix strength. This clearly indicated that the confinement offered by
ferrocast ring is vastly superior to that possible with reinforcement used as
binders and ties. The enhancement in crushing strength indicated large
ductility for core concrete. This is great step forward. The cracking tensile
stress in beam tests showed elastic behavior upto 25 MPa. This tensile capacity
can be used for the slab and beam design for tension. Both these experiments
are pointing to enhanced compressive stresses in column design and tension
capacity in the bottom flange of beams. All this will lead to substantial
savings in the construction of the building frame.
The new system consists of casting ferrcast
tubes of 25mm wall thickness. Concrete is poured into the annular space to form
the column. The beam sections are “U” forms resting on columns. Slab units are
placed on the two sides of the beam. Negative steel for continuity of slabs and
beams is tied over supports and topping concrete is poured. A perspective view
of a proposed construction with all structural elements in place is shown. The
advantages of this system are,
1. Light
Weight, leading to saving in concrete and steel
2. Advantage in
Seismic Design
3. Smaller
foundation
4. Factory
made product with assured strength
5. Form
finished, does not need plastering
6. Slab
every 7/8 days
7. Cheaper
than R.C. Frame
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