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GALVANISING
– A PROTECTOR
Hot dip galvanising is a viable means of protecting
reinforcement, particularly where the durability
of concrete cannot be guaranteed. Its use should
be considered for aggressive exposure conditions,
precast construction and prestige facades where
long life, freedom from rust staining and low
maintenance are important criteria.
Galvanised coatings provide important advantages
for the protection of reinforcement. Research
and practical experience since the 1950s have
established that the corrosion
resistance of galvanised steel reinforcement is
superior to uncoated steel and
any other type of protective measure. The corrosion
protection by the galvanised coating ensures that
the design strength of concrete is maintained
and the possibility of surface rust staining and
eventual corrosion of reinforcement and spalling
of concrete are removed.
GALVANISING PROCESS
Galvanising process is basically the process of
formation of series of alloy layers formed as
a result of a metallurgical reaction between steel
and zinc. The process of galvanisation consists
of five distinct steps, which are briefly described
below:
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Thus in the case of galvanising, the protector(Zinc)
and the protected(Steel) become one, which distinguishes
itself from other forms of coating which is merely
a physical coating over the steel held to the
surface by the adhesive forces only.
Also these alloy layers imparts the hardness and
abrasion resistance to the galvanised rebar which
prevents peeling off of the layers in handling,
and storage unlike in other coatings. This protects
the steel not only inside the concrete but also
in handling and storage at site.
The galvanising bath is typically
8 to 13meters in length; also double dipping
can be done for longer bars. Approved galvanisers
are present across India who caters to the demand
in individual geographical areas.
PROPERTIES
OF GALVANISED COATING
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| 1. |
Uniform
Coating Thickness
The coating thickness remains the same across
the entire length of the bar. Even edges
have the same coating thickness since it
is a metallurgical bond and reaction proceeds
from inside to outside. |
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| 2. |
Cathodic
Protection
This is a unique property of zinc
coating which covers up any holidays/scratches
that may occur in the coating. This is due
to the electrochemical property of zinc
which is more electro positive than steel
and has a tendency to donate electrons to
exposed steel. Hence if a holiday occurs
in the zinc coating the zinc from surrounding
area comes and covers it up.
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BARRIER AND CATHODIC PROTECTION
OF GalvanisING |
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| 3. |
Possibility
of Bending of Galvanised Steel
The rebars coated can be bend after galvanisation
without cracking/ peeling up of coating. This
is by virtue of the coating being a metallurgical
coating and zinc having good ductility. Hence
galvanised bars can be bend at the site itself. |
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| 4. |
Bond
Strength
The results of extensive programs of pull-out
testing by a number of researchers reveal
no significant difference in the bond strengths
of black and galvanised steel deformed (i.e
ribbed) reinforcing bars in concrete. |
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| 5. |
Belding
All kinds of welding (including Arc Welding)
can be performed as per the guidelines of
American Welding Society
All types of welding (including Arc Welding)
can be performed on galvanised rebars as
per CECRI and IIT Bombay
India Lead Zinc Development Association
has a vast repository of worldwide case
histories using hot dip galvanised steel
reinforcement in RCC structures and some
of them have been reproduced here. |
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