| Q.1) |
What
are the economics of galvanised reinforcement
in concrete? |
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When frequent repair costs
and consequences of corrosion damage to
a reinforced concrete building are analyzed,
the extra cost of galvanising is small.
It can be regarded as an ‘insurance
premium’, but a premium which is
low and needs to be paid once only. Currently,
the cost of galvanising of rebar is approximately
Rs.8000 per tonne of steel, and this depends
on the quantity to be galvanised, bar
diameter etc.
While the cost of galvanising
is an important factor, the cost of galvanised
reinforcement as a percentage of total
building cost is much lower than generally
realized. It is as low as 0.5 –
1.0% in many cases. When related to total
project costs, the added cost of galvanising
becomes very small indeed. Such costs
represent a very small proportion of the
cost of repairs should unprotected reinforcement
corrode. Frequently such repairs eliminate
only the visible damage and cannot be
relied upon as a long-term solution.
For most structures, even
in the most aggressive environments, the
use of galvanised reinforcement can be
confined to the exposed surfaces and critical
structural elements such as:
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Thin pre-cast
cladding elements |
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Facades of prestigious
buildings |
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Surface exposed beams
and columns |
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Windows and door surrounds |
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Prefabricated units |
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External facades of
buildings near the sea coast |
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Architectural features |
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Accordingly,
whenever
there is concern that premature corrosion
of reinforcement might occur, reinforcement
should be galvanised. The
use of galvanising however should not be
considered as an alternative to the provision
of an adequate cover of dense, impermeable
concrete. |
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| Q.2) |
What are the
supply chain dynamics?
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galvanising is done by
galvanisers under the guidance of Indian
Lead Zinc Development Association and
Hindustan Zinc and after a comprehensive
study of the facilities and quality of
work done. All the work is done in adherence
to standards prescribed by the BIS and
ILZDA. The feedback received on basis
of prior work carried out by the galvanizers
is also considered while suggesting a
galvanising facility for a project.
Currently the total capacities
available for galvanised rebars are 75,000
to 80,000 tons per year, which is approximately
6,000-7,000 tons per month. With expansions
taking place it is likely to touch 1,
00,000 tons per year.
The galvanising facilities
are located across the country, which
ensures that galvanised steel is available
with minimum lead-time. This also ensures
that the buyer incurs minimum transportation
cost.
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| Q.3) |
What are the factors
determining the durability of reinforcement? |
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Environment:
The external environment
of the concrete provides the agents, which
commonly cause corrosion in reinforcement:
oxygen, water, carbon dioxide and chloride
ions.
Marine structures and other structures close
to coastal areas are particularly at risk
from corrosion of reinforcement due to the
ingress of chloride ions from sea spray
and salt-laden air.
Away from the seacoast, corrosion of reinforcement
in concrete is mostly due to the process
of carbonation, which reduces the alkalinity
of the surrounding concrete. This process
can occur at any geographic location. The
rate of carbonation is at a maximum when
the relative humidity is about 50 per cent,
and increases with increasing temperature.
Recent surveys have shown that the corrosion
problem in relatively new buildings is worst
in coastal areas.
Carbonation
resistance:
galvanised reinforcement is better able
to resist the effects of carbonation because
of the much wider range of pH (to about
8) over which the zinc coating remains passivated.
Since black steel typically depassivates
when the pH of concrete drops below about
11.5, it is apparent that as the carbonation
‘front’ moves past a galvanised
rebar, little or no effect will occur until
the concrete adjacent to the reinforcement
is almost completely neutralized.
Chloride
tolerance:
Though zinc can be depassivated and attacked
in the presence of chloride ions, the tolerance
of galvanised reinforcement to chloride
depassivation is substantially higher than
that of black steel. In a survey of a number
of long-serving marine structures [Tonini,
DE and Cook, AR ‘The performance of
galvanised reinforcement in high chloride
environments - field study reports.’
International Corrosion Forum, NACE, Houston],
galvanised bars were shown to have been
exposed to chloride contents as high as
2.2% (by approximately weight of cement)
over periods of 10-20 years, with less than
10% loss of original coating thickness and
no record of failure. This should be compared
to chloride levels in the range of 0.2-0.3%
by weight of cement leading to severe corrosion
of black steel in similar circumstances.
Reaction
between galvanised coatings and concrete:
During initial contact of
galvanised reinforcement with wet concrete,
the outer zinc layers of the galvanised
coating react to form stable insoluble zinc
salts (Fig-1). Attack ceases as the concrete
hardens and the galvanised coating remains
intact. This surface layer increases the
bond strength of the rebar. |
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Corrosion
rate of location and planned application
life should be used to determine use of
corrosion protection |
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The above study shows that the entire coastal belt of India
(e.g. Mumbai, Chennai, Kolkatta, Gujarat,
Goa, Kerala and other coastal regions) requires
corrosion protection for the steel reinforcements
as the rate of corrosion is quite high.
Bond strength
of concrete to galvanised reinforcing bars:
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.
Tests made by the Building Research Establishment
in the UK show that, based on the work of
five investigators, adhesion to concrete
of plain reinforcing bars is on average: |
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3.3
- 3.6 MPa |
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The large spread for black steel stems from different degrees
of rust and different amounts of oxide scale
on the steel surfaces.
In the case of deformed bar, the approximate
stress at which 0.1 mm of slip occurs was
found to be:
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150
MPa |
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Hot
dip galvanised steel |
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Hot
dip galvanised steel (chromated) |
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The bond strength to concrete has also been studied in tests
conducted by the University of California
in accordance with American Concrete Institute
(ACI) standard 208-58. Both corroded and
uncorroded rebar were used. Tests were on
concrete beams with plain or deformed bars
cast inverted in the top of the beam. galvanised
rebars show equal or better bond strength
than ungalvanised rebars in all conditions
in both plain and deformed types.
Passivation
and additives:
The research into bond strengths also shows
that the addition of chromates to the concrete
mix in the ratio of 35-150 ppm by weight
of cement increases the bond strength of
galvanised plain bars significantly
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