|inchak at tatasteel.com
|Posted: Fri Jul 08, 2005 9:31 am Post subject: Message No 36 from Mr. Tom Loe
|Dear Mr. Tom Joe
You have raised a very important question:
"With FBEC ruled out as a effective method, there in an urgent need to
identify some superior coating protection to steel. Please suggest some
options which can be used and provides cathodic protection and good
On Studying Various options I have come to a conclusion that Galvanized
Rebars is the best bet considering the effectiveness, cost and reliability
aspects. In the paragraphs below I provide some arguments to support my
A barrier coating must satisfy the following conditions in order to be
effective for its stated purpose of application on reinforcement bars.
1. It must form strong bond with steel which is not easy to peel off.
2. It must not impair bonding of the rebars with concrete.
3. It must withstand normal abrasion and impact in handling storage and
4. It must offer effective resistance to chloride attack
5. It must allow Bending and manipulation of bars without peeling
With this backdrop we may proceed to examinegalvanising as a suitable
reinforcement bars, In a tropical country like ours, where the moisture
levels are high for most part of the year and the reinforcement bars have a
relatively long travelling and waiting time before they are actually
embedded in concrete, a galvanized coating is particularly useful to
prevent early rusting of the steel. GALVANIZING PROVIDES VISIBLE ASSURANCE
THAT THE STEEL HAS NOT RUSTED TILL AT THE POINT OF BEING EMBEDDED INTO
Zinc coating serves its intended purpose of protection against corrosion in
two distinct and significant ways of action - THE BARRIER ACTION AND THE
1)Barrier action: Galvanizing provides a tough METALLURGICALLY BONDED zinc
layer that completely covers the steel surface and acts as a barrier
coating, which effectively prevents the access of any aggressive corrosive
ionic species to the steel surface. The common method for galvanizing
reinforcement bars is hot dip galvanizing, in which the bars are immersed
in a bath of molten zinc, maintained at a temperature of around 450 Deg. C.
The thickness of the zinc coating increases with the duration of immersion
as well as the temperature of the bath. SINCE, THE BARS ARE COMPLETELY
DIPPED IN A LIQUID BATH (OF ZINC) ALL PARTS OF THE BARS ACQUIRE THE COATING
ALMOST UNIFORMLY. The coating is metallurgically bonded to the surface of
steel. There is a soft outer layer of almost pure zinc (Hardness 70 VPN),
which contributes to the toughness and impact resistance of the coating.
Deeper inside there is a continuous series of zinc iron alloy layersas
ZETA 94% Zn - 180 VPN
DELTA 90% Zn - 245 VPN
GAMA 75% Zn - 250 VPN
These layers are metallurgically bonded to each other and ultimately to the
base steel. THESE ALLOY LAYERS ARE FAR HARDER THAN ORDINARY ZINC, thereby
offering appreciable abrasion resistance to the coating during handling.
This strong metallurgical bond also ensures that NO UNDER-FILM CORROSION
CAN OCCUR in a galvanized bar.
Zinc has an amphoteric nature and has the ability to passivate due to the
formation of protective reaction product films in the concrete environment.
Reaction of zinc with fresh cement leads to passivity by formation of a
diffusion barrier layer of zinc corrosion products. GALVANIZED
REINFORCEMENT STEEL CAN WITHSTAND EXPOSURE TO CHLORIDE ION CONCENTRATION
SEVERAL TIMES HIGHER THAN WHAT CAUSES SEVERE CORROSION IN BLACK STEEL BARS.
While black steel in concrete typically de-passivate below a pH of 11.5,
GALVANIZED REINFORCEMENT CAN REMAIN PASSIVATED AT A FAR LOWER pH - EVEN AS
LOW AS 6.0. This largely negates the effect of the eventual progress of
carbonation. These two factors combined i.e. the chloride tolerance and the
resistance to carbonation makes galvanized coating a particularly effective
one in protecting steel embedded in concrete.
2. Sacrificial action: Zinc occupies a position higher than iron in the
galvanic series. Therefore, zinc will have a greater tendency to lose
electron compared to steel. As a result, when zinc and steel are in contact
in the presence of an electrolyte, zinc is slowly consumed by anodic
dissolution, while steel is protected as a cathode. THE SACRIFICIAL ACTION
OF ZINC IS MOST SIGNIFICANT IN SMALL DISCONTINUITIES SUCH AS COATING
DEFECTS AND CUT EDGES. The total life of galvanized coating in concrete is
the sum of the time it takes for zinc to depassivate (which itself is
longer than the time taken by bare steel) - the barrier effect, PLUS the
time it takes for the dissolution of the alloy layers in the zinc coating -
the sacrificial effect. Substantial corrosion of steel can commence only
after the zinc layer has been dissolved out of an appreciable area of steel
surface. The result of this combined effect has been verified by salt spray
test for 19 days. Bare steel was completely and severely corroded within a
few days while only specs of white rust appeared on the galvanized rebars.
CORROSION PRODUCTS OF ZINC OCCUPY A SMALLER VOLUME than the corrosion
products of ion. Therefore, these contribute little towards build up of
internal stresses in the concrete during the initial process of corrosion.
The corrosion products of zinc are also capable of migrating from the
surface of the reinforcement into the concrete matrix, thus considerably
reducing the likelihood of zinc corrosion induced spalling of concrete.
Galvanized bars are particularly convenient for storage and handling which
is especially relevant for a large country like ours with fairly adverse
climatic conditions in general. The bars may be stored outdoors (there is
no need to provide a cover for these bars) for months without rusting.
Under similar conditions a black bar would rust quite severely. These bars
can be handled and placed in the concrete in the same manner as applicable
for non-galvanized rolled bars with no special precaution or surveillance
Zinc coating has a soft pliable outer layer, which is strongly and
metallurgically bonded to the steel substrate. As such it is easy to bend
these bars without appreciable peeling and rupture of the coating. Even if
some cracks appear the sacrificial protection provided by zinc ensures that
these do not form favorable sites for initiation of corrosion.
Since zinc reacts to the wet concrete mix it leads to formation of a strong
bond with concrete. The bond strength of galvanized bars to concrete has
been studied by University of California. Tests were done on concrete beams
with plain or; deformed bars cast inverted in the top of the beam.
Galvanized rebars showed equal or better bond strength than ungalvanized
rebars in all conditions in both plain and, deformed types.
Galvanized reinforcement is quite commonly used in the Australia, New
Zealand, Canada, USA, Korea, Taiwan and several other countries and has
been found to be cost effective. A number of important structures have been
constructed such as Flatts Bridge - Bermuda, Royal Bermuda Yacht Club, Long
Dick Creek - IOWA, Key West - Florida, Seven Mile Bridge - Florida, New
Cairns Opns. Base - Queensland, Housing & Urban Development Building -
Washington D.C., Sydney Opera House, Parliament House - Canberra, Royal
Yacht Club - Victoria, Breakwater Marina - Townsville, Penno's Wharf - St.
George, Hamilton Dock, Parliament House - New Zealand, US Coast Guard
Barracks, Crocker Building - San Francisco to name a few. A study was
recently conducted to verify the effectiveness of galvanized rebars in use
for years in various marine installations in Bermuda. The results
demonstrate that over 95% of zinc remains unaffected even after prolonged
exposure to very high chloride concentration. The steel, obviously can
start corroding only when all the zinc is consumed.
Even in India, a number of structures have been constructed using
galvanized rebars. A few of these are listed below along with the quantity
of galvanized rebars used:
Lotus Temple, New Delhi - 300T
Residential Building, JNPT, Uran - 1000T
All India Institute of Physical Medical & Research, Haji Ali - 50T
Residential Building, Wadala - 50T
Mahanagar Gas Ltd, Mumbai - 50T
Guest House, Mangalore - 50T
Central Railway, Wadi Bunder - 450T
Shree Anand Sukhram Trust, Malad - 250T
Based on these experiences in the country and abroad, it can be assumed
that in near future galvanized bars will gain much more widespread
popularity as elsewhere in the world.
Galvanized rebars are adequately covered by both Indian and international
standards. In India the bars must satisfy IS: 12594, which stipulates
coating thickens of 610 g/M Sq. for Class B coating and 915 g/M Sq. for
Class A coating. Almost similar levels are stipulated also in ASTM A 767.
It can be seen that an extremely heavy coating is prescribed keeping in
view the long service life expected from the intended application.
The other Indian standards related to galvanzed rebars are listed below:
IS 1786 : Specification for high strength deformed steel bars and wires for
IS 2629 : Recommended practice for hot dip galvanizing of iron and steel
IS 6745 : Methods for determination of mass of zinc coating on zinc coated
iron and steel articles
However A word of Caution must be mentioned here. The specifications call
for a very high coating thickness. A bar unscrupulously or inadvertently
coated to a much lower thickness is almost impossible for a user to spot.
Maintenance of uniformity of coating also calls for good process control.
As such, care needs to be exercised to ensure that the source supplying the
galvanized rebars has a robust quality system and is prepared to certify
the coating thickness. Unlike the case for galvanized roofing sheets
replacement of a substandard and underperforming galvanized rebars on
detection at a later date is practically impossible. Utmost care must
therefore be taken in selection of a proven and dependable source of
The facts presented here is based on work done at TATA STEEL and the
information gathered from the following resources:
1. A Guide to specifying and inspecting hot dip galvanized reinforcing
steel - "American Galvanizers Association", Colorado
2. ACI Committee 222, "Corrosion of Metals in Concrete", American Concrete
Institute, 222R-85, 1985.
3. Adnrade, C et al. "Corrosion Behavior of Galvanized Steel in Concrete,"
2nd International Conference on Deterioration and Repair of Reinforced
Concrete in the Arabian Gulf; Proceedings Vol. 1, pp. 395-410, 1987.
4. Arup, H. "The Mechanisms of the Protection of Steel by Concrete,"
Society of Chemical Industry Conference of Reinforcement in Concrete
Construction; London, June 1983.
5. Bird, C.E. "Bond of Galvanized Steel Reinforcement in Concrete;" Nature,
Vol. 94, No. 4380, 1962.
6. Cornet, I. & Breseler, B. "Corrosion of Steel and Galvanized Steel in
Concrete;" Materials Protection, Vol. 5, No. 4, pp. 69-72, 1966.
7. Hosfoy, A.E. & Gukild, I. "Bond Studies of Hot Dipped Galvanized
Reinforcement in Concrete;" ACI, Journal, March pp. 174-184, 1969.
8. MacGregor, B.R. "Galvanized Solution to Rebar Corrosion;" Civil
Engineering, UK, 1987.
9. Roberts, A.W. "Bond Characteristics of Concrete Reinforcing Tendons
Coated with Zinc;", ILZRO Project ZE-222, 1977
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