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Message No 36 from Mr. Tom Loe

 
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inchak at tatasteel.com
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PostPosted: Fri Jul 08, 2005 9:31 am    Post subject: Message No 36 from Mr. Tom Loe Reply with quote

Dear Mr. Tom Joe
You have raised a very important question:
QUOTE
"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
abrasion resistance."
UNQUOTE
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
viewpoint.

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
application.
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
option.

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
CONCRETE.

Zinc coating serves its intended purpose of protection against corrosion in
two  distinct  and  significant ways of action - THE BARRIER ACTION AND THE
SACRIFICIAL ACTION
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
shown below:

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
called for.

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
concrete reinforcement
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
supply.

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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