Date post: | 08-Nov-2014 |
Category: |
Documents |
Upload: | sujay-raghavendra-n |
View: | 375 times |
Download: | 0 times |
1 INTRODUCTION
Reinforced cement concrete is one of the most widely used modern materials of
construction. It is comparatively cheap and readily available and has a range of attractive
properties and characteristics that makes it suitable for a variety of building and
construction applications. It is also used in a range of exposure conditions. The long-term
performance of RCC is usually assessed against two main criteria, serviceability and
durability. Serviceability refers to the ability of the concrete to resist changes in its
microstructure and properties, particularly where such changes may adversely affect the
serviceability of the element perhaps the most obvious consequence of a lack of
durability in reinforced concrete is the corrosion of the steel reinforcement, a topic that
has been widely studied and reported.
Corrosion of steel reinforcement in a concrete is an electrochemical
process that requires access of an electrolyte and oxygen to steel.
Protective measures against corrosion rely on minimizing or preventing
the corrosive electrochemical process. Four types of protective
measures as under can be identified:
a) Impeding access of deleterious materials water, oxygen, salts,
carbon dioxide etc. to the steel surface.
b) Slowing the electrochemical process through use of inhibitors.
c) Modifying the electrode through cathodic protection.
d) Providing coatings to the steel reinforcement.
1.1 CORROSION
The deterioration of a material, usually a metal, that results from a reaction with its
environment. Corrosion is the primary means by which metals deteriorate. Most metals
corrode on contact with water (and moisture in the air), acids, bases, salts, oils,
aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode
when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas,
and sulfur containing gases.
Steel embedded in concrete is normally protected from corrosion due to the presence of a
passive film on the surface of the metal. This films in the highly alkaline environment of
1
hydrated cement, with a PH in excess of about 13 and as long as the passive state is
maintained, the steel will not corrode, to ensure long term corrosion protection to the
steel, the concrete mass must be sufficiently impermeable so as to limit the transport of
species such as water, chloride ions, oxygen, carbon dioxide and other gasses through the
concrete to the depth of the reinforcement. The presence of critical levels of these
species, which are usually carried into the concrete in solution in water, either change the
nature of the concrete of alter the condition of the embedded steel. In either case,
corrosion of the steel can then initiate. Should corrosion of embedded steel in concrete
occur, physical damage to the concrete mass is likely to follow, steel corrosion products
are quite voluminous with an expansion factor of 2-10 times and typically precipitate at
the interface between the steel and the concrete. The swelling caused by this generates
stresses of sufficient magnitude about 3-4 MPa to exceed the tensile capacity of the
concrete and as a result the concrete cracks in tension. Such cracks usually run from the
bar to the nearest adjacent surface, which may be the edge of a column or precast element
or the surface of a slab or beam. Once cracking has occurred, unsightly rust staining of
the surface is often observed and further swelling usually leads to delamination of the
element or sapling of pieces of concrete from the surface, by this stage, the structure
would be in a serious state of distress, and remedial action would be in a serious state of
distress and remedial action would be necessary to extend its life. Corrosion induced
damage to RCC often necessitates early repair and occasionally complete replacement of
the structure or element well before its design life is reached. Worldwide, the costs
associated with such remedial work are massive and are expected to increase in the future
at an alarming rate. What has also become evident is that while the repair of RCC may
make good the surface deterioration of the problem. The circumstances that lead to the
initial onset of corrosion often survive in adjacent of more deeply buried regions and may
reveal themselves at some time in the future.
1.2 WHY DO METALS & ALLOYS CORRODE
Excepting for noble metals like gold and platinum etc. other occur in nature only as
compounds e.g. oxides, carbonates, & sulphides etc. and not as metals. This is because
such compounds are more stable compared to metal. In other wards compounds of metals
2
have lower energy as compared to metal itself. By spending energy the ore is converted
into metal which is then processed to yield a component or a structure. It is unstable
because it is energy rich and tends to revert back to more stable lower form namely a
compound like oxide or carbonate. This process is what we call corrosion.
1.3 CHOICE OF PROTECTION
There are many different ways of protecting steel but in general they
fall into two categories; metal coatings and organic coatings. Hot dip
galvanizing is a metal coating obtained by dipping steel or iron into a
bath of molten zinc. The iron and zinc react together to form alloy
layers which are covered by a coating of pure zinc as the work is
withdrawn from the bath. This gives an all over protection, inside and
outside, that resists knocks and abrasion yet has a probable life in
excess of 25 years. These are some of the reasons for choosing hot dip
galvanizing, but the deciding factor may well be financial or economic.
Before the economics of galvanizing can be compared with other
methods of corrosion protection, it is necessary to use the same units
of measurement. (Farm Building Research Team)
3
2 MECHANISM OF CORROSION
2.1 GENERAL
During hydration of cement a highly alkaline pore solution (PH between 13 and 13.8).
Principally of sodium and potassium hydroxides, is obtained. In this environment the
thermodynamically stable compounds of iron are iron oxides and oxy-hydroxides. Thus
on ordinary reinforcing steel embedded in alkaline concrete a thin protective oxide film is
formed spontaneously. This passive film is only a few nanometers thick and is composed
of more of less hydrated iron oxides with varying degree of Fe 2+ and Fe3+. The protective
action of the passive film is immune to mechanical damage of the steel surface. It can
however be destroyed by carbonation of concrete or by the presence of chloride ions, the
reinforcing steel is then depassivated.
2.2 INITIATION AND PROPAGATION OF CORROSION
The service life of reinforced concrete structures can be divided in two distinct phases.
The first phase is the initiation of corrosion in which the reinforcement is passive but
phenomena that can lead to loss of passivity e.g., carbonation or chloride penetration in
the concrete cover take place. The second phase is propagation of corrosion that begins
when the steel is depassivated and finishes when a limiting state is reached beyond which
consequences of corrosion cannot be further tolerated.
2.2.1 Initiation Phase
During the initiation phase aggressive substances (CO2 , chlorides) that can depassivate
the steel penetration form the surface into the bulk of the concrete.
Carbonation: Beginning at the surface of concrete and moving gradually towards the
inner zones, the alkalinity of concrete may be neutralized by carbon dioxide form the
atmosphere so that the PH of the pore liquid of the concrete decreases to a value around 9
where the passive film is no more stable.
Chloride Ions from the environment can penetrate into the concrete and reach the
reinforcement; if their concentration at the surface of the reinforcement reaches a critical
level, the protective layer may be locally destroyed.
4
The duration of the initiation phase depends on the cover depth and the penetration rate
of the aggressive agents as well as on the concentration necessary to depassivate the steel.
The influence of concrete cover is obvious and design codes define cover depths
according to the expected environment class. The rate of ingress of the aggressive agents
depends on the quality of the concrete cover (porosity, permeability) and on the
microclimatic conditions (wetting, drying) at the concrete surface. Additional protective
measures can be used to prolong the initiation phase.
2.2.2 Propagation Phase
Breakdown of the protective layer is the necessary prerequisite for the initiation of
corrosion. Once this layer is destroyed, corrosion will occur only if water and oxygen are
present on the surface of the reinforcement. The corrosion rate determines the time it
takes to reach the minimally acceptable state of the structure bur it should be borne in
mind that this rate can vary considerably depending on temperature and humidity.
Carbonation of concrete leads to complete dissolution of the protective layer Chlorides
instead cause localized breakdown, unless they are present in very large amounts.
Therefore:
Corrosion induced by carbonation can take place on the whole surface of steel in
contact with carbonated concrete.
Corrosion by chlorides is localized with penetrating attacks of limited area surround
by non-corroded areas. Only when very high levels of chlorides are present the
passive film be destroyed over wide areas of the reinforcement and the corrosion will
be of a general nature.
If depassivation due to carbonation or chlorides occurs only on a part of the
reinforcement, a macro cell can develop between corroding bars and those bars that are
still passive (and connected that is already corroding.
In structures affected by electrical fields, DC stray current in the concrete can enter the
reinforcement in some areas i.e. it passes from the concrete to the steel and return to the
concrete in a remote site. The passive layer can be destroyed in those areas where the
current leaves the steel.
On high-strength steel used in prestressed concrete but not with common reinforcing steel
under very specific environmental, mechanical loading, metallurgical and
5
electrochemical conditions, hydrogen embitterment can occur which may lead to brittle
fracture of the material.
2.3 CARBONATION INDUCED CORROSION
2.3.1 Carbonation Of Concrete
In moist environments, carbon dioxide present in the air forms an acid aqueous solution
that can react with the hydrated cement paste and tends to neutralize the alkalinity of
concrete ( this process is known as carbonation). Also other acid gases present in the
atmosphere, such as SO2 can neutralize the concrete’s alkalinity but their effect is
normally limited to the surface of concrete.
The alkaline constituents of concrete are present in the pore liquid (mainly as sodium and
potassium hydroxides) but also in the solid hydration product. Calcium hydroxide is the
hydrate in the cement paste that reacts most readily with CO2. The reaction that takes
place in aqueous solution can be written schematically as
CO2+Ca(OH)2-----------H2o,NaoH----------- CaCO3+H2O
This is the reaction of main interest, especially for concrete made of Portland cement
even though the carbonation of C-S-H is also possible when Cs(OH)2 becomes depleted,
for instance by pozzolanic reaction in concrete made of blended cement.
Carbonation does not cause any damage to the concrete itself although it may cause the
concrete to shrink, indeed, in the case of concrete obtained with Portland cement, it may
even reduce the porosity and lead to an increased strength. However carbonation has
important effects on corrosion of embedded steel. The first consequence is that the PH of
the pore solution drops from its normal values of PH13 to 14 to values approaching
neutrally. If chlorides are not present in concrete initially, the pore solution following
carbonation is composed of almost pure water. This means that the steel in humid
carbonated concrete corrodes as if it was in contact with water. a second consequence of
carbonation is that chlorides bound in the form of calcium chloraluminate hydrates and
otherwise bound to hydrated phases may be liberated, making the pore solution even
more aggressive.
2.3.2 Penetration Of Carbonation
6
The carbonation reaction starts at the external surface and penetrates into the concrete
producing a low PH front. The measurement of the depth of carbonation is normally
carried out by spraying an alcoholic solution of phenolphthalein on a freshly broken face.
The areas where PH is grater than 9 take on a pinkish color typical of phenolphthalein in
a basic environment while the colour of carbonated areas remains unchanged. The rate of
carbonation decreases in time as CO2 has to diffuse through the pores of the already
carbonated outer layer.
2.4 CHLORIDE INDUCED CORROSION
Chloride contamination of concrete is a frequent cause of corrosion of reinforcing steel.
Modern design codes for reinforced and prestressed concrete structures impose
restrictions on the amount of chloride that may be introduced from raw materials
containing significant amounts of chlorides. According to the European standard EN 206,
the maximum allowed chloride contents are 0.1-0.2% for pressed concrete. These
restrictions are thought to eliminate corrosion due to chloride in the fresh concrete mix.
In some structures built in the past, chlorides have been added in the concrete mix,
unknowingly `or deliberately, through contaminated mixing water aggregates for instance
by using sea-dredged sand and gravel without washing them with chloride-free water of
admixtures calcium chloride, which is now forbidden, in the past was the most common
accelerating admixture. Chloride contents from accelerating admixture in amounts
ranging from 0.5% to well over 2% by mass of cement have caused extensive corrosion
damage after carbonation and even in alkaline conditions.
The other main source of chloride in concrete is penetration from the environment. This
occurs for instance in marine environments or in road structures in regions where
chloride-bearing de-icing salts are used in wintertime.
Corrosion of reinforcement in non-carbonated concrete can only take place once the
chloride content in the concrete in contact with the steel surface has reached a threshold
value. This threshold depends on several parameters however the electrochemical
potential of the reinforcement, which is related to the amount of oxygen that can reach
the surface of the steel, has a major influence. Relatively low levels of chlorides are
sufficient to initiate corrosion in structures exposed to the atmosphere, where oxygen can
7
easily reach the reinforcement. Much higher levels of chlorides are necessary in
structures immersed in sea water or in zones where the concrete is water saturated, so that
oxygen supply is hindered and thus the potential of the reinforcement is rather low.
However even in atmospherically exposed structures such as bridge deck, considerable
scatter is present in threshold values. In the field and over larger numbers of structures
showing corrosion as a function of chloride content as shown in fig we will return to the
threshold in a subsequent section.
2.5 PITTING CORROSION
Chlorides lead to a local breakdown of the protective oxide film on the reinforcement in
alkaline concrete so that a subsequent localized corrosion attack takes place. Areas no
longer protected by the passive film act as anodes with respect to the surrounding still
passive areas where the cathodic reaction of oxygen reduction takes place. If very high
levels of chlorides reach the surface of the reinforcement the attack may involve larger
areas so that the morphology of pitting will be less evident. The mechanism however is
the same.
Once corrosion has initiated a very aggressive environment will be produced inside pits.
In fact current flowing from anodic areas to surrounding cathodic areas both increases the
chloride content (chloride being negatively charged ions, migrate to the anodic region)
and lowers the alkalinity (acidity is produced by hydrolysis of corrosion products inside
pits). On the contrary the current strengthens the protective film on the passive surface
since it tends to eliminate the chlorides while the cathodic reaction produces alkalinity.
Consequently both the anodic behavior of active zones and the cathodic behavior of
passive zones are stabilized. Corrosion is then accelerated autocatalytic mechanism of
pitting and can reach very high rates of penetration up to 1 mm/y that can quickly lead to
a remarkable reduction in the cross section of the rebars.
Consequences of pitting corrosion may be very serious in high-strength prestressing steel,
where hydrogen embrittlement can be promoted.
2.5.1 Corrosion Initiation
8
Initiation of pitting corrosion takes place when the chloride content at the surface of the
reinforcement reaches a threshold value or critical chloride content. A certain time is
required from the breakdown of the passive film and the formation of the first pit
according to the mechanism of corrosion described above. From a practical point of view
the initiation time can be considered as the time when the reinforcement in concrete that
contains substantial moisture and oxygen is characterized by an averaged sustained
corrosion rate higher than 2mA/m2. The chloride threshold of a specific structure can be
defined as the chloride content required to reach this condition of corrosion.
When chloride originates from the environment the initiation time of corrosion will
depend on the rate of penetration of chloride ions through the concrete cover. The
knowledge of both the chloride threshold and the kinetics of penetration of chlorides into
essential for the assessment of the initiation time of corrosion of reinforced concrete
structures exposed to chloride environments. In this regard the influence of several
parameters related both to the concrete and the environment has to be considered
( C.L. Page et.al).
9
3 CORROSION RESISTANT REINFORCEMENT
Reinforcing bars with a higher corrosion resistance than the common carbon steel rebars
can be used as a preventive method under conditions of high environmental
aggressiveness or when along service life is required. The corrosion resistance of rebars
can be increased either by modifying the chemical composition of the steel of by
applying a metallic or organic coating on their surface. There families of corrosion
resistant bars are used in reinforced concrete structures consist in respectively in stainless
steel, galvanized steel and epoxy coated rebars.
Fiber reinforced polymers rebars usually made of an epoxy matrix reinforced with carbon
or aramide fibers have also been proposed both as prestressing wires and reinforcement.
Nevertheless they are not discussed here because these applications are still in the
experimental phase and there is a lack of experience on their durability in fact while they
are not immune to other types of degradation. FRP are also used in the form of laminate
or sheets as externally bonded reinforcement in the rehabilitation of damaged structures.
3.1 STAINLESS STEEL REBARS
Stainless steel is an extended family of steel types with a wide variety of characteristics
with regard to physical and mechanical properties cost and corrosion resistance. They
have a much higher corrosion resistance than carbon steel which derives from a
chromium rich passive film present on their surface. Stainless steel bars can be used as a
preventative technique for structures exposed to aggressive environments especially in
the presence of chlorides. They can also be selectively used in those parts of structures.
Different available types of stainless steel allow the engineers to select the most suitable
in terms of strength corrosion resistance and cost
3.1.1 Properties Of Stainless Steel Rebars
Stainless steel can be divided into four categories based on their microstructure ferritic
austentic, martenstic and austntic-rerritic. Only specific grades of austenic and duplex
stainless steel are currently used in concrete although also a ferritic type with 12%
chromium has been proposed. In some countries also clad bars with a carbon-steel core
and an external layer of stainless steel are used.
10
3.1.2 Mechanical Properties
Stainless steel rebars must have mechanical properties at least equivalent to those of
carbon steel rebars in terms of characteristic yield strength elastic modulus and ductility.
The strength of an nealed austnitic stainless steels is too low to comply with requirements
for reinforcing bars and thus bars need to be strengthened. This is usually achieved by
cold working for bars of lower diameter or by means of hot rolling for bars of higher
diameter. Sufficient yield strength can be achieved with duplex stainless steels even
without any strengthening.
3.1.3 Corrosion Resistance
Although all types of stainless steels are passive in carbonated concrete the use of
stainless steel rebars in normally associated with chloride bearing environments. In fact
for structures for structures subjected only to carbonation unless an extremely long
service life required prevention of steel corrosion can be achieved with a proper design of
concrete mix and the concrete cover of if necessary by using less expensive additional
measures. Since the late 1070s many experimental studies have been carried out in order
to investigate the corrosion behavior of stainless steel in chloride contaminated structures
Pitting is the only form of corrosion expedited in practice on stainless steel in concrete.
Intergranular corrosion induced by welding is normally avoided by using appropriate
types of steel. Stress corrosion may take place only under conditions of high temperature
carbonate concrete and heavy chloride contamination which are very unlikely to occur
concomitantly. Because of the alkalinity of the pore solution and the porosity of the
cement paste, crevice corrosion also unlike on stainless steel embedded in concrete. The
corrosion resistance of stainless steel is affected by the presence of a mill scale ob their
surface this is however normally removed by pickling of a sandblasting, picking gives the
best result.
3.1.4 Stainless Steel Rebars Applications And Cost
In principle stainless-steel reinforcement can be a viable solution for preventing corrosion
in a large number of applications. The chloride threshold is much higher than the chloride
content that is normally found in the vicinity of the steel even in structures exposed to
marine environment or protection is necessary combined with normal steel at other areas.
11
Hence stainless steel bars can be used in the more vulnerable parts of structures exposed
to chloride environments, such as joints of bridges or the splash zone of marine
structures. Similarly they can be used when the thickness of the concrete cover has to be
reduced such as in slender elements. Their use may have a significant impact on the cost
of a structure. The cost of the material has decreased in recent years and further
reductions are expected due to new developments in production but stainless steel bars
are still much more expensive than carbon steel bars.
The additional cost of using stainless steel can be drastically reduced by means of a
selective use of stainless steel bars limited to the more vulnerable parts of the structure, in
the past structural designers were reluctant to use such an alternative because of the fears
regarding galvanic coupling with carbon steel. Now the combined use of stainless steel
and carbon-steel bars is encouraged in order to reduce cost referring to an intelligent use
of stainless steels. This additional cost must be compared to the cost of repair possibly
needed in the future multiplied by the probability of its occurrence.
The real use of stainless steel bars in concrete is however still rather modest among the
examples reported in the literature some documented cases are bridges subjected to the
use of de-icing salts of historical buildings. An interesting extreme example in the Guild
Hall Yard East in London which is a building hosting a Roman amphitheate Stainless
steel bars were used for new reinforced concrete walls in order to guarantee a design life
of 750 Yr.
3.2 EPOXY COATED REBARS
Epoxy coating of reinforcing bars is a protective technique developed in the 1970s in
North America Laboratory results confirmed the effectiveness of the epoxy coated bars in
many cases in preventing corrosion of reinforcement in carbonated or chloride
contaminated concrete. Recently however doubts borne out above all by negative
experience reported on structures in tropical environments.
3.2.1 Properties Of The Coating
Protection of rebars by organic coating is based on the principle of insulating the steel
and protecting it form aggressive agents that penetrate the concrete cover. The use of
coated bars should not require any changes in the structural design or during the different
12
phases of construction. The coating must be able to cover the reinforcement uniformly be
though and well adherent flexible enough to allow bending of the reinforcement able to
transmit stresses form the concrete to the reinforcement. Of all organic coatings available
the only coating types able to satisfy all these conditions are those made with epoxy
resins.
Beyond the need to insure good adhesion of the coating to the steel surface essential in
order to guarantee adequate resistance to corrosion and to allow bending of the rebar, it is
of crucial importance to have proper bonding between the coated bar and the concrete. In
fact to avoid changes in the structural design procedures it is necessary to obtain levels of
bond strength with epoxy coated bars comparable to those of bare reinforcement. Usually
coatings are less than 300μm in thickness and the reduction in bond strength to concrete
of ribbed bars with epoxy coating with respect to uncoated bas of the same geometry is
limited at least for commonly used diameters.
Requirements for epoxy-coated reinforcing bars are reported in different international
standards and recommendation the first dates back to 1981(ASTM A 775 -81) and more
recent national standards in European countries are based on it even though their
requirements are much more rigorous.
The piece of epoxy coated bars is roughly twice the price of uncoated bars.
3.2.2 Corrosion Resistance
Even though it is not completely impermeable to oxygen water and chlorides epoxy
coating of reinforcing bars can guarantee protection against reinforcement corrosion in
chloride contaminated concrete. The protection provided by the coating improves as its
thickness increase. There is however an upper limit fixed by the need to achieve adequate
bonding between the steel and the concrete. Standards specify thickness between 0.1 and
0.3 mm. effective protection depends to a great extent on the integrity of the coating in
fact any damage will expose bare metal to the aggressive environment. In the case of
chloride-contaminated concrete the attack tends to penetrate below the coating and
widens the area affected. In carbonated concrete on the other hand the attack tends to
remain in the region of the defect. Very high corrosion rates in the vicinity of defects in
the coating can occur in the presence of macro cells. A typical situation is tat of structures
in which epoxy coated reinforcement in contact with chloride-contaminated concrete is
13
coupled to non-coated reinforcement embedded in concrete that is uncontaminated of
contains a level of chlorides below the critical level. In this case the passive non coated
reinforcement can act as an effective cathode of much grater size than the anodic area
corresponding to the defects in the coating there by determining a very unfavorable anode
/cathode area ratio. The corrosion rate will be particularly high if the receptivity of the
concrete is low. For this reason design and application rules requires that the coated bars
be electrically isolated from each other.
3.2.3 Practical Aspects
As indicated above defects in epoxy coating on bars present the risk of strong local
attack. Specifications and site practice must be aimed at obtaining coated bars without
such defects particularly with regard to production handling cutting bending storage
welding and bonding. Furthermore having concrete with good chloride penetration
resistance and electrical resistance adds to the protection by making the complete system
more robust.
It is further more important to pay attention to those cases in which for economical
reasons coated steel is used only in the most critical areas of the structure. In these cases
it is important to ensure that epoxy-coated bars are electrically insulated from the
uncoated reinforcement in order to avoid macro cells.
3.2.4 Effectiveness
In recent years there have been very serious cases of corrosion damage in some structures
in tropical areas where sever attack of epoxy-coated steel has been observed only a few
years after construction. This situation has led contractors and designers to reconsider the
widespread use of this technique on structures exposed to chloride bearing environments.
Serous doubts have been expressed about whether epoxy coatings even in the absence of
any damage, can insure long–lasting protection in heavily chloride contaminated and hot
environments particularly when the concert is frequently wetted.
It should also be observed that because of the absence of electrical connection between
the individual coated bars if the coating is not effective in protecting the bar the
application of electrochemical techniques such as cathodic protection is not possible in
practice. Even the inspection of structures is difficult e.g. Potential on practice. Cannot be
14
applied if bars are disconnected. to avoid this problem for precast elements in Europe the
coating has also been applied on weld mesh or complete reinforcement cages so that
electrical connection of bars was guaranteed.
15
4 GALVANIZING
Process of coating a metal, usually iron or steel, with a protective covering of zinc.
Galvanized iron is prepared either by dipping iron, from which rust has been removed by
the action of sulfuric acid, into molten zinc so that a thin layer of the zinc remains on the
surface of the iron upon removal or by a method of electroplating. Iron is also coated
with zinc by a method in which the iron is first covered with the zinc dust and then
baked; an alloy is formed at the surface, the resulting product being known as sherardized
iron. Sheets of pure iron, copper iron, and various steels, as well as wire and netting, are
often galvanized, since the zinc coating resists oxidation and the action of moisture very
successfully. When the coating is broken or pierced some protection is still afforded,
since the zinc reacts with the corroding agent first.
4.1THE BIRTH OF GALVANIZING
Following the important discovery by Galvani and Volta that electricity is generated
through the contact of dissimilar metals, it was noticed in Volta's battery that one of the
two metals was always preserved from oxidation. Stanislaus Sorel, a civil engineer
working in Paris, filed a patent on 10 May 1837 for a method of protecting iron from rust.
The patent was for "galvanic" preservation of iron either by coating it in a bath of molten
zinc or by covering it with a so-called "galvanic paint". The method was developed by
completely coating the surface of the iron with a layer of zinc. This was the parent of the
hot dip galvanizing process
4.2 Why Galvanize!
No other protective coating for steel provides the long life durability and predictable
performance of hot dip galvanizing. An alloy of its steel base, a galvanized coating is
unique in matching the design and handling characteristics of steel.
As asset management and life-cycle costing become even more essential, after fabrication
galvanizing provides the facility to design for a predictable, engineered result.
Galvanizing is a once only process, committed to the concept of the maintenance-free use
of steel, ensuring long service life and virtually eliminating disruptive maintenance. This
long-term protection is well documented world-wide in terms ahead of any other
16
rotective coating, and galvanizing continues to find new applications in almost every
field of engineering. ( www.rustfreetrucks.com/bar)
4.3 GALVANIZED STEEL REBARS
Galvanized steel rebars can be used as a preventative measure to control corrosion in
reinforced concrete structures exposed to carbonation or mild contamination with
chloride such as chimneys, bridge substructures tunnels and coastal buildings.
Galvanized reinforcement offers significant advantages compared to carbon steel under
equivalent circumstances. These include an increase of initiation time of corrosion a grate
tolerance for low cover e.g. in slender element and corrosion protection is offered into the
reinforcement prior to it being embedded in concrete.
4.4 PROPERTIES OF GALVANIZED STEEL BARS
Galvanized bars are produced by the hot dip galvanizing process. Pickled steel bars of
welded bages are dipped in a bath of molten zinc at temperature of about 450 0c. This
process produces a metallic coating composed of various layers of iron-zinc alloys; which
has a metallurgical adhesion to the steel substrate. An external layer of pure zinc left by
the simple solidification of the liquid metal is formed on top of a sequence of inner layer,
increasingly rich in iron which is the result of formation of brittle intermetallic
compounds. The thickness of the iron-zinc layers depends on the composition of the
steel, the temperature and composition of the zinc both and the inner ion time. The silicon
content in the steel has a great influence usually it should be maintained between
0.16-.20% to limit the thickness of these brittle layers. The total thickness of the coating
should be at least 100μm and it should not exceed 150μm.
The proper execution of the galvanization process should guarantee that the temperature
and the time of galvanization do not affect the mechanical propertied of the steel bars.
The external layer of pure zinc is of primary importance with regard to corrosion
resistance of the bars. If galvanized steel is exposed to a neutral environment such as the
atmosphere, the duration of protection is primarily dependent on the thickness of the zinc
coating and its composition and microstructure has a negligible effect. Similarly for
galvanized steel bars embedded in concrete the protective properties of zinc coating are
due for the most part to the external layer of pure zinc which cam form a passive film if it
17
has a sufficient thickness. In fact a loss of thickness of 5010μm is required prior to
passivation while if the thickness is insufficient the underlying layer of Zn-Fe alloy
passivate with more difficulty.
The passivation of zinc on the PH of the pore solution. In contact with alkaline solutions
as long as the PH remains below 13.3 zinc con passivate due to formation of a layer of
calcium hydroxyzincate. However even at values higher than 12. in the presence of
calcium ions such as in concrete pore solution zinc can be passive and has a very low
corrosion rate. In saturated calcium hydroxide solutions it was found that for PH values up
to about 12.8 a compact layer of zinc corrosion products forms which will protect the
steel even if the PH changes in a subsequent phase. For PH values between 12.8 and 13.3
larger crystals form that can still passivate the bar. Finally for values above 13.3 coarse
corrosion products form that cannot prevent corrosion.
Since the PH of the concrete pore solution may vary in the where remarkable changes in
the behavior of zinc occur the behavior of galvanized steel may be influenced by the
composition of the concrete and especially by the cement type and its alkali content. in
practice however the pH of the pore solution in concrete usually is below 13.3 during the
first hour after mixing, due to the presence of sulfate ions from the gypsum added to the
Portland cement as a set refulator. a protective layer thus can be formed on galvanized
bars.
The passive film that forms on zinc not only reduces the rate of the anodic process but
even cathodic reactions of oxygen reduction and hydrogen development. In conditions of
passivity the corrosion potential of galvanized steel is therefore much lower than tat of
carbon steel. Values typically measured are between -66 and -55mV SCE compared to
values above -2—Mb usually found for passive carbon steel reinforcement.
Bonding between reinforcement and concrete is essentially for a safe and reliable
performance of concrete structures. Several factors such as concrete composition
placement curing conditions and age may affect the bond between galvanized steel and
concrete. At earl age the bond strength may be lower than that of normal steel bars due to
the hydrogen evolution at the interface and the dissolution of the superficial layer of the
zinc coating which delays the hydration of interfacial cement paste. However after a few
weeks the galvanized steel adheres with and its increased roughness improves adhesion to
18
the concrete. A higher bond with respect to bare steel could be obtained due to the
formation of hydroxyzincate seystals that fill the interfacial porosity of the cement paste
and act as bridges between the zinc coating and the concrete / in practice bond strength
for ribbed blace steel and galvanized steel bars is essentially the same because it is
mainly provided by the mechanical interlocking between the ridges of the ribbed bars and
the concrete.
Often galvanized bars are chromate tested in order to inhibit zinc corrosion and to control
hydrogen evolution.
It should be noted that hydrogen evolution is possible on galvanized bars forst of all
during pickling before galvanization then in the first hours after casting and finally in
hardened concrete in conditions of lack of oxygen. For this reason galvanizing is not
recommended a protective measure fort steel susceptible to hydrogen enmbrittlement.
Galvanized steel bars can be welded but ions of the zinc coating may take place in the
welded zone the application of a zinc rich paint should be recommended ager cleaning of
ghrt welded area.
4.5 CORROSION RESISTANCE
The passive film of galvanized rebars is stable even in mildly acidic environment so that
the zinc coating remains passive even when the concrete is carbonated. The corrosion rate
galvanize steel in carbonated concrete is approximately 0.5-0.7 μm/y therefore a typical
80 m galvanized bars remains negligible in carbonated concrete even if a low content of
chloride is present.
In chloride contaminated concrete galvanize steel may be affected by pitting corrosion. In
general a critical chloride level in the range of 1-1.5% by mass of cement is assumed for
galvanized steel compared to the value of 0.4-1% normally considered for carbon steel
reinforcement. The slightly improved resistance to chloride attack is due for a large part
to the lower value of the free corrosion potential of galvanized steel. Even if potting
corrosion has initiated the corrosion rate to be lower for galvanized steel since the zinc
coating that surround the pits is a poor cathode and thus it reduces the effectiveness of the
autocatalytic mechanism that takes place inside pits on bare steel. On the other hand it
can be observed that as long ad the zinc coating is passive it is not able to provide active
19
protection to steel as happens for galvanized steel exposed to the atmosphere and so it
consequently cracks in the zinc coating must be avoided and macroscopic defects have to
be repaired prior to casting.
The price of galvanized bars is about 2 to 2.5 times the piece of normal black steel bars.
4.6 BOND STRENGTH OF CONCRETE TO GALVANIZED
REINFORCING BARS
The results of extensive of pull-out testing by a number of researches reveal no
significant difference in the bond strengths of black and galvanized steel deformed
reinforcing bars in concrete. Tests made by Building Research Establishment in UK show
that based on the work of five investigators, adhesion to concrete of plain reinforcing bars
is on an average as follows:
1. hot dip galvanized steel 3.3 - 3.6MPa
2. black steel 1.3 - 4.8 MPa
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:
1. in black steel 150MPa
2. in hot dip galvanized steel 160MPa
3. in hot dip galvanized steel (chromated) 190MPa
the bond strength to concrete has also been studied in tests conducted by the University
of California in accordance with American Concrete Institute standard 208-58. both
corroded and uncorroded rebar were used. 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 showed equal or better bond strength than
ungalvanized rebars in all conditions in both plain and deformed types.
4.7 TEN REAL BENEFITS OF GALVANIZED STEEL
The use of galvanizing for structural steel protection gives you ten major, measurable
benefits.
20
1 Lowest first cost: Galvanizing is lower in first cost than many other commonly
specified protective coatings for steel. (The application cost of labour intensive coatings
such as painting has risen far more than the cost of factory operations such as galvanizing
- the labour component of finished paint coatings averages about 80%, compared to only
about 30% for galvanizing.)
2 Less maintenance / lowest long term cost. Even in cases where the initial cost of
galvanizing is higher than alternative coatings, galvanizing is almost invariably cheapest
in the long term (because it lasts longer and needs less maintenance). And, maintenance
causes problems and adds to costs when structures are located in remote areas, and when
plant shutdown or disruption to production is involved.
3 Long life. The life expectancy of galvanized coatings on typical structural members
is far in excess of 50 years in most rural environments, and 20 to 25 years plus, even in
severe urban and coastal exposure.
4 Reliability. Galvanizing is carried out to Australian / New Zealand Standard 4680, and
standard, minimum coating thicknesses are applied. Coating life and performance are
reliable and predictable.
5 Toughest coating. A galvanized coating has a unique metallurgical structure which
gives outstanding resistance to mechanical damage in transport, erection and service.
6 Automatic protection for damaged areas. Galvanized coatings corrode preferentially
to steel, providing cathodic or sacrificial protection to small areas of steel exposed
through damage. Unlike organic coatings, small damaged areas need no touch up.
7 Complete protection. Every part of a galvanized article is protected, even recesses,
sharp corners and inaccessible areas. No coating applied to a structure or fabrication after
completion can provide the same protection.
8 Ease of inspection. Galvanized coatings are assessed readily by eye, and simple non-
destructive thickness testing methods can be used. The galvanizing process is such that if
coatings appear sound and continuous, they are sound and continuous.
9 Faster erection time. As galvanized steel members are received they are ready for use.
No time is lost on-site in surface preparation, painting and inspection. When assembly of
the structure is complete, it is immediately ready for use, or for the next construction
stage.
21
10 A full protective coating can be applied in minutes. A 4-coat paint system requires a
week. The galvanizing process is not dependent on weather conditions
(www.rustfreetrucks.com/bar)
4.8 ECONOMICS OF GALVANIZED REINFORCEMENT IN CONCRETE
When the coats and consequences of corrosion damage to a reinforced concrete building
are analysed the extra cost of galvanizing is small. It can be regarded as an insurance
premium but a premium which is low and need be paid once only. Currently the cost of
galvanizing of rebar is approximately Rs 8000 per tonne of steel to be galvanized, bar
diameter etc.
While the cost of galvanizing is an important factor the cost of galvanized 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 and this is the correct life cycle coasting approach to the correct
life cycle costing approach to be adopted in such instances. For most structures even in
the most aggressive environments the use of galvanized reinforcement can be confined to
the exposed surfaces even in the most aggressive environments the use of galvanized
reinforcement can be confined to the exposed surface and critical structural elements such
as
Thin precast cladding elements
Facades of prestigious buildings surface exposed beams and columns
Window and door surrounds
Prefabricated units
External facades of buildings near the sea coast
Architectural features
when related to total project coasts the added cost of galvanizing becomes very small
indeed. Such costs represents 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.
Accordingly whenever there is concern that premature corrosion of reinforcement might
occur reinforcement should be galvanized. The use of galvanizing however should not be
22
considered an alternative to the provision of an adequate cover of dense impermeable
concrete. (Dr. Stephen R. Yeomans)
4.9 PASSIVATION AND ADDITIVES
The research into bond strengths also shows that there is little or no need for the current
practice of chromate passivation of galvanized reinforcement by the galvanizer or the
alternative addition chromium trioxide to the concrete mix. The addition of chromates to
the concrete mix in the ratio of 35-150 ppm plain bars significantly.
4.10 APPLICATONS IN INDIA
Convinced of the economic and technical benefits of galvanized rebars the Institute
building and construction sector has started using these in a number of projects. The
following are some of the known applications for galvanized rebars in India:
1. Lotus Temple New Delhi (300 tonnes)
2. Residential Building JNPT Uran (1000 T)
3. All Indian Institute of Physical Medical & Research, Haji Ali, Mumbai (50 T)
4. Residential Building Wadala Mumbai (50 T)
5. Mahanagar Gas Ltd. Mumbai (50 T)
6. Guest House Mangalore (50 T) ( V.R. Subramaian ).
23