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

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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

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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)

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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.

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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

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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

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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

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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

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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).

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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.

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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.

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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

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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

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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

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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.

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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

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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

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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

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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

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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.

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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.

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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

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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 ).

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