REINFORCEMENT IN CONCRETE & IT’S EFFECTS

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INTRODUCTION:

Corrosion of reinforcing steel has led to premature deterioration of many

concrete structures in the world before their design life is attained. This has

placed tremendous financial burden on many state and local transportation

agencies in their attempts to halt ongoing reinforcing steel corrosion in the

existing structures that are still functional (so that as much service life as

possible can be salvaged from these) and to replace those structures that have

already deteriorated to the point that it does not make any economic sense to

keep on maintaining them. In addition, badly deteriorated bridges have

considerable adverse effects on the nation's economic output and also place the

safety of motorists at risk.

CORROSION:

Chemical action which causes destruction of the surface of a material by

oxidation or chemical combination. Also caused by reduction of the electrical

efficiency between a metal and a contiguous substance or to the disintegrating

effects of strong electrical currents or ground return currents in electrical

systems. The latter is known as electrolytic corrosion.

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Corrosion in Steel Concrete

HOW CORROSION OCCURS..?

Mechanism of corrosion of steel is an electro-chemical process. The

electro-chemical process starts when there is a potential difference cause due to

difference in concentration of dissolved ions such as alkalies, chlorides and

oxygen, in vicinity of steel. Due to the potential difference some parts of the

metal become anodic and the other parts become cathodic. Dissolution or

pitting of iron takes place on the anodic parts and the reduction of oxygen takes

place on the cathodic parts. Rust appears on the anodic part as iron (ferrous)

gets converted to ferrous oxide or ferrous hydroxide. For this chemical process

presence of moisture and oxygen is necessary. The concrete acts as an

electrolyte and the electro-chemical process takes place as shown in fig.

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Depending on the state of oxidation, metal get converted to rust (corrosion

product) which may occupy 6 to 8 time the original size of steel. This growth

creates tensile stresses within the concrete mass surrounding the reinforcement

steel. As concrete is weak in taking tensile or expansive forces it cracks and

spalling of concrete takes place.

PROBLEM OF NATURE

Inorder to understand the mechanism behind corrosion of reinforcing steel in

concrete, one has to examine the chemical reactions involved. In concrete, the

presence of abundant amount of calcium hydroxide and relatively small amountof alkali elements such as sodium and potassium gives concrete a very high

alkalinity with pH of 12 to 13. It is widely accepted that at the early age of

concrete, this high alkalinity results in the transformation of a surface layer of

the embedded steel to a tightly adhering film, that is comprised of an inner

dense spinel phase in epitaxial orientation to the steel substrate and an outer

layer of ferric hydroxide. As long as this film is not disturbed, it will keep the

steel in passive and protected from corrosion. When a concrete structure is

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often exposed to salts, salt spashes, salt spray or sea water, chloride ions from

these will slowly penetrate into the concrete, mostly through the pores in the

hydrated cement paste. The chloride ion will eventually reach the steel and then

accumulate to beyond a certain concentration level at which the protective filmis destroyed and the steel begins to corrode, when oxygen and moisture are

present in the steel- concrete interface.

Corrosion can also occur even in the absence of chloride ions. For

example, when the concrete comes in the contact with carbonic acid resulting

from carbon dioxide in the atmosphere, carbonation of the calcium hydroxide

in the hydrated cement paste leads to reduction of the alkalinity to pH as low as

8.5, thereby permitting corrosion of the embedded steel:

The rate of carbonation in concrete is directly depends on the w/c

ratio of the concrete i.e. the higher the ratio greater is the depth of carbonation

in the concrete. In concrete of reasonable quantity, that is properly consolidated

and has no cracking, the expected rate of carbonation is very low. For example,

in concrete with w/c of 0.45 and concrete cover 25 mm, it will require more

than 100 years for carbonation to reach the concrete immediately surrounding

the steel. Carbonation of concrete or mortar is more of an issue in Europe than

in North America.

CORROSION GROWTH:

Some iron products are protective when they remain stable and are thin,

e.g. ferrous hydroxide Fe (OH)2 " Green Rust , magnetite. On the other hand,

products which do not stop corrosion growth with time. When reinforcement

corrodes, it undergoes a more or less localized dissolution, and it is covered

with unstable corrosion products (traditional rust of reddish color). In a rather

porous and wet concrete, these products migrate through the cover and can

stain the concrete surface. But, in the more traditional case, for a relatively dryconcrete, the corrosion products swell by highly deforming the cover and,

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under an effect, can crack the concrete or making spalling. The reduction of

steel cross can also area and the simultaneous rust swelling induce a more or

less significant decrease of thebonding between steel and concrete.

Due to wetting and drying cycles, heating and cooling cycles, loadingand unloading cycles, cyclic loading, leaching of lime and most importantly

additions and alterations made to the structures, isolated cracks, voids,

entrapped air and large capillary pores get interconnected and external moisture

and chlorides find their way to reinforcement steel and corrosion starts.

Of all engineering materials concrete is the most widely used one, on a

tonnage basis.

The main mechanism of degradation of steel-reinforced concrete is

corrosion damage to the reinforcing steel (rebar).

Contrary to common belief, concrete is a complex composite material,

whose structure and properties can change over time.It is generally recognized that the environmental degradation of the concrete

infrastructure is a serious, large scale and costly problem in many parts of the

world.

STAGES OF CORROSION

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Corrosion with rusting of reinforcement in concrete comprises of

two stages. In first stage, the aggressive elements such as chloride or carbon

dioxide (CO2) (Cl-) present in surrounding medium, penetrate in concrete. This

is the initiation stage. The second stage is propagation which starts when theseaggressive bodies are in rather high concentrations at the reinforcement level.

This corresponds to the rust growth which can break concrete cover.

So to describe steel corrosion in concrete, it is advisable to define

on the one hand, the penetration of the aggressive agents through concrete and

on the other hand, the conditions of depassivation of reinforcement, then the

dissolution rate of of metal and the rust growth. It should be noted that high

strength steels used for prestressing of concrete can undergo a specific cracking

by stress corrosion. This case in not treated here.

INITIATION OF REINFORCEMENT CORROSION

The reinforcement corrosion is initiated when the products formed

on steel do not protect them any more, because they become more porous. So, a

first criterion of corrosion initiation corresponds to the change of the nature of

these products. This process includes intermediate stages which gives more or

less stable products. Green rusts. A criterion for corrosion initiation, which is

more operational, corresponds to a significant modification of dissolution metal

rate (change of the corrosion activity).

DETERIORATION OF CONCRETE COVER

A concrete cover around reinforcement can be deteriorated by the

surrounding environment, because of various origins:

1. Physical causes: freezing etc.

2. Mechanical causes: concrete can crack under an excessive loading.

3. Chemical causes: for e.g. due to bodies (gas or ions) contained in the

environment.

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STEEL IN THE SOUND CONCRETE:

Before being placed in a formwork, a reinforcement is rusted, because itwas initially exposed to atmosphere. When a freshly-mixed concrete is placed

around this steel, the mixing water penetrates through the rust pores, where it

gradually forms hydrated calcium ferrite (4.CaO. Fe2O3 13H2 O). Moreover,

this water reacts with steel and forms on it a thin layer of iron and calcium

hydroxides, respectively [Fe(OH)2] and [Ca(OH)2]. All these products in the

vicinity of steel raise the pH of concrete pore solution, up to about 13. It should

be noted that in contact with an initial rust, cement hydration of is disturbed: a

transition zone is locally formed and concrete is more homogeneous, far from

this zone. So, the concrete mixing water makes it possible to form around steel,

some products, which protect it by passivation. More precisely, under

atmospherically induced rust, reinforcement is covered with a thin protective

layer of white products, containing ferrite and of hydroxide of calcium. Such a

protection vanishes, if the pore solution disappeared (e.g. when large cracks

reach reinforcements) or does not correspond any more to sound concrete.

Stages of steel corrosion in concrete induced by agents such as chlorides or the

carbon dioxide. The aggressive agent penetrates in concrete cover, and then

initiates rust. This rust grows and can crack the cover.

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DETERMINATION OF CORROSION

PROFOMETER / REBAR LOCATOR AND BAR SIZER:

Principle and Procedure:

The reinforcement bar is detected by magnetising it and inducing a

circulating "eddy current" in it. After the end of the pulse, the eddy current dies

away, creating a weaker magnetic field as an echo of the initial pulse. The

strength of the induced filed is measured by a search head as it dies away and

this signal is processed to give the depth measurement. The eddy current echo

is determined by the depth of the bar, the size of bar and the orientation of the

bar. This detection of location of reinforcement is required as a pre process for

core cutting.

Display Unit Universal Probe Display

Fig: Diagram of Profometer

Profometer is a portable battery operated magnetic device that can

measure the depth of reinforcement cover in concrete and detect the position of

reinforcement bars, Fig-. The basic principle in this method is that the presence

of steel affects the field of electromagnet. Fig- shows a typical circuitry

diagram to locate rebars and cover includes the probe unit and display unit.

In the typical Proformeter, the probe unit consists of a high permeable U-

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shaped magnetic core on which two coils are mounted. An alternating current

is passed through one of these coils and the current induced in the other coil is

measured. The induced current depends upon the mutual inductance of the

coils and upon the nearness of the steel reinforcement.Profometer is available in three models namely Model S, Model S+,

and Model SCANLOG. Model S is standard equipment and is used for

locating rebars, measuring concrete cover, storing and evaluation of data. It

displays location of rebar and concrete cover on a LCD monitor with x/y meter

scale and values obtained can be printed and down load to PC also.

Fig. : Typical Test Equipment Circuit for Profometer

Model S+ is similar but this software can print cyber scan data without

PC. Model SCANLOG is similar to S+ but it also includes integrated

software for grey-scale display of concrete cover and can give direct print out

without PC. Using any of above model rebars can be scanned over a defined

area by connecting the mobile probe first and following procedure is as

follows:

a) Select defined area from Basic Steps with scan area option

b) Set bar diameter of first layer

c) Select option Scanning Bar from menu.

d) Press start to locate the rebars over selected area.

e) The starting position of a mobile probe can be defined with the cursorand the cursor is moved with arrow keys to locate the rebars. The

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cursor position is then transferred to the measuring area. In similar

way, other rebars in first layer is marked Fig-. The rebars in second

layer is also marked by moving probe in other direction as shown in

Fig. Cover is also simultaneously measured.f) Store the diagram showing the position of rebars in first and second

layer and concrete cover. Cyber scan print out can be obtained on a

printer.

In the similar manner diameter of bar can also be determined. A typical

arrangement for measurement of bar diameter by using diameter prob. There

are various factors, which affect the profometer results. These factors are: (I)

Arrangement of reinforcement (II) Variation in the iron content of cement and

use of aggregate with magnetic properties (III) Metal ties also affects the

magnetic field. These factors should be considered in interpretation of

observations obtained from this instrument.

APPLICATION OF PROFOMETER:

Profometer is used for:

1. Determination of bar arrangement

2. Determination of cover of reinforcement

3. Determination of bar diameter

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Fig. : Diameter of Rebar in I & II Layer

RELIABILITY AND LIMITATIONS:

With this instrument a cover to reinforcement can be measured up to 100

mm with an accuracy of + 15% and a bar diameter with an accuracy of less

than 2 to 3 mm. Proper calibration of these instruments is very essential. The

factors which affect the accuracy are very closely spaced bars or bundledbars, binding wire, aggregate containing iron or magnetic properties.

1. Reinforcement less than 10mm diameter, high tensile steel or

deformed bars. In these cases the indicated cover is likely to be

higher than true value.

2. Cover measured lower than the true value when special cement,

including high alumna or added pigments is used.

3. Rebars in excess 32mm distance may require a recalibration.

CORROSION MAPPING:

Reinforcement in concrete will not corrode if the protective iron oxide

film formed by the high alkaline condition of the concrete pore fluid with a pH

around 13 is maintained. This film gets destroyed by chlorides or by

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carbonation, if moisture and oxygen are present, resulting in corrosion. In the

corrosion process anodic and cathodic areas are formed on the reinforcement,

causing dissolution of the steel and the formation of expansive corrosion

products at the anode.

HALF-CELL POTENTIOMETER:

Fig.: Half-cell Potential Equipment

DETECTION OF CORROSION BY HALF-CELL POTENTIOMETER

REINFORCEMENT LOCATION, SIZE AND CORROSION:

Steel shares about 40 to 70% of the load in RCC. During last few

decades it has been observed that, corrosion of reinforcement in severe in

structures near seashore and in the vicinity of chemical industries. A lot of

attention is needed for detecting this deterioration and protecting it with propertreatment. Thus due importance shall be given for measuring the size of bar and

the amount of corrosion.

RESISTIVITY METER (RESI):

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Fig. 2.16: Resistivity Meter with Display Unit and Resistivity Probe

CORROSION IN BUILDINGS

A variety of metals are used in buildings in many different ways. It is

for this reason that the problems of corrosion in buildings cover a very wide

range. In this brief article, only an outline or classification of the main

problems can be given, along with basic principles, to guide the designer in his

efforts to reduce the huge economic loss caused by corrosion. For specific

information on the practical problems of corrosion the reader is directed to the

extensive work of the various corrosion committees of the ASTM and of the

British Iron and Steel Research Association.

Corrosion refers to any process involving the deterioration of

degradation of metal components. The best known case is that of the rusting of

steel. Corrosion processes are usually electro-chemical in nature, having the

essential features of a battery. Dissimilar metals in the presence of a

conducting liquid, known as the electrolyte, develop an electrical potential that

cause a current to flow whenever a suitable path is provided. Such electrical

potential may also be developed between two areas of a components made of a

single metal as a result of small differences in composition or structure or of

differences in the condition to which the metal surface is exposed. Metal

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components used in buildings can be grouped for purpose of discussing

corrosion into four general categories:

1. Those used on the exterior as cladding, roofing and flashings.2. Those incorporated in the construction as structural and reinforcing steel,

masonry ties and damp courses.

3. Those used in the services to a building as piping, storage tank for a hot

water, drains and heating ducts.

4. Those buried in the ground.

CORROSION CONTROL IN

CONCRETE:

Given the importance of the costs associated with the corrosion of

infrastructures, it is extremely important that all possible methods applicable to

controlling corrosion in existing concrete bridges be developed so that these

structures will not deteriorate prematurely. Equally important is developing

methods to avoid this costly corrosion problem in all new concrete bridges to

be constructed in the future. Accordingly, the control methods can be divided

into two major areas:

Corrosion control in new concrete constructions

Corrosion control for rehabilitation of existing concrete structures

METHODS OF CORROSION

CONTROL:

Metallurgical methods

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Corrosion inhibitors

Coatings to reinforcement

Cathodic protection

Coatings to concrete

Design and detailing

CATHODIC PROTECTION:Cathodic protection is one of the effective, well known, and extensively

used methods for prevention of corrosion in concrete structures in more

advanced countries. Due to high cost and long term monitoring required for this

method, it is not very much used in India.

The cathodic protection comprises of application of impressed current ot

an electrode laid on the concrete above steel reinforcement. This electrode

serves as anode and the steel reinforcement which is connected to the negative

terminal of DC source acts as a cathode. In this process the external another is

subjected to corrode and the cathodic reinforcement is protected against

corrosion and hence the name Cathodic protection. In this process the

negative chloride ions which are responsible for the damage of the passivating

film, are drawn away form the vicinity of steel towards the anode where they

are oxidized to form chlorine gas. The environment around the steel

reinforcement reverts be back to alkaline condition which protects the steel.

The other recent development in corrosion control methods are Realkalisation

and Desalination. The realkalisation process allows making the concrete

alkaline again and passivating the reinforcing steel by electrochemical method.

This brings back the lost alkalinity of concrete to sufficiently high level to

reform and maintain the passive layer on the steel.

In the desalination process the chloride ions are removed from the concrete,

particularly from the vicinity of the steel reinforcement by certain electrical

method to re-establish the passive layer on the steel.

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METALLURGICAL METHODS

Steel can be made more corrosion resistant by altering its structurethrough metallurgical processes. Different methods such as repeat quenching of

the hot bars by series of water jets, or by keeping the hot steel bars for a short

time in water bath and by such other process, the mechanical properties and

corrosion resistance property of steel can be improved. There are many

situations where stainless steel reinforcements are used for long term durability

of concrete structures.

PREVENTIVE MEASURES:

To avoid corrosion of steel following preventive measures are to be

taken:

1. Concrete mix should be designed with as low a water cement ratio as

possible depending upon the environmental conditions in which the

structure is proposed.

2. Concrete should be made in such a manner that voids due to entrapped air or

segregation does not occur.

3. Plastic and drying shrinkage cracking of concrete should be avoided by

taking adequate care in designing concrete mixes and by proper

construction practices, specially curing.

4. Concrete mix should have good workability and cohesiveness and must be

placed and compacted properly.

5. Protective coating on the steel can be considered as a second line of defence

against corrosion.

CONCLUSION:

In civil engineering practice premature deterioration of much concrete

structure in the world before their design life is attain. Such things cause

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great financial effects on owner, i.e. Govt. or local agencies. It is occurs due to

corrosion of reinforced cement concrete.

Now days with advancement in the technology it is possible to detect the

corrosion & to overcome it or otherwise use precaution for future problems.Given the importance of the cost associated with the corrosion of infra structure

it is extremely importance that all possible method applicable to controlling the

corrosion existing concrete structure can be developed so that these structure

will not deteriorated prematurely.

The use of good construction design & procedure adequate concrete

cover depth, corrosion inhibiting admixtures & low permeability concrete alone

will not able to overcome the problems because concrete has a tendency to

crack inordinately Therefore it is very important to design the structure in all

aspects to avoid corrosion & premature deterioration as well.

REFERENCES:

Prof. R.D. Mendhe

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M. S. Shetty: Concrete Technology, Corrosion Control : "Standard Test

Method for Half Cell Potentials of Reinforcing Steel in Concrete",

American Society for Testing and Materials, West Conshohocken, PA.

J.P. Broomfield, P.E. Langford and A.J. Ewins: "The Use of a PotentialWheel to Survey Reinforced Concrete Structures", Corrosion Rates of

Steel in Concrete, ASTM STP 1065, American Society for Testing and

Materials, West Conshohocken, PA., 1990, pp.157-173.

B. Elsener and H. Bohni: "Potential Mapping and Corrosion of Steel in

Concrete", Corrosion Rates of Steel in Concrete, ASTM STP 1065,

American Society for Testing and Materials, West Conshohocken, PA.,1990, pp.143-156.

K.R. Gowers, S.G. Millard, J.S. Gill and R.P. Gill: "Programmable linear

polarisation meter for determination of corrosion rate of reinforcement in

concrete structures, British Corrosion Journal,Vol.29, No.1, 1994, pp.25-

32.

J. Flis, H.W. Pickering and K. Osseo-Asare: "Assessment of data from

Three Electrochemical Instruments for Evaluation of Reinforcement

Corrosion Rates in Concrete Bridge Components", Corrosion, Vol.51,

No.8, August 1995, pp.602-609.

M.I. Jafar, J.L. Dawson and D.G. John: "Electrochemical Impedance and

Harmonic Analysis Measurements on Steel in Concrete",

Electrochemical Impedance: Analysis and Interpretation, ASTM STP

1188, American Society for Testing and Materials, West Conshohocken,

PA, 1993, pp.384-403.

P.K. Mehta and R.W. Burrows: "Building Durable Structures in the 21st

Century", Concrete International, March 2001, pp.57-63.

J.P. Broomfield: "Corrosion of Steel in Concrete", E&FN Spon, 1997.

C.M. Hansson: "Concrete: The Advanced Material for the 21st Century",

Metallurgical and Materials Transactions A, 1995, pp.1321-1341