corrosion of steel reinforcement in concrete

8/12/2019 Corrosion of Steel Reinforcement in concrete

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By,

R.Sabitha RajP. Nayana Sri

A.Swetha

K.Shanthi sri


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Corrosion is defined as the destruction of

materials due to chemical (or electro chemical)

reaction with the environment, and also the

loss of steel due to formation of rust.Corrosion is the depassivation of steel with

reduction in concrete alkalinity.

The formation of Ferric Oxide, occupies a

greater volume than steel and results in tensile

forces leading to cracks in the concrete around

the steel reinforcement.


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Corrosion

Reaction

withChloride

Carbonation

DissimilarMetal

Corrosion


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Carbonation is a process by which Carbon

dioxide converts free lime into Calcium

Carbonate and water.CO2 penetrates into the pores of concrete and

reacts with Ca(OH)2dissolved in pore water.

In good quality concrete, the carbonation

process is very slow.


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Chloride salts can enter concrete in two ways:

i. It may be present in the concrete itself

ii. It can penetrate into the hardened concrete

wherever it is permeable and reach thereinforcement at cracks.


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Chlorides may be

introduced

deliberately as an

accelerator or in theform of natural

ingredients found in

some aggregates.

Concrete made of

beach sand.


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The concentration ofchlorides in contact with

the reinforcing steel will

cause corrosion when

moisture and oxygen are

present.

As the rust layer builds,

tensile forces generatedcause the concrete to

crack and delaminate.


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Cracks in concrete permit

corrosive chemicals to enter the

concrete and access embedded

reinforcing steel.

Chlorides are not consumed in

the corrosion process, but

instead act as catalysts to the

process and remain in the

concrete.


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o The concentration of chlorides is greatly

affected by the concretes pH.

o It was demonstrated that a threshold level of

8000 ppm of chloride ions is required to

initiate corrosion when the pH was 13.2.o As the pH was lowered to 11.6 corrosion was

initiated with only 71 ppm of chloride ions.


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ACI 224-90 presents the following table of

tolerable crack widths in reinforced concrete:

Exposure condition Tolerable crack width

(In.)

Dry air, protective membrane 0.016

Humidity, moist air, soil 0.012

De-icing chemicals 0.007

Sea water and sea water spray;

wetting and drying

0.006

Water-retaining structures 0.004


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Corrosion can take

place in concrete

when two different

metals are cast into aconcrete structure,

along with an

adequate electrolyte.

This type of corrosionis also known as

galvanic corrosion.


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Different metals have different rates of electrochemical

activity. Below is a list of metals in increasing order of activity:

1. Zinc

2. Aluminum

3. Steel

4. Iron nickel5. Tin

6. Lead

7. Brass

8. Copper

9. Bronze10. Stainless steel

11. Gold


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1) Keep concrete always dry, so that there is no H2O

to form rust. If concrete is always wet, then there is no

oxygen to form rust.2) A polymeric coating is applied to the concrete

member to keep out aggressive agents. The

embedded epoxy-coating on steel bars provide a

certain degree of protection to the steel bars and,thereby, delay the initiation of corrosion. These

coatings permit movement of moisture to the steel

surface but restrict oxygen penetration such that a

necessary reactant at cathodic sites is excluded.


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3) Stainless steel or cladded stainless steel is used in

lieu of conventional black bars.

4) Using a Fly Ash concrete with very low

permeability, which will delay the arrival of

carbonation and chlorides at the level of the steel

reinforcement. Fly Ash is a finely divided silicarich powder that, in itself, gives no benefit when

added to a concrete mixture, unless it can react

with the calcium hydroxide formed in the first few

days of hydration. Together they form a calciumsilica hydrate (CSH) compound that over time

effectively reduces concrete diffusivity to oxygen,

carbon dioxide, water and chloride ions.


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5) A portion of the chloride ions diffusing through

the concrete can be sequestered in the concrete by

combining them with the tricalcium aluminate toform a calcium chloro-aluminate (Friedels salt).

It can have a significant effect in reducing the

amount of available chlorides thereby reducing

corrosion.

6) Electrochemical injection of the organic base

corrosion inhibitors, ethanolamine and

guanidine, into carbonated concrete.


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7) The rougher the steel surface, the better it adheres toconcrete. oxidation treatment (by water immersion andozone exposure) of rebar increases the bond strength

between steel and cement paste to a value higher than that

attained by clean rebars. In addition, surface deformationson the rebar (such as ribs) enhance the bond due tomechanical interlocking between rebar and concrete.

8) As the cement content of the concrete increases (for a

fixed amount of chloride in the concrete), more chloridereacts to form solid phases, so reducing the amount insolution (and the risk of corrosion), and as the physical

properties improve, the extent of carbonation declines, sopreventing further liberation of chloride from the solidphase.

9) Electrochemical Chloride Extraction (ECE) is arelatively new technology for which long-term servicedata are limited. This method employs a temporary anodethat is operated at current density


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corrosion of steel reinforcement in concrete

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