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Corrosion

Corrosion of reinforcement is one of the main durability problems of concrete structures.

Most of the corrosion is induced by two main factors: the carbonation of the concrete cover and

the penetration of chlorides providing from marine atmosphere or from chemicals in contact with

concrete. Carbonation generally aims into uniform corrosion of the steel bar while chlorides

mainly induce localised corrosion. Both types of corrosion are of electrochemical nature.

1.1 Corrosion of Steel in Concrete due to Carbonation

The high alkalinity of cement paste, approx. pH 13, passivation of the steel surface and protects

it against oxidation (corrosion). The presence of chlorides, carbonation, acid attack or

combination of all these, reduce pH of concrete and the reinforcing steel starts to corrode.The Figure 1 shows the pH scale and divides it into three areas according to steel corrosion

occurrence.

The high alkalinity of cement paste is primarily due to the high calcium hydroxide content (lime)

the product of cement hydration as schematically shown in equation 1. Calcium silicate

compounds (CS) contained in Portland cement, when mixed with water react and form hydrated

calcium silicates (CSH) and Calcium Hydroxide (lime). Lime and other oxides of alkali earth

elements, such Sodium (Na) and Potassium (K) create the highly alkaline environment of fresh

or young concrete.

CS + H CSH + Calcium Hydroxide (lime) ------------------- [1]

As concrete ages, lime reacts with the atmospheric carbon dioxide as shown in Equation 2.

Ca(OH)2 + CO2 CaCO3 + H2O ------------------------------- [2]

This reaction "neutralizes" concrete and pH of concrete decreases. At pH level below 10 (approx.

pH 9.5-9.6) the passive protection of the steel surface due to the alkalinity of cement paste

disappears and steel starts to corrode.

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13

No corrosion, passivation12.6 Calcium Hydroxid

12

11

10 Carbonation

9.59

8 8 Calcium Carbona

passivity decreases, corrosion

start 7

6

5.55

4

Rapid and heavy corrosion

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Figure :- (1 pH Scale)

Carbonation :- Carbon-dioxide gas present in the atmosphere combines with hydrated

concrete (alkaline hydroxides) and partly neutralize the alkaline nature of concrete. This process

is known as Carbonation. Carbonation brings down the pH value of concrete from above 12-13 to

less than 9. When depth of carbonation Increases and becomes equal or more than the depth of

cover of reinforcement, it breaks down passive film surrounding steel and make steel embedded

in concrete active. Carbonation damage occurs rapidly when there is insufficient concrete coverover the reinforcing steel. Carbonation can occur even when the concrete cover to the reinforcing

steel is high. This may be due to a porous structure where pores are well connected together and

allow rapid carbon-dioxide ingress. It can also happen when alkaline reserves in the pores are

low. These problems occur due to a low cement content, high water cement ratio, poor

compaction and inadequate curing of concrete.