Corrosion of steel in concrete:
In some cases, corrosion occurs to reinforcing bars are placed in concrete. This is of course not a serious problem for the majority of reinforced structures. The corrosion is used to indicate the conversion of metals by natural agencies into various compounds. The term rusting is used to refer to the corrosion of ferrous metals. The corrosion of steel in concrete also has theories given below.
Theories of corrosion:
Various theories of corrosion of steel in concrete have been developed.
Following are two important theories of corros10n:
( 1) Chemical action theory
(2) Electrolytic theory.
( I ) Chemical action theory:
Following chemical reactions arc involved in corrosion:
Fe+0+2C02+H20 = Fe (HC03) 2 ••••.•••••••••••• (i)
2Fe(HC03 ) 2 +o ··-=2Fe(OH) C03 –t-2C02 +H20 …. (ii)
Fe(OH)C0 3 + H20 -= Fe (OH) 3 +CO2 ••••..••••.• (iii)
The combined action of oxygen, carbon dioxide, and moisture on steel results in soluble ferrous bicarbonate Fe(HC03) 2 as shown by the reaction
(i). This ferrous bicarbonate is then oxidized to basic ferric carbonate 2Fe( OH)C03 as shown by the reaction
(ii). This basic ferric carbonate is converted into hydrated ferric oxide Fe (OH) 3 and carbon dioxide is liberated as shown by reaction (iii).
(2) Electrolytic theory’:
According to this theory, metal contains anodic and cathodic regions, and these areas are corrosive when they are connected by electrolytic materials such as water, moisture, and aqueous solutions. These areas are developed in metal. Various causes, such as differences in metal composition, the typical concentration of oxygen in different parts of the metal surface.
Causes of corrosion:
Following are the factors reasons for causing corrosion of steel in concrete:
(I) improper construction methods,
(2) inadequate design procedure,
(3) insufficient cover to steel from the exposed concrete surface,
(4) permeability of concrete which depends on various factors such as water-cement ratio, size of aggregate, curing, grading of aggregates, etc.,
(5) poor workmanship,
(6) presence of moisture in concrete,
(7) presence of salts,
( 8) type of atmospheric conditions surrounding the region of concrete,
(9) unequal distribution of oxygen over the steel surface, etc.
Effect of corrosion:
The action of corrosion of steel in concrete is very slow and except under exceptional circumstances, such corrosion does not decrease the life of the concrete members.
It should, however, be remembered that the action of corrosion becomes more intensive when it is combined with adverse effects of internal and external stresses.
An important consequence of corrosion is the formation of cracks and these cracks usually progress or progress rapidly, where shear stresses are high and slipping due to bond loss.
Prevention of corrosion:
To minimize the chances of development of corrosion of steel in concrete, the following preventive measures may be taken:
( 1) avoiding the steel to come into contact with bricks, soil, wood, and other porous non-alkaline materials,
(2) avoiding the use of materials that accelerate the process of corrosion, i.e., aggregates with high salt contents, water containing salts, etc.,
(3) maintaining a high degree of workmanship,
( 4) proper structural design with the due provision of cover,
( 5) providing cathodic protection to reinforcement by some suitable method,
( 6) providing surface coatings with paints, tars, asphalts, etc.
(7) use of high quality and impermeable concrete, etc.
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