CORROSION OF REINFORCEMENT IN CONCRETE

CORROSION OF REINFORCEMENT IN CONCRETE

The damage to the concrete due to corrosion of reinforcement is considered to

be one of the most serious problems. It is an universal problem and property worth

of crores of rupees is lost every year. Due to corrosion problem in bridges, buildings

and other RCC structures, India incurs heavy loss of about Rs. 1,500 crores annually.

This paper deals with various causes of corrosion and remedial measures thereon.

Corrosion process and mechanism :

Corrosion of reinforcement steel is a complex phenomenon involving chemical,

electrochemical and physical processes. When reinforcement steel rusts, the volume

of iron oxide formed is 2-4 times greater than the steel corroded, which results in

bursting stresses in the concrete surrounding the bar. This causes cracking, spalling

and delamination of concrete. Another consequence of corrosion is reduction in cross-

Sectional area of the steel at anode, thus reducing its load carrying capacity.

For corrosion of unprotected steel, necessary precondition is the formation of

electrochemical cell which comprises two electrodes-anode and cathode, separated

by an electrolyte and connected in an electric circuit. On the surface of reinforcement

steel, areas which are subjected to higher oxygen concentration turn in to cathode

while areas of poor oxygen concentration turn in to anode. Concrete always contains

some moisture in it and acts as an electrolyte. Electrochemical cell may also form

due to variation in salt concentration in the pore water. Areas of higher salt

concentration act as anode while areas of low salt concentration act as cathode

Modern electrochemical theory of corrosion has established that a potential difference

exists between these anodic and cathodic areas. Due to this, at anode, metal oxidize

in presence of oxygen and water to form iron oxide. The positively charged ferrous

iron Fe at the anode pass in to solution while the negatively charged free electrons

e pass through the steel in to the cathode where they are absorbed by the constituents

of the electrolyte and combine with water and oxygen to form hydroxyl ions (OH)

. Hydroxyl ions released at cathode, react with Ferrous ions Fe to form ferrous

hydroxide. With further oxidation of Ferrous hydroxide (Fe (OH),) it is converted

into ferric hydroxide and finally in to rust.

Difference in environment of the concrete may also set up anodic and

cathodic areas with difference in electrochemical potential, for example where a part

of concrete is permanently submerged in sea water and a part is exposed to periodic

wetting and drying.

 The reactions involved are as follows:

Fe++ Fe + 2e

Et +2 (OH) Fe (OH)2 (Ferrous Hydroxide)

4Fe(OH)2 + 2H20 + 02 4Fe (OH)3 (Ferric Hydroxide)

Cathodic reactions :

4e+O2 +H20->4 (OH)

CAUSES OF CORROSION AND REMEDIAL MEASURES :

Various causes of corrosion and remedial measures are discussed below :

(1) Presence of cracks in concrete

Certain amount of cracking always occurs in the tension zone of RCC depending

Upon the stresses in the reinforcing steel. Through these cracks, oxygen or sea water

ingress into concrete and set up good environment for corrosion of reinforcement.

Maximum permissible width of elastic cracks in RCC members would depend upon

environmental and other factors. For normal environmental conditions, a maximum

crack width of 0.30 mm for protected internal members and 0.20 mm for unprotected

External members may be recommended

(2) Presence of moisture :

presence of moisture is a precondition for corrosion to take place because

concrete can act as electrolyte in electrochemical cell only if it contains some moisture

Corrosion can neither occur in dry concrete nor in submerged concrete.

The worst combination for corrosion to proceed is when the concrete is

sightly drier than saturated i.e. about 8-90% relative humidity with a low resistivity

and the oxygen can still penetrate to the steel. Hence in high humidity areas like

coastal India. low permeability concrete is recommended.

Electrical resistivity of concrete also depends upon moisture content of

concrete. A low, concrete electrical resistivity is required to permit electrochemical

corrosion. Values of less than 5,000 to 10,000 Ohm. cm appears critical. The concrete

of moisture content over 4% has the critical value of about 10,000 Ohm. cm. Drying

of concrete gives high resistivity. Wetting of concrete can reduce the resistivity due

to absorption of water in to dry concrete.