.
This article covers the theory of corroding in reinforcement
steel in RCC and simple measure that can be taken to limit corrosion
of steel. When the steel in reinforced cement concrete corrodes,
iron in steel is turned into rust (iron oxide). The rust occupies
up to 2 to 1.5 times more volume and causes cracks in the concrete.
The area of steel available to take up force is also reduced.
Further moisture gets into the cracks and accelerates the rate
of corrosion of the steel.
Corrosion of steel is a complicated electrochemical process.
Difference in electrochemical potential on the surface of steel
make anodic and cathodic areas on the steel surface. Metal oxides
at the anodes and this leads to corrosion. There are many types
of corrosion that can take place on the reinforcement bars.
The following are the most important:
Atmospheric corrosion that occurs in normal atmospheric
conditions.
Chloride corrosion that occurs in the presence of chlorides.
Other types such as hydrogen embritlement and stress corrosion
are important in special situations like prestressed concrete
numbers.
| Exposure Condition |
Minimum Cement Content |
Maximum water cement ratio |
| (Kg/m3) |
| |
IS |
BS |
IS |
BS |
| Mild |
250 |
275 |
0.65 |
0.65 |
| Modest |
290 |
300 |
0.55 |
0.60 |
| Severe |
360 |
385 |
0.45 |
0.50 |
|
Even though some authorities claim that corrosion can be controlled
by addition of corrosion inhibitors into cement concrete mix
or by dipping and coating steel reinforcement by suitable
resins or bituminous compound, the conventional methods of
controlling corrosion (oxide & chloride corrosion) are
by selecting suitable value of following :
Cover to reinforcement
Cement content
Cement content and water cement ratio of concrete.
Proper compaction and curing.
Minimum cover to reinforcement for mild conditional of exposure
| Cover for |
Minimum cover against
corrosion |
| Column
Bars dia<12 mm |
25 mm
|
| Column Bars dia<12
mm |
40 mm
|
| Main Bars in beams
|
25 mm
|
| Other Bars such as
stirrups |
15 mm
|
| Main
Bars in Slabs |
15 mm
|
| Foundations |
40 mm
|
| End of main Bar |
25
mm (or over) |
|
| Courtesy :
Housing News |
|  |
|
|
|
12:48:41 PM 
