The Corrosion of Steel
Reinforcement in Concrete
Corrosion :
occurs when two different metals, or metals in different environments, are electrically connected in a moist or damp concrete. Rust forms with a
volume larger than the metal consumed and in reinforced concrete the concrete cover spalls off, exposing the metal
directly to the aggressive environment.
1. Steel reinforcement is in contact with an aluminium conduit.
2. Concrete pore water composition varies between adjacent or along reinforcing bars.
3. Where there is a variation in alloy composition between or along reinforcing bars.
4. Where there is a variation in residual or applied stress along or between reinforcing bars.
5. Where there are imposed stray electrical currents.
This will occur when:
Corrosion in Reinforced Concrete
O2 H2O Fe2O3H2O (rust)
- -
- electron transfer-
anodic reaction 2Fe++
2Fe(OH)2 secondary
reaction
O2 H2O
4(OH-)
cathodic reaction
O2
H2O
4e-
anodic dissolution of iron
cathodic region
(a) Concrete is always dry, then there is no H2O to form rust. Also aggressive agents cannot easily diffuse into dry concrete.
(b) Concrete is always wet, then there is no oxygen to form rust.
(c) Cathodic protection is used to convert all the reinforcement into a cathode using a battery.
This is not easy to implement because anodic mesh is expensive, and this technology is not easy to install and maintain.
Can corrosion be avoided in reinforced concrete?
Yes if:
(d) A polymeric coating is applied to the concrete member to keep out aggressive
agents. These are expensive and not easy to apply and maintain.
(e) A polymeric coating is applied to the
reinforcing bars to protect them from moisture and aggressive agents. This is expensive
and there is some debate as to its long- term effectiveness.
(f) Stainless steel or cladded stainless steel is used in lieu of conventional black bars.
This is much more expensive than black bars.
Can we avoid corrosion?
No, not entirely:
Concrete is not usually under water or continuously dry. Aggressive agents
such as carbon dioxide, de-icing agents and/or sea water can diffuse into the
best of moist concrete, and corrosion
will eventually result.
If corrosion cannot always be avoided and economical solutions are required, the effects of corrosion can be minimized by making a better concrete.
As will be shown later, Fly Ash added to a low w/c concrete produces a much
enhanced corrosion resisting structure with no significant increase in cost.
The intrinsic nature of concrete is to be very protective of embedded steel. As soon as steel is placed in the high pH concrete
(>12), a thin dense passive layer forms that is virtually continuous and the
subsequent rate of attack is so low as to be insignificant.
Unfortunately when the carbonation front
reaches the steel or when chlorides diffuse into the steel and reach a threshold level, this coherent protective layer is replaced by a porous incoherent expansive coating.
The formerly protective oxide layer becomes an expansive porous oxide layer which
causes cracking and eventually spalling of the concrete cover layer.
The Economical Solution:
We must make concrete more protective of the steel reinforcement so that it will protect the passivating oxide layer.
Making better concrete, using only Portland cement, will not make a substantial improvement.
Fortunately Fly Ash added to a properly designed and cured concrete mixture will.