168 Damage to concrete structures
protection can be obtained by bringing the potential into the immunity
region (i.e. lowering the potential). This is called cathodic protection.
Another option is to increase the potential to bring the steel into the passivity region, which is called anodic protection. In the case of concrete structures, cathodic protection is a well-known way to mitigate corrosion of the
steel reinforcement. A detailed treatment of these protection techniques,
however, is beyond the scope of this textbook.
Other general corrosion control measures include the application of corrosion resistant alloys; these typically contain chromium. On the surface of
these special alloys, an effective and resistant passivating layer is developed.
However, these alloys are typically expensive and are only applied in very
special cases. Another option is to provide coatings on the reinforcing steel,
which provide a barrier between the steel and the corrosive environment.
Corrosion inhibitors can also be applied to the steel surface or mixed into
the concrete. They can have different actions such as forming a protective
layer on the steel surface or providing a buffer action in the surrounding
electrolyte, e.g. capturing chloride ions.
5.4.2 carbonation-induced corrosion
As illustrated in the Pourbaix diagram, the passivation layer of the reinforcing steel will become unstable in the case of the dealkalization of
the surrounding cement paste. Typically, a threshold pH value of 8 to 9
is needed to reach overall depassivation of the reinforcement steel. This
can be caused by the penetration of acids into the concrete, which will
neutralize the alkaline environment. For typical concrete structures, the
most important acid causing depassivation of the steel reinforcement is
atmospheric carbon dioxide CO 2 . It is commonly present in the air where
it has an average concentration of 0.03 vol%, but can reach levels as high
as 0.3 vol% in large cities. The carbon dioxide will penetrate the concrete and chemically react with alkaline elements (mainly Ca(OH) 2 , but
also Na(OH) or Ka(OH)) to form carbonates. This process, commonly
called carbonation, can be described by the following simplified chemical
reactions:
Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O
(5.15)
2NaOH + CO 2 → Na 2 CO 3 + H 2 O
(5.16)
2KOH + CO 2 → K 2 CO 3 + H 2 O
(5.17)
In somewhat more detail, it is to be noted that the carbon dioxide first dissolves in the pore solution according to the following reaction scheme (with
g and aq standing for gaseous and aqueous respectively):
protection can be obtained by bringing the potential into the immunity
region (i.e. lowering the potential). This is called cathodic protection.
Another option is to increase the potential to bring the steel into the passivity region, which is called anodic protection. In the case of concrete structures, cathodic protection is a well-known way to mitigate corrosion of the
steel reinforcement. A detailed treatment of these protection techniques,
however, is beyond the scope of this textbook.
Other general corrosion control measures include the application of corrosion resistant alloys; these typically contain chromium. On the surface of
these special alloys, an effective and resistant passivating layer is developed.
However, these alloys are typically expensive and are only applied in very
special cases. Another option is to provide coatings on the reinforcing steel,
which provide a barrier between the steel and the corrosive environment.
Corrosion inhibitors can also be applied to the steel surface or mixed into
the concrete. They can have different actions such as forming a protective
layer on the steel surface or providing a buffer action in the surrounding
electrolyte, e.g. capturing chloride ions.
5.4.2 carbonation-induced corrosion
As illustrated in the Pourbaix diagram, the passivation layer of the reinforcing steel will become unstable in the case of the dealkalization of
the surrounding cement paste. Typically, a threshold pH value of 8 to 9
is needed to reach overall depassivation of the reinforcement steel. This
can be caused by the penetration of acids into the concrete, which will
neutralize the alkaline environment. For typical concrete structures, the
most important acid causing depassivation of the steel reinforcement is
atmospheric carbon dioxide CO 2 . It is commonly present in the air where
it has an average concentration of 0.03 vol%, but can reach levels as high
as 0.3 vol% in large cities. The carbon dioxide will penetrate the concrete and chemically react with alkaline elements (mainly Ca(OH) 2 , but
also Na(OH) or Ka(OH)) to form carbonates. This process, commonly
called carbonation, can be described by the following simplified chemical
reactions:
Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O
(5.15)
2NaOH + CO 2 → Na 2 CO 3 + H 2 O
(5.16)
2KOH + CO 2 → K 2 CO 3 + H 2 O
(5.17)
In somewhat more detail, it is to be noted that the carbon dioxide first dissolves in the pore solution according to the following reaction scheme (with
g and aq standing for gaseous and aqueous respectively):