166 Damage to concrete structures
actively corrode in the presence of water and oxygen. However, in the case
of steel embedded in concrete, the higher pH value of the electrolyte brings
the steel into a passivation area. The reinforcing steel will be passivated and
will not actively corrode.
The passivation of steel in a concrete pore solution can be explained by
the presence of a very high content of hydroxyl ions, resulting from the
hydration process. Iron ions going into solution react with the hydroxyl
ions forming iron hydroxide. In the presence of a high content of hydroxyl
ions, iron hydroxide precipitates on the steel surface, providing a thin protective film that obstructs further dissolution of iron ions. The steel is now
passivated, due to the presence of a passivating layer of iron hydroxide. In
the case of a lower pH value of the electrolyte, the precipitated layer of iron
hydroxide becomes more porous and more permeable, so that the corrosion
process can go on actively. Consequently, it can be said that the protection
against corrosion of steel reinforcement in concrete is provided by the high
alkalinity of the pore solution. Other measures can be provided to protect
against steel corrosion, as will be briefly explained further on.
Knowing the influence of the alkalinity of the pore solution, the normal
situation of steel reinforcement in concrete is a condition of passivation.
Reinforcing steel in concrete is not expected to corrode actively. However,
seeing the widely spread corrosion damage to many reinforced concrete
structures, it is clear that some mechanisms can counteract the protective
role of the high alkalinity of the pore solution. The protective passivating
layer on the steel surface can be attacked by carbon dioxide (CO 2 ) and/or
chloride ions penetrating the concrete from the environment. This process
is called depassivation. Corrosion due to penetration of carbon dioxide, e.g.
present in the air, is called carbonation induced corrosion, while chloride
induced corrosion refers to the effect of penetrating chloride ions, e.g. ions
present in sea water or de-icing salts. Both mechanisms will be further
detailed in the following paragraphs.
Considering the required depassivation of the reinforcing steel before
active corrosion can occur and potentially cause damage to the concrete, a
two-stage corrosion process of reinforcing steel in concrete is typically considered while studying the service life, as illustrated in Figure 5.40, often
referred to as the Tuutti model (Tuutti 1982). During the initiation phase,
the reinforcing steel will be gradually depassivated by the penetration of
carbon dioxide and resulting carbonation of the cement matrix or by the
penetration of chloride ions as already mentioned. As soon as the reinforcing steel is depassivated, active corrosion can proceed during the propagation phase. The corrosion rate depends on many influencing factors, of
which the availability of water and oxygen are of main importance.
Depending on the defined corrosion acceptance level, the service life of
the structure will be the sum of the initiation time and the propagation
time. In the stage of durability design, quite often the end of the initiation
actively corrode in the presence of water and oxygen. However, in the case
of steel embedded in concrete, the higher pH value of the electrolyte brings
the steel into a passivation area. The reinforcing steel will be passivated and
will not actively corrode.
The passivation of steel in a concrete pore solution can be explained by
the presence of a very high content of hydroxyl ions, resulting from the
hydration process. Iron ions going into solution react with the hydroxyl
ions forming iron hydroxide. In the presence of a high content of hydroxyl
ions, iron hydroxide precipitates on the steel surface, providing a thin protective film that obstructs further dissolution of iron ions. The steel is now
passivated, due to the presence of a passivating layer of iron hydroxide. In
the case of a lower pH value of the electrolyte, the precipitated layer of iron
hydroxide becomes more porous and more permeable, so that the corrosion
process can go on actively. Consequently, it can be said that the protection
against corrosion of steel reinforcement in concrete is provided by the high
alkalinity of the pore solution. Other measures can be provided to protect
against steel corrosion, as will be briefly explained further on.
Knowing the influence of the alkalinity of the pore solution, the normal
situation of steel reinforcement in concrete is a condition of passivation.
Reinforcing steel in concrete is not expected to corrode actively. However,
seeing the widely spread corrosion damage to many reinforced concrete
structures, it is clear that some mechanisms can counteract the protective
role of the high alkalinity of the pore solution. The protective passivating
layer on the steel surface can be attacked by carbon dioxide (CO 2 ) and/or
chloride ions penetrating the concrete from the environment. This process
is called depassivation. Corrosion due to penetration of carbon dioxide, e.g.
present in the air, is called carbonation induced corrosion, while chloride
induced corrosion refers to the effect of penetrating chloride ions, e.g. ions
present in sea water or de-icing salts. Both mechanisms will be further
detailed in the following paragraphs.
Considering the required depassivation of the reinforcing steel before
active corrosion can occur and potentially cause damage to the concrete, a
two-stage corrosion process of reinforcing steel in concrete is typically considered while studying the service life, as illustrated in Figure 5.40, often
referred to as the Tuutti model (Tuutti 1982). During the initiation phase,
the reinforcing steel will be gradually depassivated by the penetration of
carbon dioxide and resulting carbonation of the cement matrix or by the
penetration of chloride ions as already mentioned. As soon as the reinforcing steel is depassivated, active corrosion can proceed during the propagation phase. The corrosion rate depends on many influencing factors, of
which the availability of water and oxygen are of main importance.
Depending on the defined corrosion acceptance level, the service life of
the structure will be the sum of the initiation time and the propagation
time. In the stage of durability design, quite often the end of the initiation