176 Damage to concrete structures
With this prediction formula, based on chloride diffusion in an ageing
concrete, the chloride penetration in real structures can be estimated in an
acceptable way.
When a sufficient amount of chlorides reaches the steel reinforcement,
the passivation layer can be broken down. This sufficient amount of chlorides is referred to as the critical chloride content or chloride threshold
concentration. Different options are available to define the critical chloride
content:
• based on the total chloride or on the water soluble (‘free’) chloride
content, the difference between both being related to the process of
chloride binding
• expressed as a threshold value relative to the mass of the concrete, or
relative to the mass of cement within the concrete
• expressed as a critical [Cl − ]/[OH − ] ratio for the pore solution
The latter option seems to be the most fundamental because it also
considers the alkalinity of the pore solution. A commonly considered
critical [Cl − ]/[OH − ] ratio is 0.6. Nevertheless, for practical applications,
this option is not very realistic due to experimental difficulties in determining the chloride concentration in the pore solution, which requires a
complicated expression of pore solution. From a practical point of view,
expressing the critical chloride content as a threshold value for the total
chloride content seems to be the most preferable. Most typically, the
threshold values are given relative to the mass of cement within the concrete. Unfortunately, no agreement exists on the threshold value to be
considered, as reported values for the total chloride content relative to the
mass of cement range between 0.17% and 2.5% (Audenaert 2006, Van
den Bergh 2009). Many influencing parameters are said to be responsible
for this wide variation, including type and amount of cement, chloridebinding capacity, porosity, temperature, type of cation accompanying the
chlorides, oxygen content in the pore solution, chemical composition of
the steel, and surface roughness of the steel. It can be said, however, that
an often cited value is 0.4%.
The fact that no agreement has been reached on the chloride threshold
concentration most probably is linked to the fact that the mechanism of the
breakdown of the passive film layer by chloride ions is not yet fully understood (Rosenberg et al. 1989). Nevertheless, it seems generally accepted
that the chloride ions become incorporated in the passive film, improving its solubility. Concerning the detailed mechanism, several theories have
been formulated (Van den Bergh 2009). The details of these theories however go beyond the scope of this textbook.
It is important to stress, however, that the destruction of the passive
layer typically occurs locally, rather than having an overall depassivation.
With this prediction formula, based on chloride diffusion in an ageing
concrete, the chloride penetration in real structures can be estimated in an
acceptable way.
When a sufficient amount of chlorides reaches the steel reinforcement,
the passivation layer can be broken down. This sufficient amount of chlorides is referred to as the critical chloride content or chloride threshold
concentration. Different options are available to define the critical chloride
content:
• based on the total chloride or on the water soluble (‘free’) chloride
content, the difference between both being related to the process of
chloride binding
• expressed as a threshold value relative to the mass of the concrete, or
relative to the mass of cement within the concrete
• expressed as a critical [Cl − ]/[OH − ] ratio for the pore solution
The latter option seems to be the most fundamental because it also
considers the alkalinity of the pore solution. A commonly considered
critical [Cl − ]/[OH − ] ratio is 0.6. Nevertheless, for practical applications,
this option is not very realistic due to experimental difficulties in determining the chloride concentration in the pore solution, which requires a
complicated expression of pore solution. From a practical point of view,
expressing the critical chloride content as a threshold value for the total
chloride content seems to be the most preferable. Most typically, the
threshold values are given relative to the mass of cement within the concrete. Unfortunately, no agreement exists on the threshold value to be
considered, as reported values for the total chloride content relative to the
mass of cement range between 0.17% and 2.5% (Audenaert 2006, Van
den Bergh 2009). Many influencing parameters are said to be responsible
for this wide variation, including type and amount of cement, chloridebinding capacity, porosity, temperature, type of cation accompanying the
chlorides, oxygen content in the pore solution, chemical composition of
the steel, and surface roughness of the steel. It can be said, however, that
an often cited value is 0.4%.
The fact that no agreement has been reached on the chloride threshold
concentration most probably is linked to the fact that the mechanism of the
breakdown of the passive film layer by chloride ions is not yet fully understood (Rosenberg et al. 1989). Nevertheless, it seems generally accepted
that the chloride ions become incorporated in the passive film, improving its solubility. Concerning the detailed mechanism, several theories have
been formulated (Van den Bergh 2009). The details of these theories however go beyond the scope of this textbook.
It is important to stress, however, that the destruction of the passive
layer typically occurs locally, rather than having an overall depassivation.