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alkali carbonate reaction is just a variant of the alkali silica reaction, or in
short ACR = ASR.
It should be mentioned that the dolomitic aggregate studied by Katayama
contained a substantial amount of quartz. In the case of a relatively pure
dolomitic aggregate with a very low quartz content, dedolomitization
occurs without the expansive formation of ASR gel. Due to this dedolomitization, the porosity of the dolomite increases and the mechanical properties can be reduced. This process will not be visible from the outside of
the concrete element because it does not lead to expansive crack formation. Consequently, ACR is typically considered to be harmless, neglecting
potential strength reduction due to the dedolomitization. Visible damage
will only occur in the case of simultaneous ASR. For this reason, in the following paragraphs, the focus will be on ASR.
5.3.1.2 Influencing parameters
For ASR to occur, three necessary conditions have to be fulfilled:
• Potentially reactive aggregates have to be present in the concrete.
• The alkali concentration in the pore solution has to be sufficiently high.
• Moisture has to be present in sufficient quantities.
In this respect, reference is typically made to so-called pessimum concentrations. This means that the three required elements (reactive silica,
alkalis, and water) have to be present in unfavourable quantities, meaning
quantities which are most beneficial for the reaction to proceed. As the
consequences of the reaction are not positive, the term pessimum is used
instead of optimum.
The reactivity of the aggregate particles is dependent on several parameters including the content of amorphous silica in the particles, the fraction of reactive particles, and the particle size. Typically, finer aggregate
particles show a more intense reactivity, although in the case of very fine
particles the expansion can be very limited.
The alkali concentration in the pore solution typically depends on the
alkali content of the cement, although alkalis can also be provided to the
system through admixtures, additives, mixing water, and even the aggregates. Alkalis can also enter the concrete from the environment when
exposed to sea water, de-icing salts, or industrial solutions. The term alkali
refers to the alkali metals sodium and potassium for which the combined
concentration in concrete is typically expressed in terms of the sodium
oxide equivalent, according to the following expression:
Na 2 O eq = Na 2 O + 0.685 K 2 O
(5.5)
alkali carbonate reaction is just a variant of the alkali silica reaction, or in
short ACR = ASR.
It should be mentioned that the dolomitic aggregate studied by Katayama
contained a substantial amount of quartz. In the case of a relatively pure
dolomitic aggregate with a very low quartz content, dedolomitization
occurs without the expansive formation of ASR gel. Due to this dedolomitization, the porosity of the dolomite increases and the mechanical properties can be reduced. This process will not be visible from the outside of
the concrete element because it does not lead to expansive crack formation. Consequently, ACR is typically considered to be harmless, neglecting
potential strength reduction due to the dedolomitization. Visible damage
will only occur in the case of simultaneous ASR. For this reason, in the following paragraphs, the focus will be on ASR.
5.3.1.2 Influencing parameters
For ASR to occur, three necessary conditions have to be fulfilled:
• Potentially reactive aggregates have to be present in the concrete.
• The alkali concentration in the pore solution has to be sufficiently high.
• Moisture has to be present in sufficient quantities.
In this respect, reference is typically made to so-called pessimum concentrations. This means that the three required elements (reactive silica,
alkalis, and water) have to be present in unfavourable quantities, meaning
quantities which are most beneficial for the reaction to proceed. As the
consequences of the reaction are not positive, the term pessimum is used
instead of optimum.
The reactivity of the aggregate particles is dependent on several parameters including the content of amorphous silica in the particles, the fraction of reactive particles, and the particle size. Typically, finer aggregate
particles show a more intense reactivity, although in the case of very fine
particles the expansion can be very limited.
The alkali concentration in the pore solution typically depends on the
alkali content of the cement, although alkalis can also be provided to the
system through admixtures, additives, mixing water, and even the aggregates. Alkalis can also enter the concrete from the environment when
exposed to sea water, de-icing salts, or industrial solutions. The term alkali
refers to the alkali metals sodium and potassium for which the combined
concentration in concrete is typically expressed in terms of the sodium
oxide equivalent, according to the following expression:
Na 2 O eq = Na 2 O + 0.685 K 2 O
(5.5)