Actions during service 115
shrinkage, accompanies drying shrinkage. Carbonation is the reaction of
carbon dioxide with hydration products present in the concrete. Due to
carbonation, calcium hydroxide is converted to calcium carbonate. Other
hydration products are decomposed as well (Neville 2010).
Carbonation of concrete is well-known for the initiation of reinforcement corrosion (see Section 5.4). However, carbonation also leads to a contraction of the affected concrete. The microstructure of the cement paste
is modified due to the chemical carbonation reaction. In Portland-cement
based systems, a decrease in porosity is obtained as well as a decrease
in total volume. The latter causes the carbonation shrinkage, which is
non-reversible.
In normal conditions, carbonation is a rather slow process proceeding
from the exposed surface of the concrete. As the carbonation shrinkage is
limited to the carbonated surface layer, a differential shrinkage is induced
between surface and bulk. The carbonation contraction of the surface layer
is restrained by the bulk of the concrete, potentially leading to map cracking or crazing. This crazing is only superficial as opposed to drying shrinkage cracks, which typically penetrate further into the bulk of the material.
While carbonation shrinkage is not a predominant damage mechanism
for traditional concrete, it is far more important for autoclaved aerated
concrete (AAC). Especially in the case of higher concentrations of CO 2 , carbonation shrinkage of AAC will be more important than drying shrinkage.
In this case, protective measures are recommended (Aroni 1993).
Carbonation of traditional concrete mainly leads to a reduced alkalinity and an increased corrosion risk of the rebars (see Section 5.4). This is
different in AAC, a tobermorite-based material which is originally neutral or weakly alkaline (Matsushita et al. 2004a, 2004b, 2009). Steel reinforcement in AAC is typically coated with corrosion inhibitors because the
tobermorite-based matrix has no corrosion preventive capacities (Aroni
1993). As a consequence, carbonation induced corrosion is not an issue
for AAC. It is rather the modification of the microstructure due to carbonation, inducing significant shrinkage values, which is a predominant
concern for AAC.
Carbonation of AAC leads to a decrease in strength, to an increase in
deflection, and the growth of lattice-like cracking (Matsushita et al. 2009).
For carbonation degrees below 25%, no significant carbonation shrinkage
is noticed as the double-chain silicate anion structure of the tobermorite is
well maintained (Matsushita et al. 2004b). However, carbonation shrinkage gradually increases for higher degrees of carbonation ranging from
approximately 20% to 60%, most probably due to the decomposition of
the double-chain structure (Matsushita et al. 2004b). Carbonation shrinkage values of ACC ranging between 0.1% and 1.0% have been reported,
depending on raw materials, CO 2 concentration, and relative humidity
(Matsushita et al. 2009). For real structures made of AAC, it is noticed
shrinkage, accompanies drying shrinkage. Carbonation is the reaction of
carbon dioxide with hydration products present in the concrete. Due to
carbonation, calcium hydroxide is converted to calcium carbonate. Other
hydration products are decomposed as well (Neville 2010).
Carbonation of concrete is well-known for the initiation of reinforcement corrosion (see Section 5.4). However, carbonation also leads to a contraction of the affected concrete. The microstructure of the cement paste
is modified due to the chemical carbonation reaction. In Portland-cement
based systems, a decrease in porosity is obtained as well as a decrease
in total volume. The latter causes the carbonation shrinkage, which is
non-reversible.
In normal conditions, carbonation is a rather slow process proceeding
from the exposed surface of the concrete. As the carbonation shrinkage is
limited to the carbonated surface layer, a differential shrinkage is induced
between surface and bulk. The carbonation contraction of the surface layer
is restrained by the bulk of the concrete, potentially leading to map cracking or crazing. This crazing is only superficial as opposed to drying shrinkage cracks, which typically penetrate further into the bulk of the material.
While carbonation shrinkage is not a predominant damage mechanism
for traditional concrete, it is far more important for autoclaved aerated
concrete (AAC). Especially in the case of higher concentrations of CO 2 , carbonation shrinkage of AAC will be more important than drying shrinkage.
In this case, protective measures are recommended (Aroni 1993).
Carbonation of traditional concrete mainly leads to a reduced alkalinity and an increased corrosion risk of the rebars (see Section 5.4). This is
different in AAC, a tobermorite-based material which is originally neutral or weakly alkaline (Matsushita et al. 2004a, 2004b, 2009). Steel reinforcement in AAC is typically coated with corrosion inhibitors because the
tobermorite-based matrix has no corrosion preventive capacities (Aroni
1993). As a consequence, carbonation induced corrosion is not an issue
for AAC. It is rather the modification of the microstructure due to carbonation, inducing significant shrinkage values, which is a predominant
concern for AAC.
Carbonation of AAC leads to a decrease in strength, to an increase in
deflection, and the growth of lattice-like cracking (Matsushita et al. 2009).
For carbonation degrees below 25%, no significant carbonation shrinkage
is noticed as the double-chain silicate anion structure of the tobermorite is
well maintained (Matsushita et al. 2004b). However, carbonation shrinkage gradually increases for higher degrees of carbonation ranging from
approximately 20% to 60%, most probably due to the decomposition of
the double-chain structure (Matsushita et al. 2004b). Carbonation shrinkage values of ACC ranging between 0.1% and 1.0% have been reported,
depending on raw materials, CO 2 concentration, and relative humidity
(Matsushita et al. 2009). For real structures made of AAC, it is noticed