150 Damage to concrete structures
5.3.2.1.3 Mitigation
A major measure to reduce the risk of sulfate attack is to produce a dense
concrete with reduced transport properties. This can be achieved by providing a low water/cement ratio, as confirmed explicitly by Marchand et al.
(2002) while theoretically analysing the effect of weak sodium sulfate solutions on the durability of concrete. In this way, sulfate ions will penetrate
the concrete more slowly. However, care should be taken to avoid early-age
(micro) cracking (e.g. due to autogenous shrinkage), which will, of course,
increase transport properties in spite of the low water/cement ratio.
For sulfate attack cases in which expansive ettringite formation is the
driving force leading to concrete damage, the application of a cement with
low C 3 A content is advised, or a so-called high sulfate resisting (HSR)
cement. As no significant amount of monosulfate will be formed during
hydration of an HSR cement, no subsequent transformation to ettringite
can occur. In some standards, the restriction not only concerns C 3 A, but
also C 4 AF, e.g. in ASTM C 150-2002. However, the application of an HSR
cement does not help where the C-S-H is decomposed due to the action of
the sulfate solution (e.g. in the case of magnesium sulfate).
Puzzolanic materials (fly ash, slag, silica fume, natural puzzolans) can
also be helpful in reducing the risk of sulfate attack, although different
materials might not be equally effective (depending on the alumina content, a low alumina content being beneficial) (Neville 2004). Their positive
action goes along with the densifying effect on the pore structure reducing
transport properties, with a reduction of the calcium hydroxide content of
the hydrated system reducing gypsum formation, and with a reduction of
the C 3 A content reducing the risk of ettringite formation. However, in the
case of puzzolanic materials, degradation by magnesium sulfate is more
pronounced than degradation by other sulfate solutions. In the case of the
addition of puzzolanic materials, an adequate curing of the concrete is
of main importance because lack of curing might lead to a higher risk of
(physical) sulfate attack in these cases (see further in Section 5.3.2.2).
Besides HSR cement, other special cements can significantly improve the
resistance against sulfate attack such as calcium aluminate cement, phosphate cement, alkali silicate cement, and geopolymer cement (Skalny et al.
2002). A discussion of these special cement types, however, is beyond the
scope of this textbook.
5.3.2.1.4 Example
In real structures, when expansive ettringite formation is the main cause of
damage, a chemical sulfate attack can lead to map cracking as in the case
for ASR. A major difference, however, is that cracks will typically not run
through the aggregates, as opposed to ASR-induced cracks, which are initiated within the aggregates. While ASR can show a yellowish or brownish
5.3.2.1.3 Mitigation
A major measure to reduce the risk of sulfate attack is to produce a dense
concrete with reduced transport properties. This can be achieved by providing a low water/cement ratio, as confirmed explicitly by Marchand et al.
(2002) while theoretically analysing the effect of weak sodium sulfate solutions on the durability of concrete. In this way, sulfate ions will penetrate
the concrete more slowly. However, care should be taken to avoid early-age
(micro) cracking (e.g. due to autogenous shrinkage), which will, of course,
increase transport properties in spite of the low water/cement ratio.
For sulfate attack cases in which expansive ettringite formation is the
driving force leading to concrete damage, the application of a cement with
low C 3 A content is advised, or a so-called high sulfate resisting (HSR)
cement. As no significant amount of monosulfate will be formed during
hydration of an HSR cement, no subsequent transformation to ettringite
can occur. In some standards, the restriction not only concerns C 3 A, but
also C 4 AF, e.g. in ASTM C 150-2002. However, the application of an HSR
cement does not help where the C-S-H is decomposed due to the action of
the sulfate solution (e.g. in the case of magnesium sulfate).
Puzzolanic materials (fly ash, slag, silica fume, natural puzzolans) can
also be helpful in reducing the risk of sulfate attack, although different
materials might not be equally effective (depending on the alumina content, a low alumina content being beneficial) (Neville 2004). Their positive
action goes along with the densifying effect on the pore structure reducing
transport properties, with a reduction of the calcium hydroxide content of
the hydrated system reducing gypsum formation, and with a reduction of
the C 3 A content reducing the risk of ettringite formation. However, in the
case of puzzolanic materials, degradation by magnesium sulfate is more
pronounced than degradation by other sulfate solutions. In the case of the
addition of puzzolanic materials, an adequate curing of the concrete is
of main importance because lack of curing might lead to a higher risk of
(physical) sulfate attack in these cases (see further in Section 5.3.2.2).
Besides HSR cement, other special cements can significantly improve the
resistance against sulfate attack such as calcium aluminate cement, phosphate cement, alkali silicate cement, and geopolymer cement (Skalny et al.
2002). A discussion of these special cement types, however, is beyond the
scope of this textbook.
5.3.2.1.4 Example
In real structures, when expansive ettringite formation is the main cause of
damage, a chemical sulfate attack can lead to map cracking as in the case
for ASR. A major difference, however, is that cracks will typically not run
through the aggregates, as opposed to ASR-induced cracks, which are initiated within the aggregates. While ASR can show a yellowish or brownish