142 Damage to concrete structures
limestone aggregates might sound like a sure and safe action; surprisingly,
some limestone is potentially reactive. This is the case for limestone from
the region of Tournai, Belgium, which is a siliceous limestone containing
amorphous or poorly crystalline silica.
A more practical measure is the limitation of the alkali content of the
concrete. Typically, this is achieved by prescribing a cement with a low
alkali content (a so-called LA cement), although it is clear that other constituents can also contain alkali. A safer approach is to check the total
alkali content of the concrete by considering the alkali content in all constituent materials. In many countries, the total alkali content (expressed as
Na 2 O eq ) of the concrete is required to remain below 3 kg/m³. This threshold
may be increased in case of the application of supplementary cementitious
materials (SCM), because they typically reduce the alkali content of the
pore solution in spite of containing alkali themselves.
Protecting the concrete from rain and other sources of water, or in other
words keeping the concrete dry, would also help prevent expansion due to
ASR. However, this is not a realistic approach in ‘wet countries’ such as
Belgium. Sealing the concrete with protective coatings does not seem to be
efficient because the internal moisture in the concrete element might already
be sufficient to initiate ASR. Nevertheless, coatings could reduce the expansion rate. Reducing the water/cement ratio of the concrete will reduce the permeability, and thus obstruct the water supply to the reactive aggregates. The
resulting effect, however, is a mere slowing down of the expansive reaction.
Mitigation of ASR by special chemical admixtures could also be an
option. Lithium and barium salts seem to show promising results toward
controlling ASR expansion (Feng et al. 2010, Bulteel et al. 2010). Their
preventive action is related to the formation of non-swelling lithium and
barium silicate hydrates. The required dosage depends on the alkali content
of the concrete, the properties of the potentially reactive aggregate, and the
type of lithium or barium salt. However, application of these special chemical admixtures remains very limited until now, probably because of the
high cost. Lithium treatment of ASR-affected existing structures in order
to stop ongoing ASR has also been investigated (Ueda et al. 2011), but this
is beyond the scope of this textbook.
5.3.1.4 Example
Cases of ASR can easily be found worldwide. An interesting example is the
Montreal Olympic Stadium, which was built for the Olympic Games of
1976. However, despite the fact that ASR is better known nowadays, it can
still be found in more recent structures. An example is given in Figure 5.27,
which shows concrete columns within a frame supporting a crane bridge
in open air. Severe damage due to ASR occurred within less than five years
after construction. At the top of the columns, concrete is spalled, yielding
limestone aggregates might sound like a sure and safe action; surprisingly,
some limestone is potentially reactive. This is the case for limestone from
the region of Tournai, Belgium, which is a siliceous limestone containing
amorphous or poorly crystalline silica.
A more practical measure is the limitation of the alkali content of the
concrete. Typically, this is achieved by prescribing a cement with a low
alkali content (a so-called LA cement), although it is clear that other constituents can also contain alkali. A safer approach is to check the total
alkali content of the concrete by considering the alkali content in all constituent materials. In many countries, the total alkali content (expressed as
Na 2 O eq ) of the concrete is required to remain below 3 kg/m³. This threshold
may be increased in case of the application of supplementary cementitious
materials (SCM), because they typically reduce the alkali content of the
pore solution in spite of containing alkali themselves.
Protecting the concrete from rain and other sources of water, or in other
words keeping the concrete dry, would also help prevent expansion due to
ASR. However, this is not a realistic approach in ‘wet countries’ such as
Belgium. Sealing the concrete with protective coatings does not seem to be
efficient because the internal moisture in the concrete element might already
be sufficient to initiate ASR. Nevertheless, coatings could reduce the expansion rate. Reducing the water/cement ratio of the concrete will reduce the permeability, and thus obstruct the water supply to the reactive aggregates. The
resulting effect, however, is a mere slowing down of the expansive reaction.
Mitigation of ASR by special chemical admixtures could also be an
option. Lithium and barium salts seem to show promising results toward
controlling ASR expansion (Feng et al. 2010, Bulteel et al. 2010). Their
preventive action is related to the formation of non-swelling lithium and
barium silicate hydrates. The required dosage depends on the alkali content
of the concrete, the properties of the potentially reactive aggregate, and the
type of lithium or barium salt. However, application of these special chemical admixtures remains very limited until now, probably because of the
high cost. Lithium treatment of ASR-affected existing structures in order
to stop ongoing ASR has also been investigated (Ueda et al. 2011), but this
is beyond the scope of this textbook.
5.3.1.4 Example
Cases of ASR can easily be found worldwide. An interesting example is the
Montreal Olympic Stadium, which was built for the Olympic Games of
1976. However, despite the fact that ASR is better known nowadays, it can
still be found in more recent structures. An example is given in Figure 5.27,
which shows concrete columns within a frame supporting a crane bridge
in open air. Severe damage due to ASR occurred within less than five years
after construction. At the top of the columns, concrete is spalled, yielding