Actions during service 141
The permeability of the concrete, as mainly governed by the water/
cement ratio, also has an important influence. On the one hand, the reaction rate of ASR depends on the mobility of ions and water, which in turn
depends on the permeability of the concrete. A higher permeability might
thus increase the reaction rate. On the other hand, the expansive pressures
caused by the gel formation can be reduced in case of absorption in a higher
pore volume. This means that in case ASR occurs, pressures will be lower
in a more porous system. While this statement would be supported by the
theory of Multon et al. (2010), it is not supported by the theory of Ichikawa
and Miura (2007). This again shows the debate that still exists due to the
very complex nature of ASR.
Supplementary cementitious materials (SCM) also seem to have a significant influence on expansion due to ASR. While SCMs typically contain
alkali themselves, the main mechanism by which SCMs reduce expansion
due to ASR is by lowering the alkali concentration in the pore solution of
the concrete, probably due to the presence of alumina in the SCM (Thomas
2011). With higher replacement levels, a higher reduction in the concentration of alkali-hydroxides is obtained in the pore solution. Silica fume is
very efficient in reducing the hydroxyl ion concentration, although a slow
increase is noticed again after three months. This increase is not noticed
in the case of other SCMs. Metakaolin, low-calcium fly ash, and slag also
efficiently reduce the hydroxyl ion concentration. High calcium fly ash and
high alkali fly ash show a lower efficiency and require higher dosages to
reduce the pore solution alkalinity (Thomas 2011). Following the results of
Shehata and Thomas (2000, 2002), fly ashes with low alkali and calcium
content (Na 2 O eq < 4%, CaO < 20%) are generally effective in controlling
ASR expansion, considering 25% cement replacement. For higher calcium
content (CaO > 20%), ASR expansion increases linearly with increasing
calcium content. In the case of fly ash with a high alkali content (Na 2 O eq >
5%), ASR expansion cannot be effectively controlled at a 25% replacement
level, irrespective of the calcium content.
The influence of chemical admixtures (plasticizers, retarders, accelerators) on ASR primarily seems to depend on their alkali content. Chemical
admixtures with a high alkali content typically seem to increase the risk of
ASR expansion. This is related to the overall alkali content in the concrete.
5.3.1.3 Mitigation
In order to mitigate ASR, actions can be taken to make sure that at least
one of the three required conditions for ASR is not fulfilled. The most
straightforward measure, of course, is to avoid potentially reactive aggregates. However, quite often this is neither practical nor economical because
it depends on the availability of suitable local materials. Care should also
be taken in examining the available materials. Although the application of
The permeability of the concrete, as mainly governed by the water/
cement ratio, also has an important influence. On the one hand, the reaction rate of ASR depends on the mobility of ions and water, which in turn
depends on the permeability of the concrete. A higher permeability might
thus increase the reaction rate. On the other hand, the expansive pressures
caused by the gel formation can be reduced in case of absorption in a higher
pore volume. This means that in case ASR occurs, pressures will be lower
in a more porous system. While this statement would be supported by the
theory of Multon et al. (2010), it is not supported by the theory of Ichikawa
and Miura (2007). This again shows the debate that still exists due to the
very complex nature of ASR.
Supplementary cementitious materials (SCM) also seem to have a significant influence on expansion due to ASR. While SCMs typically contain
alkali themselves, the main mechanism by which SCMs reduce expansion
due to ASR is by lowering the alkali concentration in the pore solution of
the concrete, probably due to the presence of alumina in the SCM (Thomas
2011). With higher replacement levels, a higher reduction in the concentration of alkali-hydroxides is obtained in the pore solution. Silica fume is
very efficient in reducing the hydroxyl ion concentration, although a slow
increase is noticed again after three months. This increase is not noticed
in the case of other SCMs. Metakaolin, low-calcium fly ash, and slag also
efficiently reduce the hydroxyl ion concentration. High calcium fly ash and
high alkali fly ash show a lower efficiency and require higher dosages to
reduce the pore solution alkalinity (Thomas 2011). Following the results of
Shehata and Thomas (2000, 2002), fly ashes with low alkali and calcium
content (Na 2 O eq < 4%, CaO < 20%) are generally effective in controlling
ASR expansion, considering 25% cement replacement. For higher calcium
content (CaO > 20%), ASR expansion increases linearly with increasing
calcium content. In the case of fly ash with a high alkali content (Na 2 O eq >
5%), ASR expansion cannot be effectively controlled at a 25% replacement
level, irrespective of the calcium content.
The influence of chemical admixtures (plasticizers, retarders, accelerators) on ASR primarily seems to depend on their alkali content. Chemical
admixtures with a high alkali content typically seem to increase the risk of
ASR expansion. This is related to the overall alkali content in the concrete.
5.3.1.3 Mitigation
In order to mitigate ASR, actions can be taken to make sure that at least
one of the three required conditions for ASR is not fulfilled. The most
straightforward measure, of course, is to avoid potentially reactive aggregates. However, quite often this is neither practical nor economical because
it depends on the availability of suitable local materials. Care should also
be taken in examining the available materials. Although the application of