152 Damage to concrete structures
the drying face which is in contact with the surrounding air, which typically has a relative humidity below 100%. Wick action involves capillary
suction, diffusion, and evaporation. Together with the water, sulfate (and
other) ions are also transported into the concrete. On the drying surface,
water evaporates and a salt concentration builds up near the surface because
salts cannot be carried by water vapour. The high salt concentration near
the surface causes back diffusion, leading to a nearly saturated pore solution zone near the surface (Pel et al. 2004).
In the case of a concrete element partially immersed in a sodium sulfate
solution, due to wick action and back diffusion, a very high concentration
of SO 4
2− and Na 2+ ions is obtained in the pores near the drying surface, even
higher than in the sodium sulfate solution itself (Liu 2010, Liu et al. 2012).
Thanks to this super-saturation, crystallisation pressures could occur in
the pores near the surface in the case of a reversible change of anhydrous
sodium sulfate (thenardite) into decahydrate (mirabilite) (Neville 2004).
This crystallisation process is considered to be a physical process, hence the
name ‘physical’ sulfate attack. The part of the concrete element above the
solution level is degraded significantly, showing efflorescence and cracking,
while the immersed part typically remains nearly intact.
Quite often, a parallel is seen between physical salt (sulfate) attack in concrete and physical salt attack in bricks and natural stones. However, clear
contradictions can easily be noticed (Liu 2010). Physical attack on concrete
seems to increase with increased relative humidity of the surrounding air,
while the opposite is noted in the case of bricks and stones. Furthermore,
according to the basic principles of salt crystallisation in porous materials,
a concrete with a higher water/cement ratio would have to show a better
resistance against physical sulfate attack, while experiments and field cases
show the opposite (Hime 2003).
These discrepancies illustrate that ‘physical sulfate attack’ of concrete
can be heavily debated. According to Mehta (2000) efflorescence should
not cause any damage, except under certain circumstances. This discussion has motivated Liu (2010) to study chemical aspects in ‘physical sulfate
attack’. He conjectures that the major cause for the distress of concrete
partially exposed to a sulfate environment is more likely to be chemical sulfate attack, not salt weathering, salt crystallisation, or physical attack (Liu
et al. 2011). This seems to hold both for sodium and magnesium sulfate
solutions. Salt crystallisation, however, can play an additional aggravating
role when exposed to sodium sulfate after damage initiation by chemical
sulfate attack. The conclusion of the major role of chemical attack in the
case of partially exposed structures seems to be in line with the findings
of Bellmann et al. (2012), which are based on the field performance of 20
concrete structures.
Nevertheless, in the case of previous carbonation, crystallisation of
sodium sulfate can occur in the cement paste and cause damage. Although
the drying face which is in contact with the surrounding air, which typically has a relative humidity below 100%. Wick action involves capillary
suction, diffusion, and evaporation. Together with the water, sulfate (and
other) ions are also transported into the concrete. On the drying surface,
water evaporates and a salt concentration builds up near the surface because
salts cannot be carried by water vapour. The high salt concentration near
the surface causes back diffusion, leading to a nearly saturated pore solution zone near the surface (Pel et al. 2004).
In the case of a concrete element partially immersed in a sodium sulfate
solution, due to wick action and back diffusion, a very high concentration
of SO 4
2− and Na 2+ ions is obtained in the pores near the drying surface, even
higher than in the sodium sulfate solution itself (Liu 2010, Liu et al. 2012).
Thanks to this super-saturation, crystallisation pressures could occur in
the pores near the surface in the case of a reversible change of anhydrous
sodium sulfate (thenardite) into decahydrate (mirabilite) (Neville 2004).
This crystallisation process is considered to be a physical process, hence the
name ‘physical’ sulfate attack. The part of the concrete element above the
solution level is degraded significantly, showing efflorescence and cracking,
while the immersed part typically remains nearly intact.
Quite often, a parallel is seen between physical salt (sulfate) attack in concrete and physical salt attack in bricks and natural stones. However, clear
contradictions can easily be noticed (Liu 2010). Physical attack on concrete
seems to increase with increased relative humidity of the surrounding air,
while the opposite is noted in the case of bricks and stones. Furthermore,
according to the basic principles of salt crystallisation in porous materials,
a concrete with a higher water/cement ratio would have to show a better
resistance against physical sulfate attack, while experiments and field cases
show the opposite (Hime 2003).
These discrepancies illustrate that ‘physical sulfate attack’ of concrete
can be heavily debated. According to Mehta (2000) efflorescence should
not cause any damage, except under certain circumstances. This discussion has motivated Liu (2010) to study chemical aspects in ‘physical sulfate
attack’. He conjectures that the major cause for the distress of concrete
partially exposed to a sulfate environment is more likely to be chemical sulfate attack, not salt weathering, salt crystallisation, or physical attack (Liu
et al. 2011). This seems to hold both for sodium and magnesium sulfate
solutions. Salt crystallisation, however, can play an additional aggravating
role when exposed to sodium sulfate after damage initiation by chemical
sulfate attack. The conclusion of the major role of chemical attack in the
case of partially exposed structures seems to be in line with the findings
of Bellmann et al. (2012), which are based on the field performance of 20
concrete structures.
Nevertheless, in the case of previous carbonation, crystallisation of
sodium sulfate can occur in the cement paste and cause damage. Although