Actions during service 107
5.2.1.1.3 Osmotic pressure
While the temperature is decreasing, ice formation first occurs in the larger
pores, as explained before. Due to the ice formation, mainly involving pure
water, the alkalinity of the remaining non-frozen water within the larger
pores will increase. In order to reach thermodynamic equilibrium, pore water
with lower alkalinity will move from the smaller pores to the larger pores
where ice formation occurs. As a result, the alkalinity of the non-frozen
water near the ice crystals will decrease, facilitating further ice formation.
Otherwise, due to the diffusion of solution towards the zones with higher
alkalinity, the fluid pressure will rise here, until equilibrium is obtained
with the difference in osmotic pressure between the zones with higher and
lower alkalinity. However, based on some calculations, Valenza and Scherer
(2007b) showed that the osmotic pressure is very unlikely to cause concrete
damage. Nevertheless, the osmotic pressure could be considered as an additional phenomenon, adding to the more important crystallisation pressure.
5.2.1.2 Influencing parameters
5.2.1.2.1 Degree of saturation
The degree of saturation of the concrete is calculated as the amount of free
water present in the pores, relative to the maximum amount of water when
all the pores would be filled. The degree of saturation of the concrete is
an important parameter regarding frost damage. Dry concrete, of course,
will not show any damage when exposed to freezing because no ice will be
formed. In partially saturated concrete, a certain volume of the pores will
contain no water. This empty pore volume will provide the possibility that
ice will expand or pressurized water will escape, releasing pressure.
Considering the fact that a 9% volume increase is noticed during ice formation, it would be reasonable to expect that frost damage will not occur
in concrete with a degree of saturation below about 90%. Experimental
results confirm the existence of a critical saturation degree below which
no significant frost damage will occur, although proposed values range
between 85% and 90%.
5.2.1.2.2 Air void system
Besides the amount of free expansion volume, which can be deduced from
the degree of saturation, the spatial distribution of this expansion volume
is also important. The available expansion volume is only effective when
it is easily and rapidly accessible for pressurized water or expanding ice.
In order to improve frost resistance, air-entraining agents are quite often
used in concrete, providing a large amount of small air voids evenly distributed within the cementitious system. A key parameter concerning the
5.2.1.1.3 Osmotic pressure
While the temperature is decreasing, ice formation first occurs in the larger
pores, as explained before. Due to the ice formation, mainly involving pure
water, the alkalinity of the remaining non-frozen water within the larger
pores will increase. In order to reach thermodynamic equilibrium, pore water
with lower alkalinity will move from the smaller pores to the larger pores
where ice formation occurs. As a result, the alkalinity of the non-frozen
water near the ice crystals will decrease, facilitating further ice formation.
Otherwise, due to the diffusion of solution towards the zones with higher
alkalinity, the fluid pressure will rise here, until equilibrium is obtained
with the difference in osmotic pressure between the zones with higher and
lower alkalinity. However, based on some calculations, Valenza and Scherer
(2007b) showed that the osmotic pressure is very unlikely to cause concrete
damage. Nevertheless, the osmotic pressure could be considered as an additional phenomenon, adding to the more important crystallisation pressure.
5.2.1.2 Influencing parameters
5.2.1.2.1 Degree of saturation
The degree of saturation of the concrete is calculated as the amount of free
water present in the pores, relative to the maximum amount of water when
all the pores would be filled. The degree of saturation of the concrete is
an important parameter regarding frost damage. Dry concrete, of course,
will not show any damage when exposed to freezing because no ice will be
formed. In partially saturated concrete, a certain volume of the pores will
contain no water. This empty pore volume will provide the possibility that
ice will expand or pressurized water will escape, releasing pressure.
Considering the fact that a 9% volume increase is noticed during ice formation, it would be reasonable to expect that frost damage will not occur
in concrete with a degree of saturation below about 90%. Experimental
results confirm the existence of a critical saturation degree below which
no significant frost damage will occur, although proposed values range
between 85% and 90%.
5.2.1.2.2 Air void system
Besides the amount of free expansion volume, which can be deduced from
the degree of saturation, the spatial distribution of this expansion volume
is also important. The available expansion volume is only effective when
it is easily and rapidly accessible for pressurized water or expanding ice.
In order to improve frost resistance, air-entraining agents are quite often
used in concrete, providing a large amount of small air voids evenly distributed within the cementitious system. A key parameter concerning the