Actions during service 105
from the capillary, finding its way to air voids. When enough air voids
are available within a short distance, the hydraulic pressure can be easily
relaxed (see Section 5.2.1.3: application of air entrainment to improve frost
resistance). Otherwise, the hydraulic pressure can result in damage and the
cracking of the freezing concrete.
Although the hydraulic pressure theory seems to logically explain the
occurrence of internal frost damage, Powers observed phenomena which
could not be easily explained (Powers 1953, Valenza and Scherer 2007b).
While reducing the air void spacing, i.e. facilitating the flow of pore water
when expelled by the expansive ice formation, a contraction of the concrete
is obtained larger than the thermal contraction, instead of a mere reduction
of the expansion caused by the hydraulic pressure. Furthermore, when cooling is stopped, no relaxation of the hydraulic pressures seems to occur in airvoided concrete, while the hydraulic pressure theory would predict a stress
relaxation due to migration of water to the available air voids. After further
analysis, Powers finally withdrew his own hydraulic pressure theory (Powers
1975). Nevertheless, the capillary pressure theory is useful to explain frost
damage caused by ice formation in capillary pores. In some cases, when
water-filled pores are surrounded by growing ice crystals, the resulting fluid
pressure indeed is believed to cause frost damage (Chatterji 2003).
5.2.1.1.2 Crystallisation pressure
It is important to know that in fine pores the freezing point is reduced
because the ice crystals have a high surface-to-volume ratio. At the normal
freezing temperature, a minimization of Gibbs energy will not be obtained
while forming small ice crystals because the energy gain due to solidification is counteracted by the energy of the interface between the small ice
crystals and the surrounding water (Valenza and Scherer 2007b). Based on
energy minimization, a relationship can be obtained between the freezing
point and the largest spherical crystal that can be formed in a pore with
a certain radius, taking into account a small layer (about 0.9 nm thick) of
unfrozen water which remains between ice crystals and pore wall (Valenza
and Scherer 2007b). Without going into the details of this relationship, it
can be concluded that the freezing point is reduced by 2°C for a capillary
with a radius of about 33 nm, by 5°C for a radius of about 13 nm, and by
10°C for a radius of about 7 nm (Valenza and Scherer 2007b).
As a result of the relationship between a freezing temperature and capillary radius, within a saturated cementitious material ice formation will be
first initiated in the larger pores or voids (see Figure 5.8). In the meantime,
the water in the smaller pores will remain liquid, in a state of super cooling.
This results in a thermodynamic disequilibrium, which acts as a driving
force for the water to move from the smaller capillary pores to the larger
voids where ice crystals are being formed (Mehta and Monteiro 2006).
from the capillary, finding its way to air voids. When enough air voids
are available within a short distance, the hydraulic pressure can be easily
relaxed (see Section 5.2.1.3: application of air entrainment to improve frost
resistance). Otherwise, the hydraulic pressure can result in damage and the
cracking of the freezing concrete.
Although the hydraulic pressure theory seems to logically explain the
occurrence of internal frost damage, Powers observed phenomena which
could not be easily explained (Powers 1953, Valenza and Scherer 2007b).
While reducing the air void spacing, i.e. facilitating the flow of pore water
when expelled by the expansive ice formation, a contraction of the concrete
is obtained larger than the thermal contraction, instead of a mere reduction
of the expansion caused by the hydraulic pressure. Furthermore, when cooling is stopped, no relaxation of the hydraulic pressures seems to occur in airvoided concrete, while the hydraulic pressure theory would predict a stress
relaxation due to migration of water to the available air voids. After further
analysis, Powers finally withdrew his own hydraulic pressure theory (Powers
1975). Nevertheless, the capillary pressure theory is useful to explain frost
damage caused by ice formation in capillary pores. In some cases, when
water-filled pores are surrounded by growing ice crystals, the resulting fluid
pressure indeed is believed to cause frost damage (Chatterji 2003).
5.2.1.1.2 Crystallisation pressure
It is important to know that in fine pores the freezing point is reduced
because the ice crystals have a high surface-to-volume ratio. At the normal
freezing temperature, a minimization of Gibbs energy will not be obtained
while forming small ice crystals because the energy gain due to solidification is counteracted by the energy of the interface between the small ice
crystals and the surrounding water (Valenza and Scherer 2007b). Based on
energy minimization, a relationship can be obtained between the freezing
point and the largest spherical crystal that can be formed in a pore with
a certain radius, taking into account a small layer (about 0.9 nm thick) of
unfrozen water which remains between ice crystals and pore wall (Valenza
and Scherer 2007b). Without going into the details of this relationship, it
can be concluded that the freezing point is reduced by 2°C for a capillary
with a radius of about 33 nm, by 5°C for a radius of about 13 nm, and by
10°C for a radius of about 7 nm (Valenza and Scherer 2007b).
As a result of the relationship between a freezing temperature and capillary radius, within a saturated cementitious material ice formation will be
first initiated in the larger pores or voids (see Figure 5.8). In the meantime,
the water in the smaller pores will remain liquid, in a state of super cooling.
This results in a thermodynamic disequilibrium, which acts as a driving
force for the water to move from the smaller capillary pores to the larger
voids where ice crystals are being formed (Mehta and Monteiro 2006).