Lisø PhD Dissertation Manuscript - NTNU

degree of saturation, freezing will destroy the brick. The frost resistance of concrete is
determined similarly.
As the temperature of a saturated porous material is lowered, the water freezes and
expansion takes place. Freezing of water results in an increase in volume of approximately
9%. Damage occurs when the tensile forces in the pore system exceeds the tensile strength
in the material. The extent of frost damage varies from surface scaling to complete
disintegration as ice is formed. The disintegration starts at the exposed surface progressing
through the material due to alternating freezing and thawing (see illustration in Fig. 3).
Fig. 3. A frost damaged brick wall (photo: NBI).
Repeated freezing and thawing causes progressive damage, as freezing causes
migration of water to locations where it can freeze. The first freezing may introduce fine
cracks where water may be located during the next freezing. This is how subsequent
freezing and thawing gradually enlarge cracks until visible damage has occurred.
To ensure reliable frost resistance it is of crucial importance that the pore system of
the material contains space available for expelled water close to capillary cavities in which
ice is being formed. This is why air entrainment is added in concrete. However, the most
reliable way of ensuring frost resistance of any porous material is to reduce the volume of
capillary pores. A decrease in the volume of capillary pores also leads to an increase in
tensile strength of the material, resulting in a material more resistant to ice expansion.
Hence, frost resistant concrete should have a maximum water/cement ratio governed by the
use of air entrainment as given in [18].
The degree of saturation of a façade material depends on the climatic conditions to
which it is exposed, as well as the ability to absorb water. The suction rate is commonly
used for classification of bricks. The masonry mortar composition depends to a high degree
on the suction rate of the bricks, due to the fact that a suitable amount of water should be
removed from the mortar to ensure optimal hardening conditions of the mortar itself and
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