**2. Theoretical explanations of freeze/thaw attack**

It was originally thought that the small, entrained air voids (deliberately introduced concrete to provide frost resistance) worked by providing sinks for the water

**Figure 1.**

*Left (original concept): Air voids prevent excessive pore pressure by providing sinks for displaced water. Right (revised concept): Air voids provide a site for nucleation and growth of ice crystals drawing in liquid from pores [3].*

displaced by the volume change associated with the transformation of water into ice, but it was subsequently realized that the voids serve as nucleation sites where ice crystals can grow without constraint. During the freezing process ice crystals in the air voids suck liquid from the small pores of the paste, this suction creates negative pressure in the pore liquid which puts the entire solid matrix into compression; thereby inducing compressive stress into the concrete, **Figure 1**. The shown figures have been presented at the concrete & cast stone conference in Boston, USA (2008) as well fib bulletin 53 [3].

Pore system characteristics plays an essential role in the transport properties of concrete as well as the behavior of concrete when it exposes to frost action. Capillary forces determine the absorbed water in concrete and by ice expanding hydraulic pressure increases and damage occurs. The process is directly proportional to the rate of temperature decrease, in addition, forming crystals of ice could interact with the walls of the capillary pores [4–6].
