Water can be found in concrete as product of the hydration during the mixing or as a result of filtration induced by cracking and structural joints. When this water freezes, it creates pressure in the pore network of the concrete and generate tensile stresses that produce more cracking, surface scaling and possible loss of the cover due to spalling.
According to Wittman6, frost damage can be divided into 4 different groups:
1) Volume expansion of 9% during freezing of water in large pores in saturated concrete.
2) Micro-ice lenes in coarse pores related with water migration from gel pores.
3) Changes in the thermal expansion of aggregates and hardened cement compound.
4) Thermal gradients during the cooling process; increment in tensile stress in the surface zone.
The concrete is fundamentally a permeable material, which porosity depends on the water-cement ratio, consolidation and curing conditions. Other elements may be added in order to reduce the concrete permeability and size of the capillary pores of the cementitious paste, such as fly ash and silica fume. This also influence the rate of water transportation and storing that can be absorbed.
These capillary pores can be saturated to various degrees as influenced by the remnant mix water or the absorption of water from the environment. At very low temperatures this water freezes and then expands in volume by 9%. Pores sizes are not sufficient to contain this expansion, producing pores damage and cracks formation, which reduce the strength and durability of the structure.
For the second group, during the cooling process, the gel pores shrink and expel the contained water. This water precipitates into larger pores where more expansion and deterioration is expected7.
The third case is related with the development of internal stresses as a result of notable differences between the coefficient of thermal expansion of the concrete compound ( average of 12-15 x10-6 / ??C) and the aggregates ( 7- 9 x10-6 / ??C).
The four case has effect when tensile stresses during fast cooling, generated near the surface of the element, surpass the tensile strength of the concrete. This may happen in a short period of time in the case where deicing salts are used.
As a result of the frost damage, the concrete surface facilitates the ingress of gases and liquids. In this specific case capillary absorption and carbonation absorption are analyzed.
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