Durability of Concrete
A durable concrete is one that performs satisfactorily under anticipated exposure conditions for stipulated life of the structure. The various factors affecting the durability of concrete used in normal conditions are as follows:
Permeability Almost all forms of deterioration in concrete are due to ingress of water. The ways in which durability of concrete may be affected because of permeability are:
1. The chemicals in liquid form affect the concrete by penetrating it.
2. Frost action, rusting of steel, etc.
Concrete has gel pores and capillary cavities. About 1/3 of gel pores are so small that they hardly pass any water through them. The extent of capillary cavities, which depends on the w/c ratio, is the major factor contributing to permeability. The remedies are:
1. Use of puzzolanic materials.
2. Air entrainment up to 6 per cent.
3. High pressure steam curing in conjunction with silica.
Frost Action: The concrete may be affected due to being permeable or by temperature below 0 o C. The mechanism of attack is attributed to the expansion of absorbed water on freezing. Damage can also result from movement of water within concrete on cooling below 0 o C. Ice builds up in large pores causing large expansion in local areas the others being dry cause disintegration. The conditions favouring frost attack are:
1. Horizontal surfaces open to sky absorbing more water in wet conditions and cooling quicker by radiation.
2. Low temperatures increasing the extent of migration of water resulting in freezing to greater depths in the concrete.
3. Repeated freezing and thawing.
2. Use of de-icing salts.