Decay of concrete is mainly due to following reasons:

• Concrete cement contains some free lime(CaO), which dissolves in acidic water and makes the concrete weak. As lime is more soluble in soft water than hard water, deterioration of concrete is more in soft water.
• Metal sulphates present in hard water combine with tricalcium aluminate to form sulpho aluminates which occupy more volume resulting into expansion and develops cracks, thus the concrete structure becomes weak. Further, hydrolysis also lead to decay.
• Due to the cracks developed as mentioned above, the steel gets exposed to the O2 present in the atmosphere which causes the oxidization and hence dacay; the process is cummulative.

Prevention or minimization of decay

1. By treating the surface by Silicon Fluoride(SiF4) when CaF2(which is insoluble) is formed which prevents dissolution of lime.
2. By coating the surface with epoxy resin paint or bituminous or linseed oil which makes the surface impermeable to acidic water.
3. To cover/coat the reinforcing steel bars with proper epoxy paints, resin paints.