As far as only concrete is concerned i.e. plain concrete, the effect of water seepage is very little (depending upon the grade of concrete) whereas for RCC (reinforced cement concrete) water that seeps in corrodes the reinforcement and therefore reduces the life of the structure. The defects that water seepage induces in concrete are as follows:
a) Makes capillary formation (due to the detiorating characteristics of water)
b) With this capillaries the concrete starts spalling out; i.e. the places where capillaries are produced, with even slight amount of stress that portion comes out and exposes the steel to the atmosphere
c) Concrete has a pH of about 12 -13. It also decreases the pH of the concrete when in salty water (or) when exposed to marshy areas.
d) Decreases the overall strength of concrete