in the present day models for the life of concrete structures, the causes of failure have been earmarked and role of chloride ion has been placed at the top, because of comparative ease of cl- transport in concrete structure and its extraordinary damaging effect on the existing passivating film on the rebar surface. Therefore, diffusion of cl- ions through concrete media has been given pronounced recognition and various testing techniques for the measurement of ‘diffusion’ of cl- have been developed and standardized.  this is as per standard of nt built-492 test method.