In some recommendations, including those of the JSCE and ACI standards,
the threshold content values of chloride ion on the corrosion of steel bars in concrete
are introduced, partly because of practical reasons. The readers might believe that
this value is so accurate that values over this limit will directly lead to loss of passivity
and the corrosion of steel bar can immediately start. However, this belief is not true.
This paper is aimed at reconfirming the meaning of the threshold content values,
including the statistical nature of the values and the influence of ITZ and kind of
cement on the values. Special emphasis is given to the influence of ITZ as clarified
from the intensive laboratory and exposure tests.