There is a provision in ACI 318-02/318R-02 Paragraph 10.6.4 that
limits cover based on the spacing of reinforcement, to limit surface
crack width, which is thought to contribute to acceleration of
corrosion.  Although the commentary states that "clear experimental
evidence is not available...exposure tests indicate that concrete
quality, adequate compaction, and ample concrete cover may be of
greater importance for corrosion protection than crack width at the
concrete surface," based on the current code (not the commentary),
there would be a limit to the amount of concrete cover allowed based
on the reinforcement spacing in a concrete column.
Question: what is the present thinking about excess concrete cover
over the reinforcing steel for structures that are exposed to salt and
brackish water, i.e. is the risk of micro-cracking of more concern
than the apparent benefit of having extra concrete cover on the
reinforcing steel?