Originally Posted By: jpeck
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That is spalling.
Water migrates in (along with the salt from the ocean) and rusts the steel in the concrete. When steel rusts, the surface expands (so I've been told) about 43 times in size. That is like ice damage, only it is caused by rusting steel, it pushes the concrete outward.
Once you see the cracks, there are only two options: 1) chip out the concrete, clean the steel if the steel is salvageable otherwise replace the steel - means chipping out even more concrete, patch with high strength concrete - typically it is pneumatically applied; 2) then you always have this option for everything, regardless of what it is - ignore it, but it just gets worse.
In the case of window sills, remove the windows, replace the sills, the re-install NEW windows (the old windows may no longer meet required wind loads) or at least check to see if the old windows do meet the wind codes (but once you've got the old, corroded, salt spray eaten window out, why on earth put it back in?).
Were you able to go in the crawlspace? I'd guess there was spalling on the inside too.