The gap, no more that 1/4 inch, is between a concrete sidewalk and
vertical metal walls surrounding steel metal doors that cover stairs to
the basement. The concern is water getting in the gap, then freezing in
the cold New Jersey winters, expanding and damaging the concrete.

I couldn't find any caulk in Home Depot that specified both concrete and
metal. That may be just because the situation isn't that common, and one
of the caulks may very will do the job without having to be replaced
every couple of years.

Thanks,

Ray