We were hand mixing 25 kg bags of redi-mix concrete in a wheel barrow for our fence posts however our last hole ended up only half full (4 bags only filled 18" of 36" hole) when we ran out of concrete.
Can another few bags (will be mixed first in wheel barrow) be added on top after applying some simple concrete bond to the top of the cured concrete in the hole?
The other option would be to simply backfill the remaining 18" space and rely on the bottom 18" in concrete to properly stabilize the post.
I would say yes, that would be fine. Maybe set some rebar or something similar around the post so it has something other than glue to stick to. Or if it hasn't set up yet, rough up the surface so it has some grabbbing points and not a smooth surface (if your concrete isn't going to be exposed, otherwise it will look funny.
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