28 days allows concrete to cure to around 90% of its strength.
If allowed to dry to quickly, the surface becomes "crazed" and can develop
lots of small surface cracks. This is not usually a problem with tower
installations since there is very little surface area with respect to the
concrete in hole.
I have poured quite a few bases in the hot Florida sun and have never had
one develop any cracks, even though I took no special measures to keep in wet.
My belief it is a non-issue.
I would however wait several weeks before erecting any tower after pouring
the base. Your choice.
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