Unless it's likely to rain, there's no need to cover freshly poured concrete to get a perfect finished product. If rain is likely, leave a plastic cover over it for 24 hours or until the threat of rain has passed.
Cover It Up
Covering fresh concrete with a plastic sheet has other benefits besides protecting it from rain. It can protect the new concrete from environmental cooling and heating fluctuations as it cures, and it helps resist uneven curing due to rapid moisture loss at the top. Leave it covered for 24 hours.
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Millions of yards of perfectly cured concrete are poured each year with no covering by experienced concrete workers. When covering is required because of environmental conditions, leaving it covered for 24 hours is adequate.