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If the air temp when I'm pouring concrete (small staircase) is over 90°F, if I cover the surface with (4 mil?) vapor barrier after finishing, will that be effective to slow down drying?

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Yes, but it could also mess up the surface finish.

Instead, consider dampening the concrete with water to keep it moist and cool. A garden sprinkler applied periodically will be ideal.

Another option is to do the pour very early in the morning when it is still cooler. By 10 or 11 AM you should be done.

Also a sunshade/tent fly over the concrete will help cool it and extend the curing time.

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    What's more common than a vapor barrier would be damp cloth material across the top of the concrete that's kept wetted -- you want to start off with as dry a concrete as you can then infuse the water in slowly to let the chemistry work just-so Commented Aug 24, 2021 at 23:03
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    The concrete uses water to cure , so adding water in some way is better than sealing the surface. ( It also absorbs CO2 , slowly). Commented Aug 25, 2021 at 0:29
  • Do you think this sprinkling tactic will work when it's 90°F out?
    – amphibient
    Commented Aug 27, 2021 at 15:02
  • @amphibient yes. 90F / 32 degrees C is a nice warm day and would be fine. Start early in the day helps tool.
    – Criggie
    Commented Aug 27, 2021 at 20:24
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No, because concrete "sets" like an epoxy, rather than drying out like paint, hence why it is possible for concrete to set even when it is fully submerged under water.

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