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If I start pouring concrete today at 2PM while it's sunny, and it starts to rain heavily tomorrow at 2PM... and not stop raining for the next 5 days (this is the weather forecast), will my driveway be fine?
Is 20+ hours enough time for the concrete to harden and not get "dug up" by heavy rain?
No, it isn't a problem if...
You lay poly sheeting or a tarp over it after several hours of cure time (ideally the following morning). Driving rain could degrade the surface of the slab, which is still soft even after a day.
Water running along the slab won't undermine its base.
As long as your concrete sets up normally, it should be fine if it rains 20+ hours later. I've done it several times with no problems. It would only be a problem if there is something stopping it from setting up, like freezing weather.
