Rain hitting wet forms is the situation that matters
If you are asking this question because clouds are building over an open pour, the honest answer is that we do not start into steady rain, and we stop and cover a slab if rain moves in while the crew is still working the surface. Concrete that is still plastic, meaning it has not begun to firm up, cannot defend itself against water landing on it.
That is different from asking whether rain in the forecast rules out a whole week. It does not. What matters is whether rain is going to be falling while the surface is still open and being worked, not whether a shower passes through sometime in the same seven days.
What rain does to concrete that has not set yet
Fresh concrete already has water rising to the surface as part of curing, called bleed water, and finishers wait for that water to leave before they trowel or broom the surface. Rain adds more water on top of that process at the worst possible time, and the surface ends up with too much water relative to cement right where it matters most.
The result is a weak, dusty top layer, sometimes pitted where drops hit hard, and in a bad case the surface can wash away enough to expose the aggregate underneath before anyone meant it to show. None of that is something you fix with a broom later. It has to be caught while the crew is still on site.
Why we watch the forecast before we book the date
We pour most of the year here and work around the forecast rather than the calendar, and rain is the single biggest reason a pour date moves. If steady rain is expected on your scheduled day, we call ahead of time and get you a new one rather than showing up and hoping it holds off.
Once concrete is placed and the crew is finishing it, the clock is running and there is no pausing to wait out a passing cell. That is why the decision gets made the night before or the morning of, based on the real radar, not left to chance on site.
Rain after the surface has firmed up
Once a slab has finished its initial set, generally by the day after the pour, rain landing on it is a much smaller event. The surface can shed water at that point rather than absorb the impact of it, and a normal rain shower does little more than get the top wet on concrete that is already curing underneath.
- Rain on open, unfinished concrete: real risk, the crew stops and protects it
- Rain right after finishing, same day: worth covering if it is heavy
- Rain the next day and after: minor, curing continues underneath