Q.: We had a concrete patio placed. About 3 hours after it was placed, it began to rain and continued to rain for 2 hours. Will that compromise the integrity of the concrete with respect to strength, durability, and so on, over the long term?
A.: To provide a more definitive answer, we would have to know a little more about the concrete mix, how it was finished, and whether it was covered or not.
The good news is that it sounds as if you had enough water during the initial period following concrete placement to not have to worry about the surface drying out. The bad news is that the surface of the concrete should not have been allowed to dry out for a week or more, a time also referred to as the curing period.
The biggest problem you may be faced with is the effect the rain may have had on the surface of the concrete. If it was not covered prior to the onset of the rain, if the hardening was not very far along by that time, and if there was enough rain falling with enough force, the rain may have washed some of the cement out of the concrete. You would be left with a weakened surface and the possibility of related problems in the future. They might include dusting of the surface, an unsealed surface that will allow far more water to be absorbed, and a reduced ability of the slab to resist cracking due to freeze-thaw cycles. But the most serious consequence could be surface scaling, particularly if it was a pounding rain. The result is a concrete surface somewhat like the flaky layers of a croissant. You will often see the scale right after a storm, but sometimes it is not visible until the slab has had some traffic on it, when the scale breaks or crumbles away. If you have not yet paid for your patio, this would be a reason to try getting a one-year warranty on the work from the contractor who installed it. If problems do arise resulting from surface problems left by the heavy rain, you would be covered.
On the other hand, once the concrete sets up and has been finished, keeping water on it during the hydration period is beneficial. For that purpose, you can’t have too much water. The only downside is that if ponding occurs, for example, on the surface of the concrete, it might result in some discoloration. Beyond that, its strength and durability should not be adversely affected by the presence of excess water.