Rain's effect on fresh concrete - Troubleshooting - Brief Article

Concrete Producer, The, Nov, 2003

Q. About 3 hours after our contractors poured a concrete patio, it began to rain and continued to rain for 2 hours. How will that compromise the integrity of the concrete with respect to strength, durability, and so on over the long term?

A. You ask a very good question. To provide a more definitive answer, we would need to know more about the concrete mix and how it was finished, as well as whether it was covered or not.

The biggest problem you may face is the rain's effect on the concrete's surface. If it was not covered before the rain, and if the hardening was not very far along by that time, and if there was enough rain falling with enough force, it may have washed some of the cement out of the concrete. You would be left with a weakened surface and possible future problems. These might include surface dusting, increased water absorption due to an unsealed surface, and a reduced ability of the slab to resist freeze-thaw cycle cracking.

But the most serious consequence could be surface scaling, especially 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, try getting a one-year warranty on the work from the contractor who installed it. If problems arise resulting from the heavy rain, you would be covered.

But once the concrete sets and has been finished, keeping water on it during the hydration period is beneficial. For that, you can't have too much water. The only downside is that if ponding occurs on the concrete surface, it might result in some discoloration. Beyond that, its strength and durability should nor be adversely affected by excess water.