How long for concrete to cure?

[Paul Conley]

I live in northern California and had a concrete driveway poured in September. It's almost February, and the driveway still has splotches of gray and white. Will it eventually turn all one color? If so, how long will it take?

Paul

[Joseph Meehan]

Concrete cures over years, but for all practical purposes, yours is cured. You may need a power wash to clean it up and make it look more even, or there could be other problems that are not so easy to fix.

[David Alexander]

Hi Paul,

First of all, wait until late summer to do anything. It's possible that warm weather may alleviate the problem.

The condition you're describing is called "Pinto" coloring. There are many theories about the cause of it, but it seems to be related to toweling of the surface with power trowels in combination with a lack of wet curing. This condition may be caused by trapped moisture just beneath a surface that was sealed with the trowel. I've seen this condition last for years if untreated.

To find out if the dark color is caused by trapped moisture, pour out a small amount of white vinegar on the dark area and put a chalk mark by it for later identification. Give it a few days of dry weather and check it. If the area turns a light color, then your problem is caused by trapped moisture that evaporated when the acid in the vinegar etched the surface and broke the seal.

If the vinegar works, its possible to treat the entire driveway in a similar fashion. After treatment, rinse thoroughly and allow it to dry, then spray the surface with a mixture of ammonia and water to neutralize the acid. I don't recommend any other sort of acid for this treatment because it's too easy to cause permanent damage to the concrete.

Since you live in an area subject to freeze-thaw conditions, you should seal the surface to prevent damage that can be caused by water that penetrates an unsealed surface and then freezes. Your local Sherwin-Williams or other paint store should have an adequate sealer that doesn't darken the surface. Reseal every few years.

Good luck,

David Alexander