Expert: Bruce E. Johnson - 5/19/2011

Question
Bruce,

Recently, we got a new concrete in the backyard. I's been a month, we have watered it as required.

In some places on concrete there are dark spots that don't seem to disappear.

What would you recommend to do to remedy that?

Thanks for your time.

BK

Answer
Hi B, gosh the dark spots could be anything but I am thinking that they are probably from water retention.  If you have been keeping the slab moist for long periods of time some parts of the concrete may be retaining water longer than other parts.  Let it dry out completely and see if the dark spots go away.  If there was a sealer or curing compound put on the slab right after it was finished, it's possible that the sealer was put on inconsistently which will cause temporary discoloration until the sealer dissipates which could take several months.  Also, concrete takes 28 days to cure to reach design strength (actually only seven days or less in some cases) but this doesn't mean that it is completely cured aver a month.  It will continue to cure and strengthen and harden for a long time.  It will also take a while to achieve uniform color in some cases.  There might be other factors involved such as dirt or leaves or minerals in the water used during the curing process that leeched into the concrete and stained it. Sometimes additives like "high early" can puddle up on the surface and cause discoloration for a period of time.  If it is a stain it may not come out without action on your part such as a muriatic acid or other commercial brands of concrete cleaning solutions. My advice? Leave it alone for a while and let it bleach out on it's own.  If after the summer is over you still have these spots then you can address the cleaning issue or just stain the entire slab to make it a uniform color.  I haven't seen what these dark areas actually look like so at this point in time that's the best I can do as far as advice goes.  If you'd like to take some photos and forward them along I will be happy to take a look and see if anything else comes to mind.  Sincerely Bruce E Johnson..bejohnsonconsulting.com