Slab damp spot

[USAFpj]

Got a 30 day old slab, 4in thick, under cover. No rain, 92 degrees with 42% humidity. Something I have not seen before, but a 12in damp spot in the middle of the slab. And when I say damp, basically a discolored portion that eventually dried later in the afternoon. The concrete was poured over a 10mil vapor barrier.

Can someone explain how only that one area collected the moisture from either condensation or humid air?

I've kept the front and back overhead door open for the last 30 days for air flow. Now that we're in the Summer months in the SE, it may be time to close them doors to keep the humid air out?

 

[willymakeit]

Hole in vapor barrier?

 

[DougWil]

I hope you have been watering down that slab for the last 30 days or it had a curing agent.

^In which case how would you notice a damp spot?

 

[dadsEH]

Dont have cat do ya ?

 

[Cyberbear]

Plastic moisture barrier may be collecting slab moisture which is collecting in the middle. Or, just wait and see before taking major steps.

 

[Leaflessshadetree]

After 30 days I'd stop worrying about the "cure" or any special treatment.

Actually I don't worry too much after the first day but I try and slow the curing as much as possible keep it wet/shaded for a few days and don't drive on it for a week.

 

[DougWil]

After 30 days I'd stop worrying about the "cure" or any special treatment.

Actually I don't worry too much after the first day but I try and slow the curing as much as possible keep it wet/shaded for a few days and don't drive on it for a week.

Concrete requires water to harden and cure. If it isn't kept damp, hardening and curing stop.
The curing will restart if later dampened..... even years later.

However, not keeping the initial slab damp for at least 28 days won't bring it up to design strength and worse causes slab curling.

 

[USAFpj]

Guys, I spent a lot of time researching this slab before I poured. And I also did some things different than perhaps what should have been done. I was overseas at the time, and wanted to get it complete by the time I returned.

I had the wife keep it wet for 2 weeks, but that lasted 3 days- guys, you know how that goes! It has been kept dry and covered by the completed pole barn. The doors were left open after 2 weeks, and the slab was cut in 10x10 grids.

So far, there has been no evidence of flaking, cracks, curling, etc. No wet spot this morning. I reckon only time will tell if not using rebar, 4in with fiber, and the lack of a wet cure was a mistake.

 

[Leaflessshadetree]

Doesn't sound like anything that will be an issue.
The first few days make the biggest impact and since it's indoors I assume it isn't being baked with sunlight.
Seriously I hear a lot of guys talk about keeping the slab wet for 28 days. Don't think I've ever seen it happen. I used to see them covered in plastic for a few days but now most contractors apply a curing sealer right after it's troweled, cut the grooves the next day and never return. At least around here.

 

[SarcasticDwarf]

Regarding the effect of three vs 28 days of water for curing, see this article from PCA.