Moisture Under Slab Condition 5

[zero1238]

I have a client with a home and a rear addition constructed upon a slab. The floor within the interior of the addition is hardwood and he is starting to get some minor cupping of the floor. When I looked over the outside of the slab foundation, there was some consistent hairline cracks which propagated along the mortar line of the block in close proximity to grade. I my mind, he may have some cracks within the slab that is allowing moisture to escape and crack the slab so my thought is to improve the grading and drainage along the outside, seal any cracks within the inside, and reset the floor (but allow for expansion of the floorboard by leaving a gap near the base molding. Are the any other recommendations for controlling the moisture. I believe he has a high water table there.[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1698421596/tips/IMG_1015_xgxxqt.heic[/url]

 

[structSU10]

So this is a slab no grade that is about 1' above the exterior grade? where is the cupping of the flooring occuring with relation to the exterior wall in the picture?

Another thing is there may not be a vapor barrier below the slab.

 

[JAE]

Odds are the moisture is coming UP from below the slab-on-grade vs. laterally through those cracks, unless there's been some significant pseudo-flooding around the house.

Agree with sttructSU10 that the condition of the underslab vapor barrier might be a cause/concern here.

Typically, a testing lab can do in-situ tests to determine the magnitude of moisture coming up from below. This might involve temporarily removing some flooring to gain access to the concrete slab.

Also see if there are any original drawings of the addition, or interview the builder, to see if a vapor barrier was used.

 

[zero1238]

That is correct, the slab is about a foot above grade but strangely enough, there isn’t much cupping of the floor occurring near that horizontal crack. The cupping is minimal and actually occurring more at the location where the addition is married to the original footprint; there is also a small vertical crack visible at this location from the outside. I’d be willing to bet there is no vapor barrier under the slab but we can’t determine that for sure because we can’t get a copy of the plan. One of my other questions is, what would cause horizontal cracks in the slab like that? It has to be moisture working it’s way out, no?

 

[dik]

Concrete will 'wick' moisture up for several feet. There should have been a moisture barrier at the U/S of the slab...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[lexpatrie]

Lack of a vapor barrier on the bottom of the slab, or a sand cushion underneath (to sort of informally prevent capillary action) should be considered/ruled out.

In new construction it's hypothetically possible the concrete is "too wet" for the flooring causing it to fail, but it seems less likely.

https://www.astm.org/f2170-19a.html

 

[JAE]

I had a geotech friend of mine once state that once you place a building over earth, the moisture content of the soils underneath slowly gets larger over time.

 

[zero1238]

That makes sense. I guess the first order of business is to check to see if there is a vapor barrier. Since there is very minimal moisture, I’m wondering if I don’t just have him seal the possible cracks in the slab to prevent the focused areas of escaping moisture. I can’t see the payoff of ripping up the entire slab and replacing it with a vapor barrier being worth it; it’s really only primarily in one location and it’s not near that horizontal crack along the exterior.

 

[dik]

I would not rely on sand to act as a barrier against capillary rise... nothing beats PEVB...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[XR250]

You can put a vapor barrier on top of the slab and do a floating floor. Ceramic tile also works well in these situations.

 

[zero1238]

What is PEVB?

 

[LittleWheels]

PolyEthylene Vapour Barrier

 

[dik]

Thanks... LW...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[southard2]

There are pockets of expansive clays in Texas and in North Central Florida. Those can really mess with slabs and even water tables. I was going to suggest a moisture meter but I'm honestly not sure those things really work worth a darn. When I built my house I placed a 15 mill vapor barrier under the slab. Poured the slab. I was an owner builder, so it took my like a year and a half to build the house. So over a year after the slab was poured home depot installer says he can't install the cheap vinyl plank flooring we were putting in. Said it had to have a barrier because the moisture reading on the slab was too high. So we put in the barrier and had to hire a third party to do it. The HD guy would not do it until the moisture reading was low enough.

So I got my own moisture meter and it also said the moisture content was high. Maybe it was, maybe it wasn't. I had read also that WWF in the slab can mess up the readings. Either way I really don't know if the moisture meter was working correctly or not. What I do know is never had any problems with my flooring even though the readings I was getting on my own were high.

HERE IS ANOTHER IDEA

WHAT I CAN tell you is that the air conditioning in the house had not been on for very long. In fact maybe only a week or two. Before the floor was to be put down I noticed some puddles on the slab in the bedroom area and several in the garage. (probably before I turned on the AC) OMG, I thought I had a roof leak. But I could never find a leak. Then I did an experiment. I placed a piece of plastic over the areas getting wet. Voila the next day I come and there is water under the plastic but not on top of the plastic. So it is not a roof leak. And it was quite a bit of water let me tell you. Enough to slip and fall on your butt. And in one case looked like someone had poured water on the slab.

But I have a freaking heavy duty vapor barrier how is the water getting in?

I found online that there is a condition called sweating slab. Basically it is condensation on the top surface of the slab that forms when warm moist air deposits water on the cool slab. Like condensation on the outside of your glass of water. It is more likely to happen in humid climates like where I'm at in the South cause the dew point is so high. So often you'll have cool conditions in the spring and then a warm from moves in with warmer air and humidity and voila you have puddles of water on your slab at the low points. Even sometimes I think it can just occur from cool night air and then a quick warming of the outside air while the slab is still shaded.

Either way this might have been why the moisture readings were bad. Perhaps the AC was required to really dry out the very top surface of the slab.

And it could be a source of cupping I suppose IF the the HOME ADDITION area isn't properly dehumidified by the AC system. I'd think the flooring might insulate the slab from that sort of thing but maybe not. Again think cool slab with warmer moist air and maybe that part of the slab with the cupping is also a low spot. I've seen plenty of additions where the AC system was never modified to blow in cool air or suck any air out of a room to return it to the AC. If the room feels muggy maybe it isn't getting dehumidified. Then add a cool slab. Just a speculative thought.

John Southard, M.S., P.E.

https://www.pdhlibrary.com/

 

[cliff234]

In my experience a vapor barrier (as opposed to a vapor retarder) is required under all slabs on grade located in interior conditioned spaces. (There are different ASTM numbers for vapor barriers and vapor retarders. The terms should not be used interchangeably.)

The purpose of the "sand layer" is not to inhibit moisture infiltration through the slab - it's to provide a smooth surface to allow the slab to slide slightly and inhibit shrinkage cracks.

 

[JAE]

Well not ALL interior slabs require a vapor barrier or retarder. Many industrial slabs do not have one for several reasons:
1. There's adequate interior heating and A/C to eliminate any surface moisture effects.
2. Typically industrial slabs do not have floor coverings sensitive to moisture,
3. Barriers/retarders under slabs can increase the risk of slab curling and subsequent cracking under fork lift wheel loads.
4. Humans don't occupy the spaces in the same sense of an office building or residence so moisture in the concrete isn't an issue.

We never ever use sand. Horrible material to use under a slab as it wicks moisture like a sponge and when the concrete is placed, the mudders walking on it move it around such that you have an irregular slab thickness.