Slab surface moisture issue in existing building 6

[Arizona3106]

Moisture buildup between tile and concrete floor slab (Photo attached). Shallow water conditions are known to exist in the area. Perimeter french drain was installed over 4 feet deep, however, issue now noted in central portion of building. I believe it's due to head pressure after removal of a portion of the ridge for construction of the building. Raising or removal of the floor slab for an underfloor drain is not viable. Any solutions or seal methods you're aware of that actually work well? Not too excited about about surface seals I've seen before, but maybe I just didn't see good ones...Thought I read about possibly waterproofing the underside with polyurethane, but don't recall.

 

[r13]

You need to lower the water table. I'll dig a hole and measure the seepage rate, to determine if a single sump pit will work, or need an underground water tank.

 

[Ron]

Don't immediately jump to the water table as the issue. That's rarely the case. The moisture content of the soil directly in contact with the slab below would have to be higher than the moisture content of the concrete for any significant vertical migration to occur...and that's assuming you have no vapor barrier which is also not likely.

Your photo shows that the latex based adhesive has re-emulsified and possibly is having a reaction known as saponification. You'll need to answer a few question that will help us guide you:

1. What is the structure? Residential? Commercial?
2. How large is the floor slab area and how thick is the slab?
3. What type of soil is below the slab?
4. When was the slab placed (how old is the concrete)?
5. Is the central portion where the problem occurs a replaced slab section?
6. When was the floor covering placed?
7. Was there a previous floor covering? If so, what kind?
8. Has the HVAC system been changed? If so, how?
9. Was the building previously unconditioned space?
10. Have you done any testing to check the vapor migration rate? There are a couple of tests you can run...one is a simple calcium chloride test while the other measures the relative humidity in the slab.

Please answer these questions and maybe we can work toward a solution.

 

[oldestguy]

I've seen this a few times. Each time the slab was on a "vapor barrier" and the surface covering was placed before the slab had a chance to dry significantly. If that is the case, remove the tile, let it dry and then take samples of the concrete to check for moisture. It likely is highest near the surface where drying left salts that also attract water. Might even have to remove an inch or two of concrete, to take the salts. Requires knowing the full picture to solve it. I'd not blame ground water without checking water in the slab. Follow Rons's advice.

 

[r13]

Ron's questions address the issue of a unvented space with south openings and moist subgrade during a humid season, back to home, we call the phenomenon sweating.

Here is another possibility. My front porch had the same problem with floor tile delaminating and trace of water, the root cause was found to be storm water seep through the rotten wood window frame, and worked its way to the low point of the slab.

 

[Arizona3106]

Really good thoughts here. I understand it's really everywhere, not a central issue. Per Ron's questions

The commercial building was constructed in 1986.
Exterior french drains installed about 8 years ago
Subsurface soils are typically medium dense to dense Clayey Sand with PI of 25-30
They used to have carpet. Some areas were replaced with carpet and some with vinyl.
18 months ago they installed the new flooring throughout after grinding and some type of surface sealing
Don't know about HVAC history, or concrete thickness (probably 4 inches).
calcium chloride tests info 8 and 6 years ago and showed some improvement after french drains installed, but were typically 2 to 8 (Don't think they prepped the surface well though - owners don't remember any grinding).

Issues with wavy floors were initially noted near the walls and then everywhere.

Attached are aerial with grading/topo plans....

 

[r13]

Depends on the soil type, you might have a water pond under the building. Would that be a concern?

 

[oldestguy]

Seems like some mapping of the ground water table is in order. Within the building would require some low head equipment to install observation wells. If there truly is a high groundwater table that perimeter drains cannot draw from, some wells with pumping may well have to be installed inside the building. For one job the perimeter drains didn't drain the interior and a trench actually had to be dug inside for under-drains in sand backfill. Alternative may be deep wells outside the building. However, without knowing the true ground water table, not much in the way of correction should be attempted. Check rainfall records and actual roof drainage piping outside. I've seen where underfloor piping for plumbing, etc drains run toward outside in sand filled trenches only to pick up roof drainage in the pipe trench backfill and feed that back under the building via the porous sand backfill. I'd look at this also. On a field drainage situation horizontal sub-drains can be installed in hillsides. Takes a casing first, then the perforated pipe surrounded by sand fill then withdraw the casing. Ain't easy. Might work under a building.

Edit: Before a rig inside for wells there try by hand, since water table may be near floor elevation.

 

[Arizona3106]

@Retired13 - Middle of the building is at least 10ft of cut... Chopped off a ridge. Fill in perimeter of three sides.

 

[r13]

Yeah, it wouldn't hold water after the surgery. I believe you have checked the condition of tuckpointing, and base seals.

 

[Arizona3106]

¯\_(ツ)_/¯ Probably haven't checked either, since I don't think I know what tuckpointing or base seals are

 

[oldestguy]

I'd guess this is a nursing home with lots of plumbing under floor. Any indicative of leaking pipes?

 

[Arizona3106]

@Oldestguy - It is, and they said they had that checked. Subsurface water in the area tends to be relatively thin, perched on dense, cemented material, and also flowing within thin layers.

My hypothesis was that the subsurface water was likely seeping from the cut made below the building when they removed the hilltop. It may be daylighting throughout the building area.

I'm also thinking about Retired13's comment and possible seepage through a cold joint at the masonry stem wall/footing interface. Especially since they mentioned it starting near the walls and moving inward.

Not sure the best way to rule those ideas in or out though...

 

[r13]

Do the simple inspection around building first, pay attention to cracks, missing, or loose mortar joints. Leaking service/drinking water pipes maybe checked by a pressure test (contact an experienced plumber); leaking waste water could be checked by a dye test. Just speaking loud though. This building must be well conditioned/ventilated, so it rules out the weather effect, also the hill... ruled out two already

 

[oldestguy]

If there are those porous beds, any info on strike and dip? Been a long time since I took geology courses. Thus collecting any seepage in porous layers ya gotta know a lot about layer orientation, etc. or ya miss the ones you need to tap. Also that plumbing makes it hard to do test borings and also any cure. Hand test boring work in mandatory or damage not wanted.

Edit: Likely no reasonable way to do exploration or well installation inside. Knowing the strike and dip of the area a deep well outside intercepting those layers and draining water is a thought. Would have to be pumped only when there is water. Again a fun thing.

 

[Ron]

@retired13....please stop throwing out technical terms that have no relevance to the issue. Tuckpointing and base seals have no bearing on this discussion....

@oldestguy...agree with you that more investigations of the soils and groundwater issues are warranted considering the soil classification. A core through the slab and an auger boring might provide a lot of info to help.

@Arizona...Lopping the top off a soil ridge and filling the sides should yield results opposite to what you are seeing, particularly given that the undisturbed soils in the middle will have a lower permeability than the fill soils on the sides. That should encourage lateral flow rather than vertical flow.

I would suggest at this point that you do testing of the internal relative humidity of the slab. ASTM F2170 is the relevant standard for the test. Many construction materials testing laboratories can do this test, though not all. It does require some specialty equipment and the capability to set the probe inserts properly. I have done many of these....but I'm all the way across the country from you! You can do this test in a gradient fashion to determine if your issue is in the bottom of the slab or the top of the slab. Bottom indications show groundwater influence. Top indications show HVAC influence. (In general).

 

[Ron]

Damn OG! You're good!! How did you nail this as a nursing home?

To all....OG has more knowledge in 30 seconds of his thinking than most of us have in an hour!!

 

[oldestguy]

Who else has a nice entry way and another at rear for service and hearse parking.

 

[HTURKAK]

Will you pls provide some details regarding the structural system, SOG, and architectural plan to see the partitions?


That means the problem was existing before french drains which were installed eight years ago.

I did not read the previous responds in detail ..just screened them. My opinion is ,the problem due to the capillary rise of the shallow GW which is fed by; surface runoff, roof drains, irrigation, leaking interior plumbing and leaking 6" ACP ring.

I am retired eng. in past, i always provided 8-10" coarse aggregate below the SOG and raft foundations to break capillary rise. If I were in your shoes, I will think three options;

1- Break and remove the SOG, change the old existing piping below and provide new SOG with ( 8" coarse agg.subbase+ PE fim+ heat insulation+ PE+ RC 4" slab.

2- Raise the slab with 25 mm dense rockwool+ 60 mm reinforced floating screed,

3- With some pilot excavations ,make sure the french drains are working and good condition. If so, search for leaking plumbing below SOG. If you see the french drains are not working, renew them with ( perforated pipe encased in coarse gravel ) and try to drain with gravity to the sewer system.

...and i will try to choose variant one..

Kind regards...

 

[oldestguy]

Should have read this is a nursing home and you aren't going to remove the slab and start over. I also see the capillary rise people mentioned, but when actual drains help (perimeter) it is not only a capillary rise thing. Lowering the water table under the building is the question. All those pipes, etc, means no trenching inside. Deep well MAY do it, but needs careful site evaluation first.