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Slab surface moisture issue in existing building
6

Slab surface moisture issue in existing building

Slab surface moisture issue in existing building

(OP)
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.

RE: Slab surface moisture issue in existing building

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.

RE: Slab surface moisture issue in existing building

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.

RE: Slab surface moisture issue in existing building

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.

RE: Slab surface moisture issue in existing building

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.

RE: Slab surface moisture issue in existing building

(OP)
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....

RE: Slab surface moisture issue in existing building

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



RE: Slab surface moisture issue in existing building

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.

RE: Slab surface moisture issue in existing building

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

RE: Slab surface moisture issue in existing building

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

RE: Slab surface moisture issue in existing building

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

RE: Slab surface moisture issue in existing building

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

RE: Slab surface moisture issue in existing building

(OP)
@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...

RE: Slab surface moisture issue in existing building

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 :)

RE: Slab surface moisture issue in existing building

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.

RE: Slab surface moisture issue in existing building

2
@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).

RE: Slab surface moisture issue in existing building

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!!

RE: Slab surface moisture issue in existing building

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

RE: Slab surface moisture issue in existing building

Quote (Arizona3106 ....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. ..... Raising or removal of the floor slab for an underfloor drain is not viable)


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

Quote (The commercial building was constructed in 1986. Exterior french drains installed about 8 years ago)


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...

RE: Slab surface moisture issue in existing building

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.

RE: Slab surface moisture issue in existing building

Quote:

Subsurface soils are typically medium dense to dense Clayey Sand with PI of 25-30

I don't think it is the case, but if this the soil under the slab, wouldn't be it cause problem?

RE: Slab surface moisture issue in existing building

(OP)
Foundation Plan may be useful (attached).

I should mention, the subsurface water here is not your typical groundwater table. It's more of a thin, perched aquifer that may be for example, located in a porous bed 6 feet below grade, prior to excavation of 0 to 10 feet.

As shown, top of concrete stems terminated at top of slab. Masonry walls above. That masonry/concrete interface is located at the problem elevation.

@Ron - That gradient testing of internal humidity is a great idea! How would you suggest the area be prepped to ensure results provide the desired info, i.e., is the issue from the bottom or the top?

RE: Slab surface moisture issue in existing building

I wonder is there a standard detail for preparation of the construction joint shown on foundation type 1 to 3. If untreated, it could be where the water gets in. I suggest to walk the building, and mapping the findings. Then you maybe able to connect the dots.

RE: Slab surface moisture issue in existing building

Below is a checklist of tasks that I would perform for troubleshooting.

1) Visual building inspection on building interior and exterior, and mapping damaged areas/spots.
2) Perform RH study on the slab per ASTM F2170.
3) Engage geotechnical engineer to categorize the backfill and determine water content.
4) Identify the source of ground water, through piping leakage testing, and groundwater/perched water probing.

Notes on RH probing:



Notes on water probing:

RE: Slab surface moisture issue in existing building

Water diviner here at an engineering site??? I've seen a few at work. Even I could find one, but when he is 30 feet wrong?? No need for mistakes here. We need a new rule here "you pay for space used". Only one post will come up. Ron's one sentence.

RE: Slab surface moisture issue in existing building

OG,

Please provide/impart pertinent methods of geotechnical subsoil exploration to help. I do enjoy you and Ron's ideas most of the time. The diviner thing was highlighted by the author, I left it there just for fun. Maybe I would try one day to verify the accuracy. Isn't that an engineer way (hands on) other than sitting on theories and calculations only?!

RE: Slab surface moisture issue in existing building

I for one know better than to advise a geotech on this site with how to do it. He seems well qualified. Mater of fact he now only needs a suitable answer on how to run the moisture test. I'd refer to what ever Ron says, since he has done these before. I've done some, but do not consider myself to be an expert. Once that comes in the post can be deleted. You don't see me advising structural guys even though my minor at Cornell grad school was structures.

RE: Slab surface moisture issue in existing building

Quote:

You don't see me advising structural guys

:) I've read quite a few of your responses/comments. Why not, if you know the topic, and the tricks sometimes.


RE: Slab surface moisture issue in existing building

Simple answer. Seeing my responses hopefully don't rile folks and result in them generally then not respecting my knowledge. It only takes one "goof" to develop a reputation that follows on all other posts..

RE: Slab surface moisture issue in existing building

(OP)
@oldestguy - Thank you for your insights!

@retired13 - Really appreciate the summary info on RH testing, visual survey, & damage mapping

@Ron - Would appreciate any suggestions you have on RH testing, based on the previous ones you've done, and that we only have a 4in slab. Never used it before, but is sounds like what I need to get done.
Will get a copy of F2170. I'm guessing one would remove areas of flooring & adhesive first in test zones. Would this likely require surface grinding/chipping to allow slab to reach equilibrium?. Wait at least 24hrs before drilling test hole & testing?

RE: Slab surface moisture issue in existing building

Arizona,

Suggest to explore a few articles on this webpage to get the basic on RH testing, Link.

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