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High moisture content in concrete slab on grade
5

High moisture content in concrete slab on grade

High moisture content in concrete slab on grade

(OP)
I was wondering if anyone has come across this before -
A new concrete slab on grade has been placed about 5 months ago in a new building. They want to apply an epoxy to the floor, but tests indicate that there is too much moisture in the concrete. A vapor barrier was placed below the slab and there was 6" of granular material below that.
I haven't come across this issue before and was curious to hear anyone else who has. My first thought was that it is related to condensation / humidity, but I'm assuming they have already considered that.

EIT
www.HowToEngineer.com

RE: High moisture content in concrete slab on grade

I do not know the solution, but I have heard of this happening before.

Back when I built residential homes, I would hear of problems with glue down wood floor installations every now and then. The flooring company would show up and check the moisture content of a slab on grade; and then either refuse to install or attempt to get the construction manager to sign off on installing with high moisture conditions.

Unfortunately, I don't know the outcomes. I'm interested to see what others have to say though.

RE: High moisture content in concrete slab on grade

Not uncommon. To be clear your section is slab, vapor barrier and then the 6" of granular? You don't have the granular material between the slab and vapor barrier correct?

There is no real solution other than to wait. The contractor can place fans in the building to move air or if possible turn the HVAC system on to "dry" the air inside the building. Lots of factors affect how fast a slab dries; slab thickness, environmental conditions, and whether the slab has been exposed to water after placement.

RE: High moisture content in concrete slab on grade

(OP)
Thanks for the responses.
Rabbit - Correct, Slab - vapor barrier - granular

Are you thinking this is more related to the humidity in the structure than the moisture of the slab? These seem like two different issues, right?

It seems like there are three possible situations:
  1. Moisture coming from below the slab
  2. Moisture in the slab that will leave due to the concrete curing
  3. Moisture which forms on the concrete due to condensation
I'm not sure how they are all related or which might be the issue.

EIT
www.HowToEngineer.com

RE: High moisture content in concrete slab on grade

2
The moisture you are referring to is water vapor that is present in all slabs. It's a product of the hydration required to create concrete. Every slab has it. In order to install a vinyl floor it needs to be at a certain level or it can affect the adhesive used to attach the flooring. It takes time to allow that vapor to exit the slab. Since we place a vapor barrier it all needs to come out the top because we have an impermeable membrane at the bottom of the slab.

If you have a building that has such high humidity that you have condensation on the slab surface you can't expect the water vapor to exit the slab...at least not quickly. Like I said in my first post fans or starting the HVAC system to speed up the drying process is your best bet.

RE: High moisture content in concrete slab on grade

2
@RFreund....the vapor in the floor slab mostly comes from excess moisture in the concrete when it was placed. Do two tests to determine the point where you can put a sealing type coating on the floor (epoxy is one that "seals" the floor..)

The tests are in situ relative humidity of the slab....generally, the slab should have an internal relative humidity of below 85% for placement of epoxy coatings. Some manufacturers require 75%, and...

A dessicant test can be run concurrently and is done with calcium chloride. Both tests should be done for a minimum period of 60 to 72 hours

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RE: High moisture content in concrete slab on grade

RFreund. My opinion of your three possible factors is that they are all wrong, or close to that. Ya put water in the mix to get it to the forms, etc. and you need it to get subsequent hardening also. However, the amount needed for hardening is much less than needed for placement.. The rest will evaporate eventually, leaving a little that likely stays, per Ron. When it comes to evaporation, the rules for that process may not be so nice for your area, so it takes more time..
There is another factor that perhaps Ron will comment on. When water in the concrete moves up to the surface to evaporate, it brings along dissolved salts that can come from the cement and the aggregates and even possibly the water used. Those salts don't leave with the evaporating water, but remain in the top portion of the slab. Then, with an imbalance of salt concentration where the top portion has higher salt concentration than the lower parts, that imbalance causes water in the slab to migrate to the near surface zone of high salts content. I know of two cases where the surface coating (gym floor covering) came loose due to water pressure near the top section of the slab actually pushed the covering up with bubbles. Installing glass tubes sealed in the top section of concrete water rose in those tubes demonstrating a water pressure there. In these cases, the covering was placed too early before sufficient water has evaporated. Correcting the situation involved removing the covering and some of the concrete high in salt concentration, placing a new surface zone of concrete, and finally the new covering after sufficient drying of water in the slab.

RE: High moisture content in concrete slab on grade

(OP)
This all seems to make sense. Thanks for the replies.

EIT
www.HowToEngineer.com

RE: High moisture content in concrete slab on grade

1 month per inch of thickness is a rule of thumb, so after 5 months, you may be right on track

RE: High moisture content in concrete slab on grade

You've probably heard that experience is the best teacher....OG shows us that once again. He is exactly right about bulk water and vapor migration upward in a concrete slab. Water vapor will move toward a drier area. The bottom of the slab usually has a higher moisture content than the top so vapor will move within the interstitial pores of the concrete toward the top. As it does so initially, the vapor will condense near the top when the slab is cool and the dissolved calcium and other products of both the cement and aggregate will be deposited near the top as evaporation takes place. In exterior wall where wetting and drying take place multiple times, this results in efflorescence. In floor slabs it is not so obvious and usually the whitish residue doesn't show at the surface because of how slowly this occurs.

But there's moisture in the concrete and this accumulation of salts creates a drier area and more moisture will migrate upward.

As a quicky test, tape down a 1 foot square of clear plastic. If you have moisture collect under the plastic, it is too damp to put a coating on it.

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RE: High moisture content in concrete slab on grade

The above information all sounds well grounded and sound, but I just wanted to add one thought: could there be moisture ingress at the perimeter of the slab? For example, could the slab be wicking moisture from the surroundings at its outside edge?

RE: High moisture content in concrete slab on grade

While on the subject of water in concrete here is something to think about. Early "finishing" as with a trowel will tend to bring water up and let aggregate tend to settle or densify to some extent especially at the top. This results in the top section with high water to cement ratio, tending to result in weaker concrete there. A common result is a subsequent loss of upper zones with a scaling effect perhaps a year later. In these cases of early over working a common remark of the workers is "we really tried to make a smooth surface and spent more time on that". However less troweling and waiting for a harder consistency is much nicer to the concrete in that respect.

RE: High moisture content in concrete slab on grade

Craig_H I would think that would only affect a small strip along the edge if at all. Certainly don't see it affecting the middle of the slab.

RE: High moisture content in concrete slab on grade

This is a common issue in concrete roof slabs where adhered roof membranes will start to blister or bubbled from vapor from slab if the metal deck is not vented. It's a common issue. Only option is to wait it out

RE: High moisture content in concrete slab on grade

I've run into this issue on multiple projects where it caused significant project delays and conflict within the project team. In my opinion, the best solution that I have seen is an additive to the concrete mix that effectively closes the interstitial pores that Ron mentioned during the curing process. I've only worked with one manufacturer, but to my knowledge there haven't been any flooring adhesive issues on any of the projects that used this additive. I have no affiliation with this manufacturer, and won't list the name here. I would think a Google search would turn them up fairly quickly.

As an aside, this issue has only become an issue with the switch to non-VOC adhesives in the flooring industry for environmental reasons. The moisture has always been in the concrete, but previous adhesives (with VOCs) weren't affected by it. The current adhesives (non-VOCs) are water-based, hence the adhesive failures and "new" issue of moisture in the concrete slabs.

RE: High moisture content in concrete slab on grade

birneys, you are right it's a new issue but it's been around for 10+ years. It really started to show up with the LEED craze between 2005-2010.

I worked a ton on this issue in my first job out of college. We had a special gradation to minimize shrinkage (and thus curling) and worked diligently with the contractor to make sure we kept the floor dry, clear of construction materials, and moved air across the floor (fans) almost constantly.

The admixture you are talking about must be somewhat new. Sounds like a good solution to an issue that is fairly difficult to solve because it's somewhat unpredictable.

RE: High moisture content in concrete slab on grade

This issue has been around a lot longer than 10 years! A lot of it has too do with an increasing tendency for ready mix suppliers to increase the water cement ratio of concrete. I have seen the general application concrete water cement ratios rise from 0.50 to almost 0.60 in the last 30 years. That comes from the tendency to consider only compressive strength in concrete and not durability. Compressive strength is easy to achieve....durability not so much.

Some admixtures help to decrease the interstitial voids in concrete such as silica fume and others. Most of the more effective treatments for moisture rise are done with sodium or calcium silicate hardeners that are placed after the floor is finished. There are several fairly good ones; however, I've seen some of those react with the rising water to create a soda ash residue on the surface of the concrete.

There is still no substitute for a properly designed and controlled concrete mix in solving these problems. As birneys notes, the low VOC adhesives have caused a lot of these issues due to re-emulsification when exposed to moisture.

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