Relative Humidity in Concrete Slabs and PH

[StructuralEngGuy]

Hello All,

I have a project in construction with a slab on grade that is ready to receive carpet and other finish materials. The contractor tested the relative humidy and PH values of the slab, and they were higher than those recommended by the manufacturer. The manufacturer will not honor the warranty if their product is installed on the slabs as-is. Is there any way to somehow reduce the RH and PH values in the slab? This is the first time i've heard of this kind of issue, so any help would be appreciated.

Thanks!

 

[Ron]

There are several things you can do; however, these depend on the type of floor coverings that will be used.

For carpet, neither the pH nor the relative humidity in the slab are significant. Neither will cause much if any issue with carpet.

For barrier floor coverings such as vinyl tile, the moisture and pH are significant factors, primarily because of the latex adhesives that are used. For these, the relative humidity would preferably be below 75%, although up to 85% will work. If you are using the typical calcium chloride dessicant test, the values should be below 3 lbs/1000sf/24hours; although I have had successful installations at up to 7 lbs/1000sf/24hours. As for the pH, the latex adhesives are similar to latex paints and high pH can "burn" these materials resulting in saponification, a type of reversion of the latex materials.

For barrier floor coverings such as ceramic tile that use a cementitious adhesive or mortar bed, moisture and pH are not significant issues.

If the floor covering is a wood material or composite, the moisture issue is significant and should be kept low as in the vinyl tile. The pH is not an issue.

For floor sealers such as calcium or sodium silicate compounds, both moisture and pH sensitivity are possible. These sealers/hardeners are usually applied to hard-troweled concrete and when high moisture is available, there is a tendency to have a white haze or residue develop which is essentially a precipitate of soda ash or hydrated lime. To open the hard troweled surface a bit to prevent such problems, the surface of the floor is etched with an acid wash. I've successfully used muriatic and citric acid for these applications.

An acid wash can be used to reduce the pH where needed for other applications as well. Citric acid is very controllable and not noxious.

 

[hokie66]

You can reduce the relative humidity by drying the slab. I don't know about the methods, but imagine it would involve fans, maybe heating. A lot depends on how the surface was finished, e.g. a hard trowelled surface will be more difficult in terms of releasing moisture. If the manufacturer has these specifications, he should have some suggestions as to how to achieve the humidity required. By the way, what are the RH readings?

As to pH, concrete is supposed to be alkaline, and the higher pH the better. Nothing you can do about that, except wait for years for the carbonation process to develop.

 

[Ron]

hokie66....there are several ways to remove enough moisture from the slab that it doesn't affect the floor coverings or adhesives (presuming there is no recharge of slab moisture from below). We have successfully used granular and powdered dessicants spread onto the floor, dehumidifiers, fans and playing with HVAC controls to force condensation to occur at the top of the slab rather than deeper in the slab (not as effective as the other methods, particularly for large slab areas).

As for the pH, it is only necessary to reduce the surface pH (usually no more than 1/8" to 1/4" depth). This can effectively and easily be done with a light acid wash using, preferably, citric acid. This reduces the pH to adhesive or coating tolerable levels, and it opens the pore texture of the surface to provide better adhesion and better evaporation of slab moisture. The pH reduction process should be done before the slab drying process.

 

[hokie66]

Thanks, Ron. My post crossed with your first one, but thanks for setting me straight on the issues I raised. I have never actually had to answer these questions (I know, that sounds strange, as I have played with concrete for a long time).

 

[dik]

I assume you have a vapour retarder between the soil and the concrete.

Dik

 

[StructuralEngGuy]

Thanks Ron and Hokie for your responses! The RH is currently between 92% and 96% and the PH is between 11 and 12. The manufacturer requires the RH to be between 75% and 85% and the PH between 7 and 11 to honor their warranty. We do have a vapor retarder beneath the slab. I will suggest some of your ideas to reduce the values. Thanks again!