Concrete Moisture Vapor Emission Rate (MVER) 3

[dirtdoctor]

I just finished perofrming MVER testing on a recently placed concrete slab. As many of you may know, with the increased use of water-based flooring adhesives, the MVER of a concrete slab has become increasingly important. The emission rates on the slab in question were much higher than allowed by the flooring manufacturer. I do not believe that the subgrade is causing the high readings, as it consists of an open-graded stone (no capillary rise will occur). Subgrade testing has also indicated dry conditions. The concrete is a fairly dense mix, which could cause longer to completely lose mixing water (water of convenience added during placement). The plant batch tickets also show a relatively low water/cement ratio - the placement slump was achieved by adding a superplasticizer on site.

Are there any ways to effectively reduce the vapor emission rate, i.e, 'opening' up the surface of the slab by sandblasting, etc. Any guidance would be greatly appreciated.

 

[Ron]

Dirtdoctor...you didn't give your MVER values, but I assume you are over 6 or 7 lbs/1000sf/24hr.

You're right. The water is not from the subgrade. It is interstitial water in the concrete that changes its condensing location with changes in temperature of the slab and the air over the slab.

Have you done a relative humidity test IN the slab? (See ASTM F2170) This method has, in my opinion, a greater tendency toward accuracy to get what you're looking for in the true amount of moisture that causes problems.

The calcium chloride test has some inherent inaccuracies. First, the calcium chloride is an area of extremely low humidity, so since humidity will migrate toward areas of low humidity, you are actually "pulling" moisture from the slab. For this reason, I believe it overstates the issue as I've seen many successful floor coverings go down at levels of 5 and 6 MVER. Next, you must weigh the calcium chloride reagent in the field to get an accurate weight. This involves using field scales and applying lab conditions to conditions in the field that are sometimes not even close to controlled! If you are working on an old floor that has floor coverings, you MUST scrape at least a 2'x2' area to run your test in. It has to be very clean or the first problem noted gets worse.

I have had success in the past by drying the floors using a dessicant spread across the surface after cleaning and preferably after blasting the surface with a blast-trac or other type of flooring grit blaster.

The dessicant can be oven dried sand (as fine as possible), calcium chloride pellets (expensive if large area), "DrySorb"...a commercial absorbant, or even drilling mud (dry of course!).

Clean the floors with several passes of vacuuming. Don't use water to clean. You may wipe down with a mop dipped in acetone or other high flash solvent that won't leave a residue.

 

[zman44]

We are looking at options for installing (?) a floor coating (epoxy) over a recently placed slab. One product which has been recommended is a Koester product which binds with and blocks a large percentage of water vapor. This material is purported to reduce the moisture vapor level to levels which the epoxy floor coating can handle. It is reported that this product can reduce levels from the 20# range to the single digits. Does anybody have any experience with Koester?

 

[dik]

Further to Ron's comments... I usually use 6mil poly vapour retarder and don't rely on coarse aggregate to provide the elimination of moisture from the U/S of the slab...

Dik

 

[johnnyceramic]

ceramic john

i"ve used for years on epoxy floor installations and glued floor laminates a product form international chemcrete they call it sulfix 700 it has done a great
job
best of luck Ceramic john

 

[golf39]

See Structural Thread 507-179186 Concrete Cover w/Vapor Barrier. Here are some references that address this issue.

http://www.post-tensioning.org/pdfs/sog-faq3.pdf

http://www.nrmca

Concrete In Practice CIP29, Vapor Retarders Under Slab on Grade.
ACI recommends vapor retarders for all slabs which recive moisture sensative flooring, which to me includes most carpets, VCT, etc. The first two references recommend vapor barriers be installed for all interior slabs on grade, which I concur with.
Hope This Helps.