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New slab moisture readings before epoxy is installed over — there is a vapor barrier under

Jamie_K | Posted in General Questions on
We were getting ready to put down our epoxy finished flooring; however, in order to double check our slab (new build/construction) was dry enough we performed some calcium carbonate tests and our moisture reads coming up through slab were consistently about 4lbs per 24 hours.  In talking with some epoxy people in the industry, they all said that is high and would expect it to be below 3lbs as it was poured 6 months ago. Our thoughts are either:  vapor barrier since it is so effective is taking longer to dry upwards than typical slabs with lesser or no barrier under slab, or possibly wicking in from a little 3-5″ wicking up sides but it’s really not much and reads were in middle of 40’x60′ shop.
Slab details:  plenty sufficient grade sloped away with gravel up against (which has few inches of wicking with heavy rain/snow but nothing visual seen coming in through slab or stem wall into interior) > 5″ gravel as cap break > Viper 2 (10 mil) commercial grade vapor barrier with seams taped well > 4 to 5″ thick slab poured 5 sac with mesh concrete late July 2018
Question:  Do you know or know who would know the following (as the epoxy guys know epoxy adhesion wise but not moisture entrapment / mold prevention / etc)?
  • Any thoughts if there are any other moisture concerns we are not thinking of where these reads should be concerning?
  • It is possible to primer slab & epoxy just fine (flooring adherence wise).  My question that no one seems to know thus far is how much is too much moisture to sandwich in (lock in) between a good vapor impermeable barrier below and epoxy surface impermeable barrier above?
  • Will it:  1) dry out sides between sandwiched in 40×60 slab, 2) stay where it is and with minimal acceptable moisture level sandwiched staying at that level is fine, or 3) at/above X amount of moisture you don’t want to sandwich it in because will promote mold growth?

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  1. Expert Member
    Peter Yost | | #1

    HI Jamie -

    If your slab is 6 months or more old, that transmission rate is high. I would be sure to test your slab in more than one location and use ASTM F2170 (Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes).


    1. Jamie_K | | #2

      Peter, thank you for responding. What we are hearing though is most surface moisture tests come back below 5 lbs/24hrs & rarely come in below 3lbs/24hrs. Also, that it depends a lot on denseness of concrete factors: ours being a 5 sack with fiber mesh 4-5" thick will by nature read higher. We did do 3 locations and all came back 4.11, 4.13, 4.15 in same relative range. We are going to proceed doing RH with probes done in a few locations done this afternoon or Mon. We know we can get epoxy to adhere, that's not what we are questioning.

      - We are wondering what is an acceptable amount of concrete moisture to sandwich in between two impermeable layers (vapor barrier below & epoxy above sandwiched in concrete). Surface moisture below 5lbs/24 hrs (per one ASTM test) as well as what type of RH read (per other ASTM method)? RH at or below 90% should be dry enough or need to be lower?

      - Granted there should be no mold growth risk on underside of epoxy coating because may always be moisture in concrete but there should be no oxygen or food source correct?

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