[Shop-talk] Testing for moisture in a concrete floor

[Lee Daniels]

Jim - note that concrete does not actually ever "dry". It cures - over months
and years. The water becomes part of the structure, and it's important that
there's *enough* water. If you pour a driveway in a sunny hot dry climate,
it's important to spray water over it every day for a week or two so that
there's enough H2O to complete the curing.

Note that once included in the structure, it's no longer in the form of "H2O",
but is part of a silicate structure.

Here's a pretty good YouTube description of how it works:
 https://youtu.be/2qegAzbrI1U

I'm not sure if any of this helps, but what you really want is to give the
structure enough time to reach a reasonable equilibrium with its environment.
Since the bottom of your slab has access to plenty of moisture, and it's
slightly porous, this seems to be no problem.

 - Lee

On Sun, May 21, 2017 20:43, Jim Stone wrote:
> Are there any chemists here?  Or experts in testing for moisture in concrete?
> I have a fairly technical question re: testing for moisture with calcium
> chloride tests.  I can supply much more detail if needed, but here is the
> issue in as small a nutshell as I can make it. I am doing the test prior to
> painting my new garage floor.  The garage is located on Eastern Long Island,
> so the climate is fairly temperate, but we are also surrounded by salt water.
>  The garage floor was poured at the end of December, so I am well past the 90
> days that is usually recommended before painting. The floor is plumbed for
> radiant heat, with insulation between the concrete and the ground and around
> the perimeter.  The heater was installed the beginning of April and the floor
> has been maintained around 60-65 degrees since then. As I noted in response to
> someone else's recent question, after researching the topic I am planning
> on using Polyurea from GarageFlooringLLC.com <http://garageflooringllc.com/>
> On paper, there shouldn't be any reason not to go ahead and paint the floor
> but I don't want to waste the time and money involved and decided to do a
> calcium chloride test to be sure.  I did the test in the recommended three
> spots a week ago and the results weren't good.   The three tests all came
> back between 6.5 and 7.5, more than twice the recommended result for painting
> a floor. (FWIW, I bought a cheap scale on Amazon and did everything myself.
> The scale was cheap, but the starting weight of the kits was exactly what the
> manufacturer said they should be, so I don't think my scale is the
> problem.)   I had two test kits left (I'd bought 5 just in case I screwed
> up) so I did those too, but with the same result.
>
> Based on those results, I shouldn't coat the floor.  However, before simply
> assuming the problem is my floor, I would like to add a couple of potentially
> important points:
>
> The test kit says to do the test when the humidity is between 40% and 60%.
> That isn't possible here.  It was much higher for the first test, probably
> 60-90% over the three days and included a day of rain.  This test was 60-70%,
> about normal around here.  While the space is heated, it isn't
> air-conditioned and it didn't occur to me that I could have put a
> dehumidifier there until just now. I decided to do the "plastic square" test
> in between the two calcium chloride tests.  There was absolutely no trace of
> moisture on the concrete after more than 24 hours. I've had boxes of
> flooring sitting in one corner of the garage for several months.  Each box
> weighs about 40 pounds and the cardboard is painted or otherwise coated and
> repels moisture.  I moved some of the boxes and there was no trace of
> moisture under the boxes either.  There were three boxes on top of each
> other, so there was quite a bit of weight keeping it flat on the floor.
>
> My guts tell me that the floor is dry and the humidity is the problem, but
> coating the floor involves several days work and about $1000, so I don't
> want to screw it up.  So, here is the question for the chemists and/or
> experts:  I know from my research that humidity does impact the test results,
> but don't know by how much.  Is it possible to factor that into the
> formula?  FWIW, here is the formula they use:
>
> [Gain in weight (grams) x 2.057 x 24 x 1000] / [Hrs. exposed x 454]
>
>
> Any help will be appreciated.
>
>
> Thanks,
>
>
> Jim_______________________________________________
>
>