How thick is your floor?

[CC Solutions]

A flooring contractor used Wagners to test a new slab and the GC wanted a second opinion.

I was called in and tested a few locations using the same Rapid RH (is there anything else? ) and concurred with the flooring sub that the floor was too wet for the flooring they were installing.

The floor was put in anyway, using a latex sealer and 'special' glue. It is failing just a few months later.

I was called in again and asked to evaluate the slab, starting with RH. This is where things get more serious. We have a failure so now we have to ramp it up a bit. Even though I am being told the slab is 4" thick, I need to verify that it really is. Well guess what?

It isn't.

I drilled depth holes approximately 8" from my new test holes and measured the slab between 4" and up to 8" thick.

Now that there is a problem, I am rethinking my original testing. I did say the slab was too wet so I am kind of safe there, but if they go off my original readings and based their sealer on that, was I off? Either way, my readings were over 90% and even the Wagner can vary by 3% at that high of RH so we could have been at 94% to 97% easily.
I sure never said the slab was at or below 90%. And we were looking for 75% so if my probes were too shallow and reading 90%, going deeper would have been even wetter.

Perhaps there are grade beams in the ground that were not noted. Maybe there are grade beams down low, then vapor retarder was installed and the top 4" is separated from the thicker concrete. If that is the case my new readings could be erroneous because I have some at 3.2" deep!

It's too early to tell, but there will be a lot of questions asked in the near future. Slabs on grade are notoriously uniform in my experience, unless there was something buried or beefed up. No concrete installer wants to pay for extra slab thickness and no GC is going to let him pour less than he bid. Slabs just don't tend to vary more than half an inch on grade and a good sprinkling of readers will catch any discrepancy.

It could be that all the original probes were put in 4" concrete and I just happened to hit the thick parts now because I am installing along supporting walls to stay out of the way.

I'll let you know what comes out in the wash.