Q. I will be doing moisture testing (internal relative humidity per ASTM F 2170) on a slab on ground for a small library (6,500 square feet) in Colorado that was placed 3 months ago and dried in 2 months ago. We will be using Wagner Rapid RH sensors, which Wagner was kind enough to contribute to the project. The slab has a Stego moisture retarder directly in contact with the concrete—also a very kind contribution. Where in the building should we locate the sensors?
A. Scott Tarr, North S.Tarr Concrete Consulting provided these recommendations:
We normally suggest doing initial “pilot” testing so we don’t have to abide by ASTM requirements (F 2170) on number of tests. If you can’t pass three, you can’t pass 103 so why spend the money? We would typically install three to six unless there was a reason to install more – different rooms, different floors, different pours (with substantially different dates – that is, drying times). If they pass, we just say in our report that ASTM requires additional testing but the flooring system manufacturer can waive that if they want.
Since it’s only been dried in a couple months, you can start off with only doing three tests. But you only have 6,500 sf so it’s not a lot more work to do the full eight tests – no need for the “fraction thereof” for relative humidity so it’s just three for the first 1,000 square feet and one for each additional 1,000 sf. I wouldn’t necessarily put them all at the perimeter but I would put a few there – maybe look at the exterior and see if there is any wall that looks like it might be better depending on the drainage (sometimes a little more water collects on one side of a building).
But I’d just spread them out – the library likely has several rooms so maybe three in the largest couple of rooms and two in another, or something like that. Another thing you can consider, if they know there will be different finish flooring materials going in, maybe put more probes in an area where the flooring system requires a lower RH number. It’s likely carpet tiles or something like that but they may have an entry with sheet vinyl, linoleum, or rubber that is more moisture-sensitive.
If it’s only been dried in for a couple months, I’m guessing you’re still in the 90s, but your area is very dry so maybe it’s lower.
