Video 19 – In-Situ RH Stability

Previous Video  Next Video

I want to talk for a minute this next slide about the sensitivity of relative humidity to ambient conditions.

We saw before that with calcium chloride kits, the air temperature and relative humidity can have really a significant effect on the result. Well, it turns out that based on some co-luminary work done at several universities around the world, relative humidity is not nearly so sensitive to ambient floor conditions.

Some work done in Denmark, and here at the University of Illinois, and in Sweden in Lund University, all suggests that when you make a relative humidity measurement, if the building temperature changes, the relative humidity in the slab will change very little.

There’s more work going on right now, we’re hoping to develop tables that you can use to correct your results. But, the idea would be to go out to a building under construction. Well before, at service conditions before the HPAC system is running, put probes in the slab, get your results and correct them to what you’ll find to be the service conditions.

As an example on this next slide, I can show you some numbers based on some this preliminary work. For example, if you go out to a building at 55 degrees because the heat is not on yet, and we measured let’s say 80% relative humidity in the floor, it turns out that when that floor gets up to 73 degrees at service conditions, the humidity in the floor will only go up 2 points up to 82%.

So, potentially we’ll be able to go out and make these measurements before work service conditions which is going to be much more useful than other test methods. I want to talk about on this next slide, the difference in results and a little more detailed between Surface Test and the In-Situ probes.

And why it is important to put the probes at a correct depth into the concrete.

This graph shows the relative humidity on the left-hand vertical axis versus time across the bottom. These are measurements made on a slab, here on our facilities several years ago, over the course of about 5 or 6 months. You see the dates at the bottom run from May through October. And the relative humidity on the vertical axis runs from about 95% down to about 55%. The top blue line shows the relative humidity at 40% at the depth of the slab and going from about 95% relative humidity when the slab was first cast down to about 70%, 65% to 70% after 5 or 6 months. That’s the typical sort of measurement you will find in a concrete floor slab if you follow it over time. So if you put a probe in, when the slab is new, and go out and make a reading once a week, you can actually plot a curve that shows how the floor is drying-out over time and predict if the floor would be dry enough when you’re ready to install your flooring.

In that way, contractors, flooring installers can make some measurements and ahead of time, predict if the floor would be suitable or if you have to take some remedial actions, such as drying-out the floor or putting-in a sealer.

The thin black line that’s very jagged that you see on the graph, that was a measurement made with the relative humidity probe set very close to the surface of the concrete. What we did was, we wanted to understand how the moisture goes in and out at the surface of the slab. What we found, as you see by the jaggedness of this line, is that, everyday moisture is going into or out of the floor and that really depends on the ambient conditions.

So, during the day, when it may be warm and dry, moisture is moving out of the slab. Overnight, when it’s damp and cool, moisture is actually going back into the floor. And so, everyday it’s like two steps forward, we dry the slab a little, and at night, it’s one step back, moisture goes back into the floor slab. If you’re in Houston, it may be very difficult to dry the slab quickly. And if you’re in Phoenix, it may dry very quickly.

So, new to surface, you’re not getting a true picture of what’s happening deeper down, and this graph shows that. And why it’s so important to go ahead and put probe at the proper depth on the slab, generally around 40% of the full thickness of the slab.

Now, the test method was really developed for typical normal floors, 4 to 5, 6 inches thick. If you’re measuring floors that are much thicker, you may have to make some other decisions. You certainly can put the probes to any depth that you’d like, but, typically, if you were down 2 to 3 inches in the slab, the moisture picture below that depth would be fairly constant. Follow the ASTM F2170 procedure and put the probe at 40% of the depth.

Previous Video  Next Video