So let's go ahead and talk about what is relative humidity, why is it useful, why we should trust it. First, let's understand a little bit about what actually relative humidity is.
Relative humidity is the ratio of how much moisture is actually in the atmosphere compared to what the air could hold. In simple terms, the relative humidity we measure in the concrete is very much like the humidity you feel in the air when you're walking outside. On a humid day, it feels damp. The humidity is higher. On a dry day, it feels more comfortable. That's exactly what's going on in a hole that we might drill into the concrete.
Technically speaking, relative humidity is the ratio of the actual pressure of water vapor in the air compared to saturation water vapor. Again, generally speaking, it is the amount of moisture that we're feeling in the air compared to what the air could hold if it were saturated. When we drill a hole in the concrete and put a moisture sensor in there, we're simply measuring the relative humidity in that hole in the concrete.
Okay. Let's go ahead and talk about why relative humidity is truly a useful technique. On this next slide, we see a schematic diagram of a concrete floor system. The gray area represents the concrete. There are little blue bubbles representing moisture in the slab. Underneath the slab there's a vapor retarder. The idea with the vapor retarder is it cuts off communication of moisture between the subbase and the concrete. So there's no passage of moisture or very little passage of moisture from the ground up into the slab. We're only concerned with the drying out of the concrete itself.
At the top of the picture, we see a bubble that indicates the drying rate of the concrete at the surface depends on three things. First of all, the relative humidity in the air over the concrete, the temperature, and the air flow across the surface. If we want to maximize the drying of a concrete slab, the things we're going to do are have a dry environment. Lowest relative humidity possible over the floor, warmer air, and move the air around. If we can do those three things, we can accelerate the drying of the floor.
Now, in a building under construction before it's all closed in, those ambient conditions are going to change day by day. Some days it's going to be cooler and drier, and some days it might be warmer and wetter. Humidity is actually going to flow from the air into the concrete when the air is damper and the concrete is drier. When the air is drier, then the concrete moisture will move out of the concrete. So, day by day, hour by hour, you'll find moisture moving in and out of the concrete. We have a slide that we'll look at in a few minutes that shows that in a little more detail.
On the next slide, you see that the concrete is beginning to dry, and schematically we represent that by moisture in the concrete moving out at the surface. There are actually three different regimens that the concrete goes through in the process of drying. First, beginning when the concrete is wet and freshly placed and it's full of liquid water, that liquid water has to evaporate at the surface. Eventually, when that liquid water at the surface is gone, liquid water a little deeper down begins to move, and eventually moisture vapor diffuses from deep in the concrete up through the concrete and evaporates at the surface.
What you take away from this is that the concrete must dry at the surface first before moisture can move out from deeper down. So, at any point in time, as we see in the next slide, the concrete's going to be drier at the surface and wetter deep down. This is true for virtually any concrete slab after it's been drying for anywhere from a few days to many months. The surface of the concrete will tend to equilibrium with the air. The bottom of the concrete will tend to be more moist.
The only way you can get the true moister picture then is by putting a probe down into the concrete and actually measuring that moisture within the body of the concrete. If we go ahead and put a floor covering on top of this slab, as shown in the next slide, that floor covering and adhesive or if it's a coating, it initially sees what was a drier surface, and it doesn't see all that moisture deeper down in the concrete.
I'm going to put a little animation into motion here by pressing the button and moving to the next slide. You see how the moisture has now spread out through the slab. What was originally drier at the top is now wetter, and if you had done a surface test, such as a calcium chloride or with a handheld meter, you may have thought that the concrete was dry. But deeper down, you had a higher level of moisture which has now moved up. The slab is roughly equilibrated, and if the slab is at the same temperature top to bottom, the moisture eventually will be the same top to bottom.
In the real world, we find that there's a very slight gradient, maybe 1% or 2% or 3% drier at the top, underneath floor coverings or coatings, and a little wetter deeper down at the bottom. The reason is the bottom of the floor slab is usually a little cooler than the surface because it's in contact with the soil or subbase.
On the next slide, what we're going to show you is where to put the relative humidity probe. Why do we put it at a particular depth? This slide shows the distribution of moisture from the top of the concrete deeper down. You can see that it's drier at the top and wetter at the bottom. If you go to about 40% depth and put a probe into that depth, that's the value that eventually will equilibrate throughout the slab after it's covered with the floor covering.
This number is based, actually, on a standard developed in Scandinavia at Lund University in Sweden. They found by measuring concretes the relative humidity at roughly 40% of the depth is what will come to equilibrium eventually. It makes it very easy for us. On a 5 inch thick slab, you drill a hole 2 inches deep. If the slab is a little deeper, you drill a little bit deeper. That way you measure the true moisture picture that will eventually be seen by the floor covering or the adhesive or the coating at the surface.