Video 3 – Moisture Is Always An Issue In Concrete Slabs

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Let’s talk first of all about what it is that we want to measure. Floor covering as you see in this slide, over on the right-hand side is a picture of the different components in a floor system. There are really quite a number of individual components that make up a floor system. We usually think just about walking on the surface of the floor. The floor covering, the carpet, or the vinyl as being the floor, but the floor system is made up of a lot of components.

Starting at the bottom, you’ve got subgrade soil. You may have a compacted subbase and vapor retarder, a capillary break, then the concrete floor itself, and above that patching and leveling compounds, adhesives, primers, and then finally the finished flooring.

All of these components are sensitive to moisture one way or another. Generally, when they are wetter, they expand, and when they’re drier, they’ll contract. So you get dimensional changes in floor systems. Floor coverings, adhesives, primers, patching compounds can all be sensitive to moisture and high PH and can break down when there’s a problem.

So what we’re going to try to do is to understand the moisture condition. Not right at the surface, but actually down in the flooring system, in the middle of the concrete. That’s best measured by relative humidity.

On the next slide, you see a list of the different types of floors in terms of their location relative to the ground. Floors above grade can be generally considered elevated floors, and there are generally two types – structural floor slab, which is a concrete floor that supports itself, or we can have concrete in steel deck. Normally that building is framed in steel, and corrugated or ribbed steel sheets support the concrete floor. Those are all considered elevated floors.

Slabs-on-ground, often called slabs-on-grade, slabs below grade, also on ground but such as a basement. Whether it’s residential, commercial, or industrial doesn’t matter. All these types of floors can potentially have moisture problems and need to be tested for moisture.

There are many different sources of moisture in the slabs. This next slide gives you some idea broken up into three categories – above the slab, below the slab, and in the slab.

Moisture above the slab can come from the ambient relative humidity in the room. The air space over the floor, during construction, may be uncontrolled. On hot, humid days, moisture goes into the concrete. On drier days, it’s coming out. We can have precipitation in the form rain or snow or fog that can get onto the slab. Water that’s used for curing the concrete also soaks into the concrete and has to dry out eventually. We also have construction water and accidents. Washing, spills during the course of construction can get floors wet again.

Now, below the slab, we find moisture coming in, in the form of subslab vapor. That’s the number one cause of problems. It’s not liquid water below the floor that actually gets in contact with the slab from the underside, but rather just moisture vapor that’s down in the soil and comes up into the bottom of the slab. That’s the reason for needing a subslab vapor retarder.

In the slab, we’ve got the curing water and the batch water. That’s the number one factor in determining how long it takes a concrete floor to dry out is the ratio of water to cement that was originally batched into the concrete

There many of these different sources of moisture, and they can all be measured by looking at the relative humidity actually down in the concrete with a probe set into the floor slab.

Let’s go ahead and talk about why would we consider measuring relative humidity at all. What’s this all about? Well, it turns out based on 20 years of experience measuring relative humidity in slabs and troubleshooting the performance of different kinds of flooring, we found that RH really gives the most accurate, useful picture of the true moisture condition within a slab. We’re actually getting down into the concrete, measuring the moisture in the concrete, and you’ll find out in a little while that that relates very well to the performance of floor coverings and adhesives.

It also turns out that it’s the most useful predictive tool for floor covering and adhesives. We can actually determine the bond strength of adhesives or coatings to a concrete floor under different moisture conditions. Therefore, we can establish in the laboratory the different levels of humidity in the floor under scientifically controlled environment, how floor coverings and adhesives perform, and then relate that to the actual performance out in the field.

In that way, relative humidity turns out to be the most useful and scientifically valid tool of all the different techniques that have been tried over the years for looking at the interaction of moisture and concrete and floor coverings or coatings.

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