Understanding Relative Humidity in Concrete

Relative humidity (RH) is often mistaken for moisture; however, these are two completely separate terms. Moisture content represents the amount of water in liquid form, while humidity levels represent the amount of water in vapor form.

In ASTM F2170, relative humidity is defined as "the ratio of the amount of water vapor actually in the air compared to the amount of water vapor required for saturation at that particular temperature and pressure, expressed as a percentage."

Simply put, relative humidity represents the amount of water vapor that is present in the air in comparison to the amount that the air can hold.

The Difference Between Moisture and Relative Humidity

To better understand the concept of relative humidity, imagine a glass of water that is sealed.

  1. If the glass is full of water, the moisture level and the relative humidity level will both be 100%
  2. If half of the water is removed, the amount of moisture is now at 50% but RH is still at 100% because the air is fully saturated with water.
  3. Now, remove almost all of the water from the glass. With lower moisture content, the RH value will decrease as the air is no longer saturated.

What About Relative Humidity in Concrete?

A similar principle can be applied to concrete. If your concrete looks dry at the surface, it doesn't necessarily mean that it's dry throughout. There may still be important sources of water inside the concrete (in the form of relative humidity).

Explaining Relative Humidity of Concrete

Sources of Moisture in Concrete

Multiple factors can influence the level of relative humidity in a concrete slab, thus contributing to high relative humidity 6 months or more after pouring concrete:

  • The initial amount of water in the concrete (from a high water/cement ratio);
  • Ambient conditions such as high ambient relative humidity, cold temperatures, additional sources of water, and limited air movement;
  • Wet curing;
  • The use of pozzolans, such as silica fume, which have a tendency to retain water and take longer to dry; and
  • Missing or inadequate moisture barrier in a slab on grade, causing outside moisture to penetrate the slab.
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    Prevent Flooring Failures With Concrete Humidity Meters

    Before a flooring system is installed, it is crucial to measure the relative humidity of the concrete. Only when the relative humidity has reached a certain level is it safe to install a floor while avoiding failure.

    Once the floor is installed, it seals the surface and prevents further drying of the concrete underneath. If the floor is installed too early, excess relative humidity will be trapped inside the concrete. It will try to evaporate, leading to pressure and changes in the alkalinity at the flooring and concrete interface. Typical concrete failures due to relative humidity include:

    • Delamination of the floor adhesive;
    • Blistering of the epoxy coating;
    • Re-emulsification of the adhesive (stain); and
    • Curling, cracking, or bubbling of flooring materials

    Inappropriate concrete relative humidity testing prior to flooring installation can lead to these types of recurring and costly failures.