Understanding Relative Humidity

Explaining the Concept and Sources of Relative Humidity in Concrete

Relative humidity is often mistaken for moisture and humidity. Moisture represents the amount of water in the liquid form, while humidity represents 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 (RH) represents the amount of water vapor that is present in the air in comparison to the amount that the air can hold.

To better understand this concept, 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 by relative humidity in the air is still at 100% because the air is fully saturated with water.
  3. Now, remove almost all of the water from the glass. At low moisture content the relative humidity value will decrease as the air is no longer saturated.

A similar principle is applicable to concrete. If your concrete looks dry at the surface, it doesn’t necessarily mean it’s dry throughout. There might still be important sources of water inside the concrete in the form of relative humidity.

Relative Humidity explained

Sources of moisture

There are multiple factors that can influence the level of relative humidity in a slab and can contribute to a high level of relative humidity, even 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 temperature, additional sources of water, and little to no air movement.
  • Pozzolan such as silica fume has a tendency to retain water inside the concrete, requiring a longer drying time.
  • Wet curing.
  • For slab on grade, if an adequate moisture barrier is not provided, water sources can reach the slab and increase the level of RH, this can be observed on a short- or long-term period.
  • Floor failure

    Before installing the flooring system, it is very important 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 on the concrete, it seals the surfaces and prevents further drying of the concrete element. If the floor is installed prematurely, that excess relative humidity trapped inside the concrete can change to moisture and tries to evaporate, which can cause pressure and changes in the alkalinity at the flooring and concrete interface. Typical concrete failures are delamination of the floor adhesive, blistering of epoxy coating, re-emulsification of adhesive (stain), curling, cracking, etc. Because of inappropriate relative humidity testing, these type of failures occur on a regular basis and are costly to the industry.