Understanding Concrete Shrinkage and Humidity

Concrete shrinkage is the reduction in volume that occurs as concrete hardens and dries. This process is significantly influenced by the surrounding environmental conditions, specifically the relative humidity (RH).

Humidity's Role in Concrete Shrinkage

Shrinkage occurs primarily due to the loss of water from the concrete to the surrounding environment. This water loss causes the cement paste to contract.

• When the relative humidity of the environment is less than 100%, there is a moisture gradient between the concrete and the air.
• This gradient drives water vapor out of the concrete, leading to drying shrinkage.
• The lower the ambient relative humidity, the faster the rate of drying and the greater the potential for shrinkage.

Condition for No Shrinkage

Zero shrinkage occurs only when there is no moisture loss from the concrete. This happens when the surrounding environment is fully saturated with water vapor, meaning the relative humidity is 100% (or 1.0).

Therefore, if concrete is placed in an environment with a relative humidity of 1 (equivalent to 100%), no shrinkage will occur because there is no driving force for water to leave the concrete.

Answer Derivation

Based on the principle of moisture movement:

• Option 1 (RH = 1 or 100%): No moisture loss, hence no shrinkage.
• Options 2, 3, and 4 (RH < 1): Moisture loss will occur, leading to shrinkage.

The condition required to prevent shrinkage is a relative humidity of 1.