After casting, an ordinary cement concrete on drying–
💧 Understanding Moisture Movement in Concrete
Concrete is a porous material. Its volume is not constant and is highly influenced by the moisture content within its pores. This leads to two opposing phenomena:
- Drying: When concrete is exposed to air with lower humidity, the water held in the pores of the cement paste starts to evaporate. The loss of this water causes a reduction in volume. This is known as shrinkage.
- Wetting: Conversely, if dry concrete is exposed to water, it will absorb the moisture and undergo a slight increase in volume. This is known as swelling or expansion.
📝 Detailed Analysis of the Options
(a) Expands
This is incorrect. Concrete expands when it gets wet, not when it dries. The only exception is expansion due to unsound materials, which is a defect, not a normal property.
(b) Mix
This option is irrelevant to the physical process of drying.
(c) Shrinks
This is the correct answer. The loss of water from the capillary pores of the hardened cement paste to the atmosphere is the primary cause of drying shrinkage. This volume reduction is an inherent property of all Portland cement concretes. If this shrinkage is restrained (e.g., by reinforcement or adjacent structures), it can induce tensile stresses that may lead to cracking.
(d) None of these
This is incorrect as option (c) accurately describes the behavior.
📊 Types of Shrinkage in Concrete
| Type of Shrinkage | When it Occurs | Primary Cause |
|---|---|---|
| Plastic Shrinkage | In fresh, plastic concrete (first few hours) | Rapid evaporation of surface water faster than it can be replaced by bleed water. |
| Drying Shrinkage | In hardened concrete (days to years) | Loss of adsorbed water from the cement paste to the environment. |
| Autogenous Shrinkage | In hardened concrete (ongoing) | Volume reduction due to the chemical process of hydration itself (water consumed by reaction). |
💡 Study Tips
- Drying = Shrinking: This is the fundamental rule. Think of how a wet sponge or clay shrinks as it dries out. Concrete behaves similarly.
- Wetting = Swelling: Remember the opposite is also true. Concrete expands slightly when it absorbs water.
- Shrinkage is a Problem: In structural engineering, shrinkage is a major concern because it can cause cracks. Proper curing is essential to minimize the effects of early drying shrinkage.