When concrete dries after casting, it:
Correct Answer: C. Shrinks
📚 Detailed Explanation: Concrete Shrinks on Drying After Casting
Why C (Shrinks) is correct: When concrete dries after casting, it undergoes drying shrinkage — a reduction in volume caused by the evaporation of free and adsorbed water from the hardened cement paste. This contraction is the standard behaviour of all Portland cement concrete. While some expansion can occur with certain expansive cements or additives, normal OPC concrete always shrinks on drying.
Volume Changes in Concrete Over Time
| Stage | Volume Change | Reason |
|---|---|---|
| During hydration (sealed) | Slight contraction (autogenous shrinkage) | Chemical combination of water with cement reduces total volume slightly |
| On drying (exposed to air) | Shrinks (drying shrinkage) | Evaporation of capillary and gel water; capillary tension causes contraction |
| On wetting (re-saturation) | Swells slightly | Water re-enters gel pores; gel expands; reversible component |
| Under sustained load | Increases in deformation (creep) | Time-dependent flow of cement paste under stress |
Types of Shrinkage in Concrete
| Type | When Occurs | Cause | Magnitude |
|---|---|---|---|
| Plastic shrinkage | Before initial set (0–4 hours) | Rapid surface evaporation; wind, sun | Small; causes surface cracking |
| Autogenous shrinkage | During hydration | Chemical contraction; water consumed by C-S-H formation | Small; significant only in very low w/c mixes |
| Drying shrinkage | After initial drying begins | Evaporation of free and gel water | Largest; IS 456 design value = 0.0003 |
| Carbonation shrinkage | Long-term exposure to CO2 | Carbonation of Ca(OH)2; volume reduction | Small; significant in thin members |
- Concrete on drying after casting: shrinks (drying shrinkage).
- Drying shrinkage is the dominant shrinkage type in normal concrete structures.
- Shrinkage joints in slabs and walls accommodate this movement and prevent uncontrolled cracking.