Compacting of concrete is done to:
Correct Answer: B. Remove air bubbles
📚 Detailed Explanation: Purpose of Compaction — Remove Entrapped Air
Why B (Remove air bubbles) is correct: When fresh concrete is placed, air inevitably becomes entrapped within the mass — between aggregate particles, in the paste, and at the interface with formwork. Compaction applies mechanical energy (vibration or pressure) to temporarily reduce the viscosity of the paste, allowing air bubbles to migrate upward and escape at the surface.
What Happens During Compaction
| Stage | Process | Effect |
|---|---|---|
| Before compaction | Air trapped between particles | Porous, weak concrete |
| During vibration | Vibration reduces inter-particle friction; concrete liquefies temporarily | Particles rearrange; air rises |
| After compaction | Particles pack closely; air expelled at surface | Dense, strong, impermeable concrete |
Effect of Entrapped Air on Strength
| Entrapped Air (%) | Strength Reduction |
|---|---|
| 0 (fully compacted) | None — maximum strength |
| 1% | ≈5% reduction |
| 5% | ≈30% reduction |
| 10% | ≈50% reduction |
Signs of Adequate Compaction
- Cement paste (cream) appears at the surface — air has been displaced.
- Bubbling at the surface stops or slows significantly.
- Concrete settles uniformly around reinforcement and into corners.
- Compaction is done to remove air bubbles — not to place, spread, or introduce air.