Proper compaction of freshly prepared concrete results in: (i) Achieving maximum strength (ii) Expelling entrapped air (iii) Change in fineness modulus of aggregate
Correct Answer: B. Both (i) and (ii)
📚 Detailed Explanation: Effects of Proper Concrete Compaction
Why B (i and ii only) is correct: Compaction mechanically removes trapped air voids from the concrete mass. This densification directly achieves maximum strength. However, the fineness modulus (FM) is a property of the aggregate grading curve — it cannot be changed by any mixing or compaction process.
Analysis of Each Statement
| Statement | Verdict | Explanation |
|---|---|---|
| (i) Achieves maximum strength | Correct | Expelling air removes voids that would otherwise weaken the concrete matrix. 1% entrapped air → ~5% strength loss; 5% entrapped air → ~30% strength loss |
| (ii) Expels entrapped air | Correct | The primary mechanical purpose of compaction. Vibration causes temporary liquefaction, allowing air bubbles to rise to the surface |
| (iii) Changes fineness modulus of aggregate | WRONG | FM is determined by sieve analysis of the aggregate before mixing. Compaction cannot alter particle size distribution |
Air Content vs. Strength Loss
| Entrapped Air (%) | Approx. Strength Loss |
|---|---|
| 0 (fully compacted) | 0% — maximum strength |
| 1% | ~5% |
| 2% | ~10% |
| 5% | ~30% |
| 10% | ~50% |
- Compaction expels entrapped air (ii) and maximises concrete strength (i).
- Fineness modulus cannot change through compaction — it is a fixed gradation property of the aggregate.
- Benefits of compaction also include: increased density, durability, impermeability, and bond with reinforcement.