The primary purpose of concrete compaction is to:
Correct Answer: A. Increase the density
📚 Detailed Explanation: Compaction Increases Density of Concrete
Why A (Increase the density) is correct: The core purpose of compaction is to expel air voids from the fresh concrete mass. As air is removed, the solid components (aggregate, cement paste) pack more closely together — this increases the bulk density of the concrete. Higher density = fewer voids = stronger, more impermeable concrete.
Why Density Matters
| Property | Effect of Higher Density |
|---|---|
| Compressive strength | Increases (fewer load-transferring voids) |
| Impermeability | Increases (fewer capillary pathways) |
| Durability | Increases (less penetration of chlorides, sulfates, CO2) |
| Bond to reinforcement | Increases (concrete fills around bars completely) |
Why Other Options Are Wrong
| Option | Why Wrong |
|---|---|
| B. Increase weight | Weight (mass) of concrete is fixed by the materials batched; compaction does not add material |
| C. Increase voids | Opposite! Compaction reduces voids by expelling air |
| D. Decrease setting time | Setting time is governed by cement chemistry, temperature, and w/c ratio — not by compaction |
- Compaction = primarily to increase density by removing air voids.
- Every 1% air removed → approximately 5% strength increase.
- Fully compacted concrete at correct w/c ratio achieves maximum design strength.