Workability of concrete mix with low water cement ratio is determined by:
Correct Answer: C. Compaction factor test
📚 Detailed Explanation: Compaction Factor Test for Low W/C Mixes
Why C (Compaction factor test) is correct: Low w/c = stiff mix = very low slump (near zero). At near-zero slump, small changes in workability produce no measurable slump difference — the slump test cannot discriminate. The compaction factor test is more sensitive at low workability because it measures the energy (gravity) needed to compact concrete, which varies significantly even when slump is near zero. Options A and D (strength tests) measure hardened concrete, not fresh concrete workability.
Workability Test Selection Guide
| Mix Type | Recommended Test | Range |
|---|---|---|
| Very stiff (VB >20s) | Vee-Bee test | VB: 20–40 s |
| Stiff / Low w/c | Compaction factor test | CF: 0.75–0.87 |
| Normal to high workability | Slump test | 25–150 mm |
| Very fluid (SCC) | Flow table / slump flow | >600 mm |
- Compaction factor test = best for low-workability mixes where slump test gives zero/unstable results.
- Vee-Bee test is even better for very stiff (near-zero CF) mixes.