Which of the following statements is true? A. Workability reduces with time. B. Workability does not reduce with time. C. Workability remains constant with time.
Correct Answer: A. Only A
📚 Detailed Explanation: Loss of Workability Over Time
Why A (Only A) is correct: From the moment water contacts cement, hydration begins. Over time:
1. Cement hydration consumes free water and forms C-S-H gel, stiffening the paste.
2. Water absorption by dry aggregates reduces free water available for lubrication.
3. Evaporation in hot/dry conditions removes surface moisture.
All three mechanisms progressively reduce workability from the moment of mixing. Workability never increases after mixing without adding water or admixtures (retarders/plasticisers). Statements B and C are false.
1. Cement hydration consumes free water and forms C-S-H gel, stiffening the paste.
2. Water absorption by dry aggregates reduces free water available for lubrication.
3. Evaporation in hot/dry conditions removes surface moisture.
All three mechanisms progressively reduce workability from the moment of mixing. Workability never increases after mixing without adding water or admixtures (retarders/plasticisers). Statements B and C are false.
Rate of Workability Loss
| Time After Mixing | Typical Slump Reduction |
|---|---|
| 0 min (reference) | 0 (design value) |
| 30 min | 10–25 mm drop |
| 60 min | 25–50 mm drop |
| 90+ min | Significant; concrete may be unusable |
- Workability always decreases with time — never stays constant or increases naturally.
- IS 456 requires concrete to be placed within 2 hours of mixing (30 min for hot weather).
- Retarders can slow (but not stop) workability loss; re-dosing with superplasticiser restores workability temporarily.