For given workability, the grading requiring the least amount of water is one that gives
Correct Answer: B. least surface area for the given cement and aggregates
📚 Detailed Explanation: Grading and Water Demand
Why B (least surface area) is correct: Water in concrete is used for:
(1) Wetting/coating aggregate surfaces.
(2) Providing free water for lubrication (workability).
If total aggregate surface area is minimised (through optimum grading using larger particle sizes), less water is consumed for surface coating → more free water remains for lubrication → the target workability is achieved at a lower total water content. This also means a lower w/c ratio and higher strength for the same workability. Option A (greatest surface area) is the opposite — it would require more water for the same workability.
(1) Wetting/coating aggregate surfaces.
(2) Providing free water for lubrication (workability).
If total aggregate surface area is minimised (through optimum grading using larger particle sizes), less water is consumed for surface coating → more free water remains for lubrication → the target workability is achieved at a lower total water content. This also means a lower w/c ratio and higher strength for the same workability. Option A (greatest surface area) is the opposite — it would require more water for the same workability.
Surface Area and Water Demand
| Grading/Size | Specific Surface | Water Demand (same workability) |
|---|---|---|
| All fine aggregate | Highest | Highest |
| Mixed grading | Medium | Medium |
| Optimal grading (min. surface) | Lowest | Lowest |
- Least surface area = least water demand = lowest w/c for same workability = highest strength.
- This is why well-graded aggregates using larger maximum size are preferred for structural concrete.