Which of the following increases the workability of concrete?
Correct Answer: D. Increasing the aggregate size without any change in the mix of concrete
📚 Detailed Explanation: Larger Aggregate Size Increases Workability
Specific surface area (surface area per unit mass) of aggregate decreases sharply as particle size increases. At a constant water content, larger aggregates leave more free water for lubrication, improving workability.
Why D (Increasing aggregate size) is correct: A 40 mm aggregate has far less surface area per kg than a 10 mm aggregate. With the same water content in the mix, larger aggregates require less water to wet their surfaces, so more water is available to lubricate the mix — higher workability. Option A (increasing A/C) decreases workability (less paste per aggregate). Option B (decreasing w/c) decreases workability (less free water). Option C (angular instead of round) decreases workability (more friction).
Effect of Aggregate Size on Workability
| Max. Aggregate Size | Specific Surface Area | Water Needed to Coat | Workability |
|---|---|---|---|
| 10 mm | High | High | Lower |
| 20 mm | Medium | Medium | Medium |
| 40 mm | Low | Low | Higher |
- Larger aggregates = lower specific surface area = less water to coat = more free water = better workability.
- IS 456 limits max. aggregate size to 1/4 of the minimum section dimension or 5 mm less than cover.