The maximum size of aggregate to be used in a gravity dam should not exceed
Correct Answer: A. 40 mm
📚 Detailed Explanation: Maximum Aggregate Size for Gravity Dam
The maximum size of aggregate (MSA) to be used in a structural element depends on: (1) the minimum dimension of the element, (2) the cover to reinforcement, and (3) the spacing of bars. In mass concrete structures like gravity dams, where cross-sections are very large and reinforcement is minimal, larger aggregate sizes are permitted.
Why A (40 mm) is correct: For gravity dam concrete, IS 456 Table 1 and standard dam construction practice specify an MSA of 40 mm for the main body of mass concrete. Large aggregate reduces: the cement content per m³ (less cement = less heat of hydration — critical in mass pours to prevent thermal cracking), the water demand, and the overall cost. In contrast, for reinforced concrete beams/slabs, MSA is typically 20 mm; for columns with closely-spaced bars, 12.5 mm or 10 mm is used. Some dam projects use even larger nominal sizes (80 mm for the interior mass), but the standard design reference value is 40 mm.
Maximum Aggregate Size by Structure Type
| Structure | MSA (IS 456) | Reason |
|---|---|---|
| Slabs | 1/3 of slab thickness or 20 mm | Thin sections |
| Beams / columns | 25 mm or 3/4 × bar spacing | Reinforcement gaps |
| Gravity dam (mass concrete) | 40 mm (standard) | Large section; reduce heat |
| Dam interior (special) | Up to 80–150 mm | Cyclopean/mass concrete |
Key Concepts for Students
- IS 456 Rule: MSA ≤ 1/4 of minimum dimension of the section; ≤ 3/4 of minimum clear spacing between bars; ≤ 5 mm less than clear cover.
- In gravity dams, sections are metres thick — so 40 mm aggregate easily satisfies the 1/4 rule.
- Large MSA in mass concrete reduces heat of hydration, but increases mix design complexity (gap grading risk).