For the construction of retaining structures, the type of concrete mix to be used is:
Correct Answer: C. 1:1.5:3
📚 Detailed Explanation: Concrete Grade for Retaining Structures
Why C (1:1.5:3 = M20) is correct: Retaining structures are:
• Reinforced: IS 456 mandates minimum M20 for any reinforced concrete member.
• Exposed to moisture/soil: Requires at least moderate exposure grade = M25 in severe conditions, but M20 (1:1.5:3) is the standard minimum for standard retaining walls.
• 1:3:6 (M10) and 1:2:4 (M15) are too lean for reinforced structural elements. 1:1:2 (M25) is correct for harsher conditions but over-specifying for standard retaining walls.
• Reinforced: IS 456 mandates minimum M20 for any reinforced concrete member.
• Exposed to moisture/soil: Requires at least moderate exposure grade = M25 in severe conditions, but M20 (1:1.5:3) is the standard minimum for standard retaining walls.
• 1:3:6 (M10) and 1:2:4 (M15) are too lean for reinforced structural elements. 1:1:2 (M25) is correct for harsher conditions but over-specifying for standard retaining walls.
Concrete Grade Selection for Structural Elements
| Structure Type | Recommended Grade | Nominal Mix |
|---|---|---|
| Blinding / levelling | M10 | 1:3:6 |
| Foundation (non-RCC) | M15 | 1:2:4 |
| Retaining structures (RCC) | M20 | 1:1.5:3 |
| General RCC (slabs, beams, columns) | M20–M25 | 1:1.5:3 to 1:1:2 |
- Retaining structures = minimum M20 = 1:1.5:3 per IS 456 for reinforced construction.