Concrete Mix Selection for Retaining Structures For the construction of retaining structures (such as gravity or RCC retaining walls), which nominal concrete mix proportion is most commonly specified in traditional practice for structural work?

Introduction / Context:
Retaining structures require concrete with adequate strength, durability, and workability. Historically, nominal mixes (by volume) such as 1:2:4 have been associated with grades around M15–M20 depending on materials and control. While modern design prefers designed mixes with target mean strength, exam contexts often refer to nominal proportions.

Given Data / Assumptions:

• Traditional nominal-mix notation, not a designed mix.
• General retaining wall structural concrete (not mass lean blinding).
• Ordinary exposure conditions unless stated otherwise.

Concept / Approach:
Nominal mixes convey a rough cement content and aggregate ratio. 1:2:4 is a common structural concrete proportion offering a balance between strength and workability for footings, slabs, and retaining wall stems or footings in older specifications. Leaner mixes like 1:3:6 or 1:4:8 are used for mass concrete foundations or bedding layers, not for primary structural elements where higher strength is required.

Step-by-Step Solution:
Eliminate very lean mixes (1:3:6, 1:4:8) → typically for mass/bedding.Consider richer mixes (1:1:3, 1:1:2) → often used where very high early strength or special conditions are needed; not the standard baseline.Select 1:2:4 as the most common nominal proportion for structural retaining elements in traditional practice.

Verification / Alternative check:
Legacy textbooks and PWD schedules list 1:2:4 for many structural members before the widespread adoption of design mix concrete.

Why Other Options Are Wrong:

• 1:3:6 and 1:4:8: lean mixes, typically for non-structural mass concrete.
• 1:1:3 and 1:1:2: richer than commonly necessary for general retaining structures; higher cement content implies higher cost and heat.

Common Pitfalls:
Using nominal mixes indiscriminately; always verify design strength and durability requirements and prefer design mixes in modern practice.

Final Answer:
1 : 2 : 4