Nominal mixes — determine the corresponding legacy grade If the nominal volumetric proportion of concrete ingredients (cement : sand : coarse aggregate) is 1 : 3 : 6, the corresponding legacy grade designation is:

Introduction / Context:
Traditional Indian practice associated nominal mixes with legacy “M” grades (kg/cm² based), e.g., 1:3:6, 1:2:4, 1:1.5:3. Understanding these helps interpret older drawings and specifications still in circulation.

Given Data / Assumptions:

• Nominal proportion 1 : 3 : 6 by volume.
• Standard workability and curing conditions assumed.
• Legacy grade mapping (approximate characteristic strength).

Concept / Approach:

Historically, 1:3:6 corresponds to about 10 MPa characteristic strength, labeled M 100 (≈ 100 kg/cm²). Likewise, 1:2:4 maps to M 150, and 1:1.5:3 maps to M 200, acknowledging variability due to materials and site control.

Step-by-Step Solution:

Verification / Alternative check:

Older handbooks and IS-era tables list this mapping for guidance; modern design prefers designed mixes (MPa-based) rather than nominal mixes.

Why Other Options Are Wrong:

• M 150, M 200, M 250, M 300 correspond to richer mixes than 1:3:6.

Common Pitfalls:

• Confusing modern MPa grades (M20, M25) with legacy M 200, M 250 labels.

Final Answer:

M 100.