IS 456 (1978) grade notation — which grade is NOT recommended in that code? Considering the grade notation used in IS:456-1978 (legacy kg/cm²-based naming), identify the grade that was not recommended.

Introduction / Context:
Older editions of IS:456 (1978) used grade labels such as M100, M150, M200, M250, M300, M350, and M400 (based broadly on kg/cm²). Later revisions adopted MPa-based grades like M20, M25, M30, etc. Recognizing which legacy grades existed avoids confusion when reading older drawings and texts.

Given Data / Assumptions:

• The question references IS:456-1978 specifically.
• Within that edition, listed grades up to M400 were common.
• Higher grade M500 was not part of the recommended series then.

Concept / Approach:

IS:456-1978 commonly cited concrete grades up to M400 for reinforced concrete. M500 belonged to higher-strength concretes beyond coverage of typical prescriptive provisions at that time and was not recommended as a standard grade in that edition.

Step-by-Step Solution:

Verification / Alternative check:

Historic tables and handbooks referencing IS:456-1978 align with this grade band, prior to MPa-based modern designations.

Why Other Options Are Wrong:

• M100, M200, M300, and M400 were part of the conventional range then.

Common Pitfalls:

• Confusing older kg/cm² labels with modern MPa designations (e.g., M20, M25).

Final Answer:

M 500.