Ordinary concrete — which grade is NOT typically classed as ordinary? In the conventional Indian grading system where M-grades denote characteristic compressive strength (e.g., M10, M15, M20, M25, M40), which of the following is <em>not</em> usually referred to as ordinary concrete?

Introduction / Context:
“Ordinary concrete” in common site parlance refers to normal-strength mixes used for regular building elements (e.g., M10–M25). Higher grades require more stringent control of materials, water–cement ratio, and curing to achieve high strength and durability, and they are not typically labelled “ordinary.”

Given Data / Assumptions:

• M-grades indicate characteristic compressive strength in kg/cm² or MPa (e.g., M400 ≈ 40 MPa).
• Ordinary building work often uses M10–M25 grades.
• High-grade concrete (≥M40) is used for special performance requirements.

Concept / Approach:
Lower M-grades (M100, M150, M200, M250 in kg/cm² notation) fall within conventional applications such as blinding layers, slabs, beams, and columns of standard buildings. M400 corresponds to a much higher strength class (≈40 MPa), commonly treated as high-strength/structural grade beyond the “ordinary” label in everyday usage and requiring enhanced quality control and possibly admixtures.

Step-by-Step Solution:

Verification / Alternative check:
Typical schedules for RCC in regular buildings show M20/M25; high-rise or prestressed elements often specify ≥M40, reinforcing the distinction.

Why Other Options Are Wrong:

• M100, M150, M200, M250: broadly found in ordinary concrete use cases.

Common Pitfalls:
Confusing legacy kg/cm² notation with MPa; assuming “ordinary” is a strict code term rather than widely used practical terminology.

Final Answer:
M 400