In concrete mix classification for reinforced concrete design, which grade is generally not used for reinforced concrete (R.C.C.) structural members?

Introduction / Context:
Concrete grades (M10, M15, M20, M25, M40, etc.) denote the specified characteristic compressive strength in N/mm^2 at 28 days. In reinforced concrete design, minimum grades are stipulated to ensure adequate strength, durability, and bond with reinforcement. Lower grades are typically restricted to plain concrete works such as blinding, levelling courses, and non-structural applications.

Given Data / Assumptions:

• We are selecting the grade that is generally not used for R.C.C. members.
• Typical practice requires at least M20 for R.C.C. in many modern codes for durability and strength.
• Lower grades like M10 are used for plain cement concrete (P.C.C.), not for reinforced members.

Concept / Approach:
Reinforced members must meet minimum compressive strength to develop rebar bond and sustain design actions with adequate safety and serviceability. Therefore, grades below the threshold (e.g., M10) are unsuitable for R.C.C. and are reserved for non-reinforced works. Higher grades such as M25 and M40 are common for beams, slabs, columns, and high-performance elements, respectively.

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