Concrete grade designation: meaning of M 30 For M 30 grade concrete, the specified 15 cm cube compressive strength at standard age corresponds to 30 N/mm² after how many days of curing?

Introduction:
Concrete grades like M 20, M 25, and M 30 are commonly used shorthand for specified compressive strengths. Understanding exactly what age and specimen these values refer to is essential for acceptance criteria and quality control on site.

Given Data / Assumptions:

• M 30 means a characteristic compressive strength of 30 N/mm².
• Standard specimen is a 150 mm cube (15 cm cube) tested under standard conditions.
• Standard age for grading is the conventional curing period for characteristic strength.

Concept / Approach:

By definition, the grade M 30 refers to the characteristic strength at the standard age of 28 days. Acceptance testing and mix design both revolve around this benchmark, although early-age tests (3 or 7 days) can be used for trend checks.

Step-by-Step Solution:

Verification / Alternative check:

Typical strength gain curves show ~65–75% strength at 7 days, confirming 28 days as the reference for grade designation.

Why Other Options Are Wrong:

3, 7, and 21 days are interim ages; 56 days can be relevant for some cements but is not the standard grade reference.

Common Pitfalls:

Confusing cube strength with cylinder strength; mixing up early-age target values with characteristic strength at 28 days.

Final Answer:

28 days