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?
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A3 days
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B7 days
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C21 days
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D28 days
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E56 days
Answer
Correct Answer: 28 days
Explanation
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:
1) Identify the definition of grade: M x = x N/mm² characteristic strength.2) Recognize standard age: 28 days for acceptance.3) Conclude that M 30 corresponds to 30 N/mm² at 28 days.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