Quality control of concrete: for M 20 grade concrete, what should be the minimum compressive strength (on 150 mm cubes) at 7 days of curing?

Introduction / Context:
Concrete strength gain is time-dependent. While the characteristic grade (e.g., M 20) is defined at 28 days, site quality control frequently checks 7-day strengths to assess early performance and curing adequacy. Empirical correlations guide the minimum expected 7-day strength that indicates the mix will likely achieve its 28-day target if proper curing continues.

Given Data / Assumptions:

• Grade: M 20 (characteristic strength 20 N/mm2 at 28 days on 150 mm cubes).
• Specimens: 150 mm cubes tested at 7 days.
• Objective: minimum expected 7-day strength for acceptance in routine control.

Concept / Approach:
Typical strength-gain curves show that 7-day strength is roughly 60–75% of the 28-day strength for ordinary Portland cement mixes under normal curing. For M 20, this places the 7-day expectation around 12–15 N/mm2. Many exam standards adopt 15 N/mm2 as the convenient minimum checkpoint threshold for M 20, signaling acceptable early hydration and mix proportioning.

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