Difficulty: Easy
Correct Answer: 15 N/mm2
Explanation:
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:
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.
Step-by-Step Solution:
Target 28-day strength: f28 = 20 N/mm2.Take 7-day fraction ≈ 0.7 (typical) → f7 ≈ 0.7 * 20 = 14 N/mm2.Adopt the widely used minimum checkpoint: 15 N/mm2.Verification / Alternative check:
Early-age tests combined with curing records (temperature, moisture) help refine predictions; values at or above 15 N/mm2 generally indicate the mix is on track for M 20.Why Other Options Are Wrong:
7 or 10 N/mm2: too low for M 20 under normal curing; would signal potential issues.20 N/mm2: equals the 28-day characteristic strength; unrealistic at 7 days for ordinary mixes.12 N/mm2: borderline low compared to the adopted exam threshold.Common Pitfalls:
Equating site acceptance at 7 days with compliance at 28 days; 7-day checks are indicative, not definitive.Final Answer:
15 N/mm2
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