Difficulty: Easy
Correct Answer: 75%
Explanation:
Introduction / Context:
Core testing is used when standard cubes/cylinders are not representative or are unavailable. Codes specify how to judge acceptability of in-situ concrete based on core strengths compared with the specified grade's cube strength at the designated age.
Given Data / Assumptions:
Concept / Approach:
Most codes derived from IS practice require both: (1) average core strength threshold and (2) minimum individual core threshold. The widely used limits are 85% (average) and 75% (individual). These limits acknowledge the inevitable reduction in core strength due to extraction, size, and moisture effects, while still ensuring structural adequacy.
Step-by-Step Solution:
Step 1: Identify the governing acceptance clause for cores (average ≥ 85%).Step 2: Check the accompanying requirement for individual cores so that a single weak core does not pass the lot.Step 3: The minimum for any single core is taken as 75% of the specified cube strength.Step 4: Therefore, the correct limit for “no individual core” is 75%.
Verification / Alternative check:
Typical national standards and many QA manuals list 85% (average) and 75% (single core) as paired criteria for core-based acceptance, reflecting empirical reductions observed for cores versus standard cubes.
Why Other Options Are Wrong:
90%: Too conservative for individual cores; not the standard limit.
85%: This is the average criterion, not the minimum per-core value.
80%: Not a standard threshold in IS-based acceptance for individual cores.
70%: Too low; would risk accepting under-strength concrete.
Common Pitfalls:
Final Answer:
75%
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