Concrete core acceptance by equivalent cube strength (IS-based): For accepting in-situ concrete by core tests, the average equivalent cube strength of the extracted cores must be at least 85% of the specified cube strength of the concrete grade at the specified age. No individual core result shall be less than what minimum percentage of the specified cube strength?

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:

• Acceptance is based on equivalent cube strengths from drilled cores.
• Average of cores must reach at least 85% of the specified characteristic cube strength.
• There is also a lower bound for each individual core result.

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:

• Confusing the average (85%) with the minimum individual limit (75%).
• Comparing core strengths directly to cube strengths without applying the code-prescribed criteria.

Final Answer:75%