In concrete mix design and quality control, the characteristic strength fck is defined as the compressive strength below which not more than what percentage of test results are expected to fall?

Introduction / Context:
Characteristic strength, denoted fck, is the statistical backbone of design in reinforced concrete. It links laboratory test variability to field safety by defining a strength level that most test results will exceed. Codes use this concept to choose mix proportions and set acceptance criteria for site cubes or cylinders.

Given Data / Assumptions:

• Concrete strength tests show normal-like variability.
• Design employs a lower fractile (a percentile) to account for scatter.
• Quality control aims so that only a small fraction of results fall below fck.

Concept / Approach:
By definition, characteristic strength is the value below which not more than 5% of test results are expected to fall. This corresponds to the 5th percentile of the population distribution. Mixes are proportioned so that the mean exceeds fck by a margin related to standard deviation, ensuring compliance despite fluctuations in materials and workmanship.

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