Difficulty: Easy
Correct Answer: 5% of results fall
Explanation:
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:
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.
Step-by-Step Solution:
Recognize fck as a fractile, not the mean.Use acceptance criteria that relate target mean = fck + t * s, where s is standard deviation and t depends on selected risk level.Confirm that the adopted percentile is 5%, hence Option “5% of results fall.”Verification / Alternative check:
Check cube test histograms; approximately 95% of compliant results should exceed fck if the process is centered at the target mean.Why Other Options Are Wrong:
10% or 2%: not the conventional percentile used for fck in standard practice.“None of the above”: incorrect because 5% is the standard definition.Common Pitfalls:
Treating fck as an average; ignoring required margin above fck in mix design.Final Answer:
5% of results fall
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