Definition of characteristic strength of concrete The characteristic strength is defined as the value below which not more than what percentage of test results are expected to fall for a specified size specimen at 28 days?

Introduction:
Concrete strength is inherently variable. The concept of characteristic strength provides a statistically meaningful design value that ensures a low probability of falling below a specified threshold. This question checks whether you know the conventional percentage associated with this definition.

Given Data / Assumptions:

• Standard 28-day test on 150 mm cubes is considered.
• Normal quality control and sampling frequency are in place.
• Statistical basis uses the lower tail of the strength distribution.

Concept / Approach:

The characteristic strength fck is defined such that not more than 5% of test results are expected to be less than fck. This corresponds to a lower fractile of the population strength distribution and underpins acceptance criteria and mix design margins.

Step-by-Step Solution:

Verification / Alternative check:

Quality control formulas (mean target strength = fck + k * standard deviation) commonly use k near 1.65, which corresponds to the 5% fractile for a normal distribution.

Why Other Options Are Wrong:

2–4% do not match the conventional definition; 10% would be too lenient and reduce safety margins.

Common Pitfalls:

Confusing fck with average (mean) strength; forgetting that acceptance involves both individual and group criteria tied to variability.

Final Answer:

5%