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Characteristic strength is defined as that level of strength below which a specified proportion of all valid test results is expected to fail.Unless otherwise stated, this proportion is taken to be 5%.
Due to the variability of constituent materials and testing, the concrete must be designed to meet a target mean strength, i.e. a margin above the characteristic strength is required to give a 95% confidence in achieving the characteristic. The margin is based on 1.64 standard deviations (sd). Typical production sd for cocnrete may be 6MPa but well controlled production can be much lower.
Acknowledgement: Concrete Society
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