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.