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Characteristic strength
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 concrete may be 5MPa but a well controlled modern computerised batching plant should be lower. Above 6MPa indicates poor control or a plant issue.
Acknowledgement:
Concrete Society
Concrete Bookshop - Members receive 40% discount on Concrete Society publications
Concrete practice
- guidance on the practical aspects of concreting - GCG8
