Building Materials, Third Edition

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This approach is contrary to the recommendations given in IS: 10262–1982, where the
recommended standard deviation increases with the increase in specified characteristic strength.
The BRE approach seems to be more logical and is backed up by observation of test results of
concrete made at a large number of ready mix plants in the UK. In order to arrive at the target
strength for mix design which is always higher than design strength established statistical
concepts are used. The results of cube strength of concrete cast at site on random sampling of
concrete follow the normal distribution curves when results of a large number of cube strengths
are plotted against the frequency of occurrence. Figure 11.7 shows the properties of normal
distribution curves.

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