Standard deviation is a measure of the variation of concrete strength of a set of data from its mean. If the strength results are away from the mean, the deviation will be higher and vice a versa. Its a statistical measure for Re-designing the mix base on results achieved. In initial mix...
To check the standard deviation, you need to have minimum 30 results (30 samples, 1 sample made of 3 cubes) of the concrete which is in question.
Once you have 30 results, you can plot it and see the control level.
following possibilities can be seen in deviation report
SD is on higher side - it shows the very poor control on either production, sampling, cube casting, curing or deviation in cube testing machine.
SD is Good and results are on higher side - indicates the need of optimizing the mix so that you can reduce the national resource wastage.
SD is good but results are falling below required or very marginal - indicates the need for optimization of mixes.
There is lot you can see by plotting the results or by using line control graph to see deviation every day and doing corrections in process wherever required.
SD is a mathematical calculation figure, so might be a value of SD can be excellent or good but your concrete might be having bad control which giving you some time very high results and sometime very low results.
We are the processor of the data and we need to detect it so that we can avoid failures.