Actually I have exposed concrete cubes in sulphates salts up to 180 days and found compressive strength and weight loss of the specimens with respect to water cured specimens...How many minimum samples I have to test for std deviation calculation?
E.g If have n=15 no.of cubes.you have tested and you have also strength of each cube. First find out mean strength summation of all strength divided by 15. Once you find it find deviation in strength of each by subtracting obtained strength from mean strength.then use formula of STD deviation.Square root of summation of all deviations of 15 cubes divided by 15 minus 1