How to calculate average strength of cubes with many samples?

[CivilTechie]

Can some one kindly explain to me on how to calculate overall average strength of cubes for more than 5 samples. Also how to calculate the overall average strength of cubes if we get more than 15% variation in single individual cube of a sample. Should we discard the sample with variation ? But if we go with IS 516 part 1 sec 1 it says take the nearest average values of 2 cubes if we dont have more samples left. Kindly help me in clearing my doubts regarding the variation part mentioned in IS 516 Part1 Sec1

 

[dhirajnbachhav]

As per IS 516 (Part 1/Sec 1), the average compressive strength is normally calculated from three cubes of the same sample.

• If the variation between individual cube strengths is within the permissible limit, the average of all three cubes is reported.
• If one cube shows more than 15% variation from the average of the three, it is treated as an outlier. In such cases, as per IS 516, the result may be taken as the average of the two nearest values, provided there is a valid reason to believe the third result is anomalous (e.g., casting, handling, or testing error).
• The entire sample should not be discarded automatically. The decision should be based on technical judgment and the provisions of the code.
• For multiple samples (e.g., many sets of 3 cubes), first determine the average strength of each sample, then calculate the overall average by averaging the accepted sample averages. Do not average all individual cube results together if they belong to different samples.

In short, the 15% variation rule applies within a single sample, not across all samples. Each sample should be evaluated independently before calculating the overall average.