Whenever testing for cube strength, it is always advised to make and test three specimens from the same mix. This is done to reduce or average out the error due to manually introduced differences in the three cubes.

Now, concrete and cement are always tested at both 7 days and 28 days for compressive strength. The compressive strength at seven days is known to be 65% of the final (28 days) strength. This double testing is done so that if the seven days strength is found inappropriate, the cubes may be straight away discarded to save 21 days’ time because the 28-day strength will also probably be below par. Now, a time even earlier than seven days is not chosen because, at that age, strength gain is very steep, and there’s a higher margin of error. Also, seven days is a round figure.

For both the tests, 3 cubes each are employed. Hence, a total of 6 cubes go through the compression test.

6 cubes are casted for testing the compressive strength because first 3 cubes are tested for 7 days and another for 28 days. The cubes tested for 7 days should gain 70% of strength and cubes tested for 28 days should gain 99% of strength.