Interpretation of Cube Test Results for Acceptance of Concrete
IS 456 elaborates on sampling requirement and acceptance criteria for concrete. However, the criteria is often misunderstood, leading to incorrect interpretation of test results. Many times a concrete that meets the criteria mentioned in IS 456, is still considered as a failure and vice versa.
Clearly it is the sample average and not individual cube result that is to be used for finding standard deviation and acceptance of concrete. Individual cube results become nonrelevant after finding the sample average.
Average of Group of 4 consecutive, nonoverlapping test results is considered for acceptance. Individual sample may fall short of grade of concrete by 3 Mpa but average of Group of 4 consecutive samples needs to be above the grade of concrete by a margin. This margin is defined as greater of 3 MPa or 0.825 times the standard deviation. The criteria is based on the assumption that concrete members resist forces as a group and not as individual members.
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