If the cube test results show the concrete strength failure, that was an awkward moment for an Engineer. 

If you get the failure result of a concrete cube test, no need to worry about it! 

There is a way to find out if the cube really failed or not. Sometimes we may implement the cube test wrongly, or the cube was cast with more slurry; it is called human error.

We are going to discuss the criteria mentioned in the IS code to accept or neglect the concrete cube test results.

Before getting into the discussion, you must read the Method of Concrete Tests.

Reject the Cube Specimen when you notice the following points!.

  • If it has more slurry
  • If it is porous or honeycomb
  • If it is not in even shape
  • If the weight of the concrete cube is below 7.5 Kg

Avoid the cube specimen when you meet any one of the above criteria because the cube results will surely show failure results.

Acceptance Criteria of Concrete Cube Test Results

We are well aware that the Compressive strength value should be taken as the average strength of the three specimens.

specimen under testing machine

  • The individual variation of the specimens should not be more than ± 15% of the average strength if more than the test specimens are invalid.

For example, If you get the three specimens Crushing Strength Value 20,25,30, respectively. The average compressive strength value is 25, but the individual variance between the sample is more than 15%.

Hence the Specimen sample was invalid.

The acceptance criteria have illustrated for both concrete compressive and flexural strength.

Compressive strength

  • The mean compressive strength of concrete should be considered from four consecutive test results, and it should comply with the limits, as shown in the table below.
  • The individual test result should comply with the appropriate limit, as shown in the below table.
Acceptance Criteria Concrete Cube Test Results Mean Strength in N/Sq.mm Individual Test Results in N/Sq.mm
M15 ≥ Fck+0.825 x Std Deviation or Fck+3 N/Sqmm Whichever is greater ≥ Fck-3 N/Sqmm
M20 or Above  ≥ Fck+0.825 x Std Deviation or Fck+4 N/Sqmm Whichever is greater ≥ Fck-4 N/Sqmm

Flexural Strength

  • The mean flexural strength of concrete should be considered from four consecutive test results exceeding the specified characteristic strength by at least 0.3 N/Sqmm.
  • The determining flexural strength should not be less than the compressive strength of less 0.3 N/Sqmm.

Want to know about the Calculation of Standard deviation of concrete. Check out the post.

Author

Bala is a Planning & QS Engineer working at Megha Engineering & Infrastructure Limited who has more than 9+ Years of experience in Building & Infrastructure. He is the author, editor of Civil Planets.

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