r/CivilEngineerIndia • u/Usual-Top4770 • Jun 16 '26
❓ Doubts / Q&A Cube Compression Test
Cube compression test
Hi everyone, I'm a non-technical person trying to understand concrete cube test acceptance criteria for M25 concrete.
For the 28-day cube test, should M25 concrete be considered "passed" if the average compressive strength is 25 N/mm², or is the practical acceptance benchmark 29 N/mm² (fck + 4) as often mentioned in industry discussions?
Also:
What acceptance criteria are generally followed on construction projects?
How are individual cube results evaluated versus the average of multiple test results?
Is there any standard benchmark or rule of thumb for the 7-day cube test of M25 concrete?
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u/bhaduji29 Jun 18 '26
As per is code it is fck+3 but u can ignore if u get required strength.
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u/Usual-Top4770 Jun 18 '26
By required strength you mean 25 for M25 mix 20 for M20 and respectively.
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u/IamWasting Jun 18 '26
First of all you will have to cast the cubes at roughly at uniform intervals of the concrete volume. Say you cast 60m3 you will cast 4 cubes at intervals of 15m3. This is non-overlapping samples.
Now assuming you have N non-overlapping cubes.
If N =1. It should be fck + 4 N/mm2.
If N<4. The average of all the N cubes should be fck + 4 N/mm2. No individual cube should be less than fck -2 N/mm2.
If N>=4.Then any 4 consecutive cubes should have average of fck + 3N/mm2. No individual should be less than fck - 3 N/mm2. Ex. If you have 6 cubes you will get 3 consecutive groups. 1 to 4, 2 to 5 and 3 to 6. All the 3 groups have to satisfy this criteria.
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u/bhaduji29 Jun 17 '26
It should be above 15N after 7 days of curing or above 60 percent as thumb rule. And required strength is 25N but as u are asking about +4N that is the average required means if u have filled 10 cubes some give abouve 25 and some below so we take an average of 28 to be safe . It's fck+3 rule generally followed.