The crushing of concrete cubes or cylinder samples is required in construction especially concrete structures. The reason for this is to verify the strength given in the design mix. Let’s look at an example of a concrete column. Commonly, it has 60 mega pascals of specified strength. When you crush concrete cubes, the result should be equal to or more than 60 Mpa.
The specified strength should be achieved at a test age of 28 days but sometimes we experience failure results at 28 days. That’s the reason we have spare cubes or cylinder samples. The spares are the alternatives that you can check at 42 days or 56 days when a sample fails at 28 days test.
Now, there are many questions about why we crush concrete cubes at 28 days. And you might be asking why. I was once the person asking about this until I came to know the reason. Read further and, you’ll find the answer later in this blog.
You also might be asking, why we crush cubes or cylinders at 7 days. In most common concrete elements, such as columns, beams, slab-on-grades, and conventional suspended slabs, we start crushing cubes at 7 days. The reason is that it is the time the cast concrete can be loaded, and we can proceed with the following construction activities, but only if the compressive test report at 7 days has passed the condition of 80 percent of the specified strength.
If a 7-day compressive test result is “failed” by having less strength than the condition stated above, then you can use the spare cubes or cylinder samples to test at 14 days and then most probably you’ll get the desired result. But sample test failures rarely happen at 7 days, so 14 days test does not matter.
Concrete elements are designed on the basis of a 28-day cube’s crushing strength. A 28-day cube’s strength actually represents the characteristic strength of the concrete. It is mandatory to test the concrete cubes or cylinders at the age of 28 days as per almost all building code requirements.
Now, once your 7-day test and even your 14-day test have passed, you will then wait for the 28-day test. The question now is: “Why” 28 days?
Now here is the answer.
The studies show that the characteristic strength of concrete reaches 99% at 28 days of age. And it is said that the final strength will be achieved after 1 to 2 years. This means that a concrete slab or beam which has a specified strength of 40 megapascals or Mpa (N/mm2) shall be almost attained at 28 days of age in cubes or cylinders.
Now, you might have questions like:
What if the test fails at 28 days? What will we do?
If the cube has a failed test result at 28 days, then your spares will be used to test on either 42 days or 56 days, depending on what test age you prefer beyond 28 days.
Two cubes will be tested (or crush concrete cubes) again with the same cube ID number, but these will be spares.
So most probably, a test beyond the 28-day test is likely to pass. In my experience, it’s very rare to get a failed test result at 42 days.
Here are some answers that you can consider following:
Hayrettin said”
“Although the ultimate strength varies with the kind of cement and curing duration, the concrete compressive strength in 28 days, is very near to its final strength that could be reached in one or two years. The usage of 28 days compressive strength for design strength is dependent on the Law of Averages, propounded by JACOB BERNOULLI.”
Wael said:
“It is believed that the 28 day consideration for the specified strength is based on the studies and test trials of the cube or cylinder sample. It is found that at 7 days you will get the 80 percent of the specified strength and at 28 days you will get 99 percent of the specified strength.”
Related Article:
Did you know why we crush concrete cubes? If so, please write down your experience. Love to know it!
You can share your answer in the comment box below. Let me see your answers.
Thanks for reading.
Sir i got 99% concrete compressive strength in 7 days . What are the reasons that strength is very high after 7 days?
It is normal especially if you use concrete admixture and cement replacements GGBS and microsilica. If these three will work together your concrete will gather high strength at a very early age. How much more after 28days.
Dear Mr.Neol,
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Welcome and thanks too.
Hello Noel,
Your all posts are very good and helpful for me.
Thanks for sharing good knowledge.
Good evening sir
Very useful information
Sir, I have question regarding crushing of cylinders #7 or 28 days…..
Why we crush cylinders @7 and 28 days??? Why not at 10 or 30 days???
As you said above that we curing the concrete at 7 days, so we tested the cylinders at 7 days if it shows the strength 80% @7 days.
But my question is still the same why 7 days, why not 6 or 8 days???
What are the specific reasons???
Is there any cement, aggregates reactions or what??? Or strength gaining procedure? ??
Time frame???
Or is there heat of hydration effects up to 7 days???
I’m sorry, simple saying that standard codes mentioned like that can’t be satisfied? ? Bcz the codes also written by someone and it can be elaborate or explained by the research….
Thanks and regards
Tayyab Malik
Please kindly improve your grammar to better understand your explanations. There are certain grammatical errors which cannot be understood by common persons. This is not to offend you but to help other readers understand simple grammatically correct English.
Hi Jil,
Thank you for reminding me and I admit there was a grammar error but I already corrected that.
Thanks again.
Hello Noel,
What to do and what will going on if the 42 or 65 days test get failed result?
Thank in advanced.
Hi Sey,
You can still send your spare cubes for a 56-day test and for over 65 days, you must discuss with your concrete supplier what the best thing to do is, so you come out with the best decision. You may also want to conduct coring to verify the actual concrete strength. Because sometimes cubes are not properly made, that is one of the reasons for a failed test report. I had an experience where I tested the cubes for 90 days and the results were too high, meaning the concrete would still gain strength after 3 months and beyond.
I’m sorry, according to my opinion the crushing of cylinder can’t give us 100% surety that our concrete is good. The crushing of cylinders only can give us some idea or some satisfaction that our concrete quality is good…..
My opinion is; we should concentrate more on fresh concrete quality tests… If our fresh concrete quality tests results are within limits then the crushing of cylinders if shows variation then it’s not a big problem. ..
If cylinders fail then qe can’t say I your concrete quality is not good or if cylinders are passed also we can’t say our concrete quality is very good…
Taking cores also can’t give us exact result if our all poured concrete.
In my experience, Most of the client they will request to conduct core testing if the 28 days cubes failed. And they will decide if need to demolish or accept based on the result.