The crushing of concrete cubes or cylinder samples is required in a 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 megapascals of specified strength. When you crush it, 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 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 cylinder 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 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 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:
“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.”
“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.”