Why We Crush Concrete Cube or Cylinder Samples at 28 days

Why We Crush Concrete Cube or Cylinder Samples at 28 days The crushing of concrete cubes or cylinder samples is required in a construction especially concrete structures. The reason why this is to verify the strength given in the design mix. Let say for example for a concrete column commonly it has  60 megapascals of specified strength. When you crush it, the result should be equal or more than 40 Mpa. The specified strength should be achieved at 28 days test age but sometimes we experience failure results at 28days that’s the reason we have spare cubes or cylinder samples. The spares are the alternatives that you can send at 42 days or 56 days when sample fails at 28 days test. Now, there are many questions why we crush concrete cubes at 28days. And you might be the one who’s asking. I was once who are asking about this until I came to know the reason. Read furthermore, you’ll find the answer later on this blog. You also might be asking, why we crush cubes or cylinder at 7 days. In most common concrete elements such as column, beam, slab-on-grade, conventional suspended slab, we start crushing cubes at 7 days. Well, the reason is that it is the time the cast concrete can be loaded and can proceed succeeding construction activities, if only if the compressive test report at 7 days are passed from the condition of 80 percent of the specified strength. If 7 days compressive test result is “Failed” less than the condition above, then now you can use the spare cubes or cylinder samples to test for 14 days and then most probably you’ll get the desired result. But sample test failures rarely happen at 7 days so does 14 days test not matter. Concrete elements are designed on the basis of 28 days cube crushing strength. 28 days cube 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, since your 7 days test and even 14 days are passed. You will then wait for 28 days test. The question now is “why” 28 days. Now here is the answer. The studies show that characteristics strength of concrete is at 99% at 28 days age. And it is said that the final strength will be achieved after 1 to 2 years. It 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 age of cubes or cylinder. Now, you might have questions like: a. What if the test failed at 28 days? What will we do? If the cubes got a failed test result at 28 days test age 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 no. but it’s a spare. So most probably the test beyond 28 days test age is passing. In my experience, it’s very rare to get a failed test results 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 2 years. The usage of 28 days compressive strength for design strength is dependant on Rules of Averages propounded by JACOB BERNOULLI.” Wael said, It is believed that the 28days consideration for the specified strength is based on the studies and tests trials of the cube or cylinders 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 the 99 percent of the specified strength.