Why Do Civil Engineers Test Compressive Strength of Concrete at 28 Days?

# Why Do Civil Engineers Test Compressive Strength of Concrete at 28 Days?

Use the below table to find out the compressive strength gained by concrete after a specified age in relation to the grade of concrete used.

Viewing the table, you can discover that concrete gains 16 percent strength in one day, 40 percent in 3 days, 65 percent in 7 days, 90 percent in 14 days and 99 percent strength in 28 days.

Therefore, concrete gains strength rapidly in the first two weeks after casting (90% in just 14 days). In 28 days, the concrete’s strength will have reached 99% and continues to gain strength in the future. However, the rate of gain advances at a much-reduced pace when compared to the first 28 days.

Because concrete compressive strength is at 99% in 28 days, it is very close to its final strength which could be in 1 or 2 years time. So engineers rely on the results of compressive strength test after just 28 days and use this number in any design calculations.

As per IS 456 and BS CP 114 concrete gains 120 % -124 % strength in one year. This is a pretty long time to test concrete and commence the project. Hence the principle of the ‘ Law of Averages’ ( LOA) is adopted.

The LOA states: “the principle that, in the long run, probability as naively conceived will operate and influence any one occurrence.

The LOA was formalized as a law of large numbers (LLN) and was first proved by Jacob Bernoulli. It took him over 20 years to develop a sufficiently rigorous mathematical proof which was published in his Ars Conjectandi (The Art of Conjecturing) in 1713. He named this his “Golden Theorem” but it became generally known as “Bernoulli’s Theorem”. This should not be confused with Bernoulli’s principle, named after Jacob Bernoulli’s nephew Daniel Bernoulli., who gave the most important principle in Fluid Dynamics. Thus by Statistical principle the 28 day compressive strength of Concrete is relied upon by The Art of Conjecturing, propounded by Jacob Bernoulli.

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7. Great explanation.. especially for upcoming Engineers

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