In this article, I will describe why we test concrete strength after 28 days.
As we know the strength of concrete is normally tested after 28 days. As time passes concrete gains strength but it is still unknown how much time concrete takes to gain 100 % strength. In the first 28 days, concrete gains its 99 % strength.
The percent strength of concrete for 1, 3, 7, 14, 28 days is as follows.
| Age of Concrete | Strength % |
| 1 day | 16 % |
| 3 days | 40 % |
| 7 days | 65 % |
| 14 days | 90 % |
| 28 days | 99 % |
From the above table, you can see concrete achieve 16% strength in 1 day, 40% in 3 days, 65% in 7 days, 90% in 14 days, and 99% in 28 days.
After 28 days this process slows down that is why we calculate the strength of concrete on the basis of 28 days.
It is still not known how much time concrete takes to gain 100% strength but it is assumed that it takes one year to achieve 100% strength.
99% is very close to the final strength of concrete that is why we consider 99% strength for design calculation.
That was the phenomenon behind the question of why we test concrete strength after 28 days.