Concrete is a mixture of cement, sand, aggregate, and water, which hardens with time, when we change the proportion of these components, the strength of concrete changes accordingly.
Take the case of a building. Various structural elements are there in a building like footing, columns, beams, one way slab, etc.
Depending on the type and height of the building, the strength of concrete required for constructing these elements changes, so it is necessary to choose the correct grade of concrete for building structure.
The grade of concrete is nothing but a strength of concrete of that particular mix, a mixture of cement, sand, aggregate, and water in 28days. M denotes the grade of concrete, and M stands for combination.
For example, for M5 grade, the mix proportions 1:5:10 where 1 is the cement ratio, 5 is the ratio of sand, and 10 is the coarse aggregate ratio.
Based on the volume or weight of materials, at least three concrete cubes of 150 ×150 × 150 mm are cast for each mix at 27 degrees Celsius, and they are tested in a laboratory at 28 days. Its mean strength is calculated at 28 days, which is the concrete strength for that particular concrete mix.
Now the question is, why 28 days? Why not 20 or 40. This table shows the compressive strength of concrete with its edge.
Concrete Age Strength %
1 Day 16%
3 Days 40%
7 Days 65%
14 Days 90%
28 Days 99%
Here you can see concrete achieve 40 per cent strength in 3 days, 90 per cent in 14 days, and 99percent strength in 28 days. So, it is clear that concrete gains strength rapidly in the initial days after casting, that is, 90 per cent in only 14 days, and after that, concrete gains only 9 per cent strength in the next 14 days, so the rate of gain of strength decreases since concrete achieve 99 per cent strength in 28 days.
Hence, it's almost close to its final strength, and that's why we use this strength at the base for our design and evaluation.
