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 in building various structural elements that are there like footing, columns beams, slabs, 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. Grade of concrete is nothing but a strength of concrete of that particular mix that is a mixture of cement, sand, aggregate, and water in 28days. M denotes grade of concrete, and M stands for the mix.
For example, for M5 grade, the mix proportions 1:5:10, where 1 is the ratio of cement, 5 is the ratio of sand, and 10 is the ratio of coarse aggregate. 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, and that is the strength of concrete for that particular 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%
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. 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 so it's almost close to its final strength and that's why we use this strength at the base for our design and evaluation.
Now, coming back to concrete grades, this table shows the proportion of cement, sand, aggregate, and water in different grades of concrete mix ratio. For M5 grade, the mix proportion is 1:5:10, and the quantity of water required shall be 60 litres. In the same way, you can see the mix proportions of other grades of concrete up to m25. Up to M25 nominal mix can be made, which shall be used for small-scale construction where actual consumption is not high, but for M 30 and above grade design mix should be used.
Here you can see as the strength of concrete increases. The cement content is constant in proportion, but sand, aggregate, and water content are reducing. So, if you add more water to the concrete, the strength of concrete decreases and vice versa. I have observed that in most cases, the concrete grade of foundation and columns will be more than slabs and beams, please note that this is not the general rule that's why I said in most cases
The reason number one is foundations and columns are compression members whereas beams and slabs are tensile members
The reason number two is the load shall be transferred from slabs to beams, beams to columns and columns to foundations.
The third reason is it is economical to reduce the grade of concrete for slabs and beams.
Friends, I hope you have understood the different grades of concrete and their uses like this article if you learn something and be sure to share it with someone who might benefit from it.