Concrete is a mixture of different ingredients like cement, sand, aggregate, and water. When they are mixed with each other, it begins to harden with time. The strength of different grades of concrete depends upon the proportion of different ingredients used in the concrete.
When cement is increased in the right amount its strength increases. In the same way when the sand has increased the strength of concrete decreases.
Concrete is used to construct different members like the beam, column, and slab of the building. Among them, different members need different strengths to make the structure economical and safe.
If we take an example of a building, then the column needs to be constructed stronger than the beam and slab. Because columns have to bear all load from the superstructure and transfer it to the soil.
Moreover, the strength of the column to be prepared also depends upon the load and the position of the column in the building. For example, the column which is at the lowest part of the building needs to be stronger than the upper column of the building.
Due to these reasons, different grades of concrete is necessary to prepare for different strength. So, the strength of concrete is very necessary for economical and safety purposes. If the grade of concrete is unnecessarily high then the structure will be uneconomical. If the grade of concrete is low then the structure will be unsafe. So, concrete is carefully designed for the required load.
The strength of concrete is measured in 28 days. It has symbols like M35, M30, M25, M20, M15, and so on. Here, “M” denotes the mix ratio of concrete and numerical value shows the value of grade of concrete.
For example,
If we take the M20 grade of concrete then, M shows that it has a mixture of cement, sand, and aggregate in 1:2:3 in proportion. And 20 shows that the compressive strength of concrete is 20 KN/mm2
There are different processes to measure the strength of concrete. The strength of concrete is measured in the laboratory at different intervals of time till 28 days. The compressive strength of concrete is generally measured after 7 days, 14 days, and 28 days.
To measure the strength of concrete numbers of cubes having dimensions 15 cm x 15 cm x 15 cm are prepared and tested at 27 degrees celsius.
According to IS code, Concrete takes 40% of its strength in 3 days, 90% of strength in 14 days, and 99% of its strength in 28 days. So, we can conclude that the concrete gains most of its strength in the initial 14 days and only 9% in the next 14 days.
And, this is the final strength of concrete which is used as the base for our design and evaluation of the structure.
Mix proportion of Concrete grades
| S.N | Grade of Concrete | Mix Ratio (Cement:Sand:Aggregate) | Compressive Strength of Concrete |
|---|---|---|---|
| 1 | M5 | 1:5:10 | 5 Mpa |
| 2 | M7.5 | 1:4:8 | 7.5 Mpa |
| 3 | M10 | 1:3:6 | 10 Mpa |
| 4 | M15 | 1:2:4 | 15 Mpa |
| 5 | M20 | 1:1.5:3 | 20 Mpa |
| 6 | M25 | 1:1:2 | 25 Mpa |
In the above table, we can see 4 columns with different headers. The first column is the serial number. The second column is the grade of concrete. The second column has values M5, M7.5, M10, etc.
Let’s take one of the values M15. Here in M15 M indicates the mix ratio of cement, sand, and aggregate which is given in column three, and 15 indicates the strength of concrete in Mpa or N/mm2 which is given in column four.
Different grades of concrete and their uses
The different grades of concrete and their uses are given below.
M5 and M7.5 grades of concrete
This type of concrete is used as a filling material like sand and clay. It is also used as the floor concreting.
M10 grades of concrete
This type of concrete is used to construct the pathway ie. RCC Roads, wall, non-load bearing structure, and boundary wall column.
M15 grades of concrete
M15 grade of concrete is used to construct low load-bearing beam, slab, and foundation PCC works.
M20 grades of concrete
M20 grade of concrete is used to construct beam, slab, and also a column for residential as well as a commercial building having up to 3 to 5 storey.
M25 grades of concrete
M25 grade of concrete is used to construct the column of the building as well as the structure like a culvert, load-bearing wall, etc.
M30 grades of concrete
M30 grade of concrete is durable and weather-resistant concrete. So, This grade of concrete is used to construct underground foundations for tall and heavy buildings.
M35 grades of concrete
M35 grade of concrete is used to construct high-strength structures like bridges, piles, columns of multi-storey buildings.
M40 grades of concrete
M40 grade of concrete is high strength concrete, It is used to construct the underwater structure like pile or pier of the bridge, a column of the multi-storey building, RCC or PCC Roads, Retaining wall, etc.
Compressive strength of different grades of concrete at 7 and 28 days
The compressive strength of different grades of concrete at different intervals of time in N/mm2 or Mpa are given below.
| S.N | Grades | 7 Days | 14 Days | 28 Days |
|---|---|---|---|---|
| 1 | M 10 | 6.5 N/mm2 | 9 N/mm2 | 9.9 N/mm2 |
| 2 | M 15 | 9.75 N/mm2 | 13.5 N/mm2 | 14.9 N/mm2 |
| 3 | M 20 | 13 N/mm2 | 18 N/mm2 | 19.8 N/mm2 |
| 4 | M 25 | 16.25 N/mm2 | 22.5 N/mm2 | 24.8 N/mm2 |
| 5 | M 30 | 19.25 N/mm2 | 27 N/mm2 | 29.7 N/mm2 |
| 6 | M 35 | 22.75 N/mm2 | 31.5 N/mm2 | 35.6 N/mm2 |
| 7 | M 40 | 26 N/mm2 | 36 N/mm2 | 40.6 N/mm2 |
| 8 | M 45 | 29.25 N/mm2 | 40.5 N/mm2 | 45.6 N/mm2 |
| 9 | M 50 | 32.5 N/mm2 | 45 N/mm2 | 49.5 N/mm2 |
Percentage of Compressive strength of different grades of concrete at 1, 3, 7, 14, and 28 days.
| S.N | Age | Strength Percentage |
|---|---|---|
| 1 | 1 Day | 16% |
| 2 | 3 Days | 40% |
| 3 | 7 Days | 65% |
| 4 | 14 Days | 90% |
| 5 | 28 Days | 99% |
Water cement ratio for different grades of concrete
Water cement ratio in concrete depends upon exposer and types of structure to be constructed. Like reinforced structure (Having Steel) or plain concrete (Without Steel).
According to IS Code 456:2000, the data are given below.
| S.N | Grade of Concrete | Water Cement Ratio |
|---|---|---|
| 1 | M7.5 | 0.40 |
| 2 | M10 | 0.45 |
| 3 | M15 | 0.50 |
| 4 | M20 | 0.55 |
| 5 | M25 | 0.60 |
For more detail on the water-cement ratio, Read this article
Slump value for different grades of concrete
There is no actual value of slump for different grades of concrete. But, we can take the approximate value as per experience by civil engineers given below.
- Slump value for M10 to M20 can be taken as a minimum of 20 and a maximum of 30.
- Slump value for M20 to M30 can be taken as a minimum of 25 and a maximum of 75.
- Slump value for M30 to M40 can be taken as a minimum of 50 and a maximum of 100.
- Slump value for M40 to Above can be taken as a minimum of 75 and a maximum of 150.
I hope this article on “Grades of concrete | Mix proportion | Water cement ratio | Slump value” remains helpful for you.
Happy Learning – Civil Concept
Read Also,
Mix design of concrete for Grade M35, M30, M25, and M20 using IS Code
Factors affecting workability of concrete | How to increase workability
Thermal properties of concrete | Shrinkage, Creep, and Strength with Age
High Strength Concrete | How to Prepare High Strength Concrete?
