Grade of concrete can be defined as the minimum compressive strength of the concrete mix that it acquires or possesses after 28 days of construction under proper quality control. This strength is tested in the lab by testing 15cmx15cmx15cm cube samples of concrete mix that has been cured over 28 days by keeping it moist in a wet gunny bag.
This grade of concrete is denoted by an M which signifies mix followed by the 28 days strength of the mix in numerical form. For example, a grade of concrete M20 here shows that this concrete mix has its minimum or characteristic compressive strength as 20N/mm2(or MPa) after 28 days of controlled curing and quality control.
Different grades of concrete have different proportions of cement, fine aggregate(sand), and coarse aggregate mixed with water. For example, M20 grade concrete is made from a mixture of cement, sand, and coarse aggregate in a 1: 1.5: 3 ratio. The water mixed with it is also determined by a set of standard values of water-cement ratio given depending on the cement content and type of concrete used, i.e., plain or reinforced cement concrete.
The above method of specifying characteristic concrete strength complies with Indian Standards (IS). In other countries, such as the United States and European countries, this method of displaying the compressive strength of concrete is done differently. For example, in European countries, concrete grades are represented as C16 / 20 where C indicates a concrete strength class of 16 in MPa, which is the minimum concrete strength of cylinder test samples (15 cm in diameter, 30 cm in height) at the end of 28 days, 20 is also given in MPa, which represents the minimum strength of concrete in the cube sample (15 cm side) at the end of 28 days.
Similarly, the compressive strength of concrete is expressed as C20 in British Standards (BS) and 2000 in US Standards where 2000 is the strength of concrete in pounds per square inch (psi).
Uses and Classification of Grades of Concrete
The different grades of concrete can be clubbed together under individual categories for better classification and specific usage. The grade of concrete can be classified as Lean Concrete, Ordinary Grade of Concrete, Standard Grade of Concrete, High Strength Grade of Concrete.**
Ordinary Grade of Concrete
M10, M15, M20
Standard Grade of Concrete
M25, M30, M35, M40, M45, M50, M55
High Strength Grade of Concrete
|Concrete Type||Grade of Concrete||Grade Mix Ratio(Cement:Sand: Coarse Aggregate)||Compressive Strength at 28 days(MPa or N/mm2)||Application|
|LeanConcrete||M5||1:5:10||5||Providing uniform surface to foundation concrete and prevent its direct contact from the soil|
|Ordinary Grade of Concrete||M10||1:3:6||10||Used for constructing leveling course, bedding for footing, concrete roads, etc.|
|Standard Grade of Concrete||M25||1:1:2||25||Used in the construction of columns, beams, footings, slabs, etc for mild exposure.|
|M40||Design Mix||40||Used in the construction of Prestresses concrete, slabs, footings, columns, beams, etc.|
|M45||Design Mix||45||Used in the construction of runways, concrete roads(PQC), RCC Cloumns, Pressurised beams, and concrete girders.|
|M55||Design Mix||55||Used in the construction of pressurised concrete piers and girders.|
|High Strength Grade of Concrete||M60||Design Mix||60||Used in the construction of high compressive strength requiring RCC structures like high rise buildings, long span bridges, spillways for dams, etc.Used in harsh construction conditions.|
- Design Mix - The concrete when prepared by mixing cement, sand, and coarse aggregate in a ratio which has been decided after proper trial and error of several options in the laboratory.
The grades of concrete M5, M7.5, M10, and M15 can only be used for Plain Cement Concrete(PCC) and all the other above grades of concrete can be used for Reinforced Cement Concrete.