The grading of concrete is a system that classifies concrete based on its compressive strength. The compressive strength of concrete is the amount of force it can withstand before it breaks. Concrete is graded using a letter and number system, where the letter M stands for “mix” and the number represents the compressive strength in megapascals (MPa). For example, M20 concrete has a compressive strength of 20 MPa.
How to select suitable concrete grade for construction?
Grade of concrete construction is selected based on structural design requirements. There are two types of concrete mixes, nominal mix and design mix. Nominal mix concrete are those which are generally used for small scale construction and small residential buildings where concrete consumption is not high.
Nominal mix takes care of factor of safety against various quality control problems generally occurring during concrete construction. Design mix concrete are those for which mix proportions are obtained from various lab tests. Use of design mix concrete requires good quality control during material selection, mixing, transportation and placement of concrete.
This concrete offers mix proportions based on locally available material and offers economy in construction if large scale concrete construction is carried out. Thus, large concrete construction projects uses design mix concrete. So, suitable grade of concrete can be selected based on structural requirements. Nominal mixes for grades of concrete such as M15, M20, M25 are generally used for small scale construction. Large structures have high strength requirements, thus they go for higher grades of concrete such as M30 and above. The mix proportions of these concretes are based on mix design.
The grading of concrete is important because it helps engineers and builders to select the right type of concrete for a particular project. Concrete with a higher compressive strength is more durable and can withstand more weight. However, it is also more expensive. Concrete with a lower compressive strength is less expensive, but it is not as durable.
The following are the 7 grades of concrete:
- M10: This is the lowest grade of concrete and is used for non-structural applications such as footpaths and driveways.
- M15: This grade of concrete is used for light structural applications such as retaining walls and foundations.
- M20: This is a popular grade of concrete that is used for a variety of structural applications, such as beams, columns, and slabs.
- M25: This grade of concrete is used for more demanding structural applications, such as bridges and buildings.
- M30: This is a high-strength concrete that is used for very demanding structural applications, such as high-rise buildings and dams.
- M35: This is an ultra-high-strength concrete that is used for the most demanding structural applications, such as nuclear power plants and aircraft hangars.
- M40: This is the highest grade of concrete and is used for extreme applications, such as deep foundations and offshore structures.
The grading of concrete is also affected by other factors, such as the type of cement used, the size and shape of the aggregates, and the water-cement ratio. The water-cement ratio is the amount of water used in the concrete mix relative to the amount of cement. A lower water-cement ratio will result in a stronger concrete.
The grading of concrete is an important factor to consider when designing and building structures. By selecting the right grade of concrete, engineers and builders can ensure that their structures are safe and durable.