Grade of Concrete
The desired properties of concrete are its compressive strength, tensile strength, shear strength, bond strength, density, impermeability, durability, etc. Among these, the property that can be easily tested, and is perhaps the most valuable (from the viewpoint of structural design) is the compressive strength.This is measured by standard tests on concrete cube (or cylinder) specimens.Many of the other important properties of concrete can be inferred from the compressive strength, using correlations that have been experimentally established.
The quality or grade of concrete is designated in terms of a number, which denotes its characteristic compressive strength (of 150 mm cubes at 28-days), expressed in MPa (or, equivalently, N/mm 2 ). The number is usually preceded by the letter ‘M’, which refers to ‘mix’. Thus, for example, M 20 grade concrete denotes a concrete whose mix is so designed as to generate a characteristic strength of 20 MPa; the meaning of this term is explained in the next section.
In the recent revision of the Code, the selection of the minimum grade of concrete is dictated by considerations of durability, and is related to the kind of environment that the structure is exposed to. The minimum grade of concrete in reinforced concrete work has been upgraded from M 15 to M 20 in the recent code revision.However, this is applicable only under ‘mild’ exposure conditions. An exposure condition is considered ‘mild’ when the concrete surface is protected against weather or aggressive conditions and is not situated in a coastal area. Under more adverse environmental exposure ‡ conditions, higher grades of concrete are called for. For ‘moderate’, ‘severe’, ‘very severe’ and ‘extreme’ exposure conditions, the minimum grades prescribed are M 25, M 30, M 35 and M 40 respectively, for reinforced concrete work [Cl. 6.1.2 of the Code].It should be noted that the higher grades specified here are dictated, not by the need for higher compressive strength, but by the need for improved durability.
The need is for ‘high performance’ concrete, and it is only incidental that this high performance (obtained, for example, by reducing the water-cement ratio and adding mineral admixtures such as silica fume) is correlated with high strength. In practice,although M20 is the minimum grade specified for reinforced concrete, it is prudent to adopt a higher grade.
However, there are specific applications that may call for the grade of concrete to be decided on the basis of considerations of strength, rather than durability.For example, the use of high strength is desirable in the columns of very tall buildings, in order to reduce their cross-sectional dimensions; this is desirable even under ‘mild’ environmental exposure. Similarly, high strength concrete is required in prestressed concrete construction [refer IS 1343 : 1980]. The definition of the term ‘high strength’ has been changing over the years, with technological advancements resulting in the development of higher strengths. The present Code (in its recent revision) describes grades of concrete above M 60 as ‘high strength concrete’.Concrete grades in the range M 25 to M 55 are described as ‘standard strength concrete’, and grades in the range M 10 to M 20 are termed ‘ordinary concrete’