Grade of Concrete

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’