Learn why Mix Ratio of M25 Grade is NOT 1:1:2 | Engineering Blunders

Learn why Mix Ratio of M25 Grade is NOT 1:1:2 | Engineering Blunders

Many entry-level civil engineers and students believe that the Mix ratio of M25 grade concrete is 1:1:2. This is a very common misconception. There are two reasons why the M25 concrete Mix ratio is not 1:1:2 and they are explained in this article.

M25 Grade Concrete Is not a Nominal Mix Concrete

M25 grade concrete is strictly a Design Mix Concrete and not a Nominal Mix concrete. It should be designed through the Design Mix procedure alone. The codebook or textbook did not mention in anyplace that the volumetric Mix ratio of M25 grade concrete is 1:1:2. In fact, IS456 (clause 9.3) codebook clearly says that Nominal Mix can be used only for M20 concrete or lower.

Nominal Mix concrete may be used for concrete of M20 or lower.

– CLAUSE 9.3, IS456

High Cement Content than the allowed limit

The maximum cement content allowed by the IS456 codebook is 450 kg/m³. Using excess cement will increase the risk of shrinkage cracking, thermal cracking, and damage due to alkali-silica reaction.

Cement Content not including fly ash and ground granulated blast furnace slag in excess of 450 kg/m³ should not be used unless special consideration.

– CLAUSE 8.2.4.2, IS456

LET’S CALCULATE THE WEIGHT OF CEMENT PER CUBIC METER FOR ALLEGED M25 MIX RATIO AS PER NOMINAL MIX METHOD

If you want detailed calculation you can refer to this post for calculation procedure for Nominal mix concrete.

• Alleged M25 Mix Ratio = 1:1:2
• The volume of cement required per cubic meter for M25 grade ratio concrete= (1 / (1+1+2)) * 1.5 = 0.375 m3
• Bulk density of cement is 1440 kg/m3
• Therefore weight of cement required = 0.375 * 1440 = 540 kg/m3

As you can see the weight of cement calculated as per the Nominal Mix method for M25 grade concrete is 540 kg/m3, which is 90 kg more than the allowed limit by Clause 8.2.4 of IS456 codebook.

what we can do?

Engineers don’t calculate or batch materials for M25 grade concrete using the Nominal Mix method in the construction site. They use the design mix method or opt for ready mix concrete. It is disheartening to see many websites misguide students and freshers for mere page views.

Please Share this information to curb the misconception about M25 grade concrete that spreads among the civil engineering community. Thank you.

Er.SP.ASWINPALANIAPPAN., M.E.,(Strut/.,)

Structural Engineer

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