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

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

M25 Grade Concrete Is not a Nominal Mix Concrete

M25 grade concrete is a high-strength concrete and should be designed as a Design Mix concrete, not a Nominal Mix concrete. Nowhere in the Codebook or in textbook is it stated that the Volumetric Mix ratio of M25 grade concrete is 1:1:2. In fact, the code book IS456 (clause 9.3) prohibits the use of Nominal mixed concrete for grades greater than M20.

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

– Clause 9.3, IS456

High Cement Content than the allowed limit

Maximum cement content allowed by the IS456 codebook is 450 kg/m³. Excessive cement use increases the risk of damage due to shrinkage cracking, thermal cracking and high 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 us calculate the weight of cement per cubic meter for M25 Mix Ratio as per Nominal Mix Method

• The alleged M25 Mix Ratio = 1:1:2
• The volume of cement required per cubic meter of 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 according to the nominal mixing method for M25 grade concrete is 540 kg / m3, which is 90 kg more than the allowable limit 450 kg/m³ as mentioned Section 8.2.4 of the IS456 code book.

What we can do?

Experienced and skilled engineers will not calculate or batch materials for M25 grade concrete using Nominal Mix method at the construction site. They use design mix method or choose ready mix concrete. It is disheartening to see so many websites misleading students and freshers just for page views.

Please Share this information to prevent the misconception about M25 grade concrete that are spreading among the civil engineering community. Thank you.