As per the IS: 456-2000 the grade of concrete above the M-20 must and should be the design mix. And in the IS 456-2000 there is nothing mentioned about mix ratio 1:1:2 for M-25 grade of concrete. If we consider the ratio1:1:2 for M-20 then the cement content comes in the concrete matrix is 528 kg/m³.

Whereas IS: 456-2000 on the page 19 clause 8.2.4.2 mentioned that the cement content in the concrete not including fly ash and ground granulated blast furnace slag in excess of 450 kg/m³ should not be used in concert matrix unless special consideration has been given in the mix design. Otherwise increased risk of cracking due to drying shrinkage in thin sections, or to the early thermal cracking and to the increased risk of damage due to the alkali silica reactions.

The structural concrete surfaces is exposed to severe rain, alternate wetting and drying such as RCC overhead water tanks comes to severe exposure environment for which the minimum grade of concrete shall be M-30, and minimum cement content 320 kg/m³ and the maximum free W/C ratio is 0.45. 

MIX DESIGN FOR     M-25