W/C ratio in concrete

As per IS 456 table 5 for RCC - M 20 : 0.55, M 25 : 0.50, M 30 : 0.45, M 35 : 0.45, M 40 : 0.40

C2S present in cement requires 21 % of water and C3S requires 24% water for complete reaction in concrete. The C-S-H gel requires 15% water to fill all voids inside concrete. Hence average of (21+24 /2 = 23%) + 15% = 38% water is at least required in concrete. However it may be lowered by using admixtures.

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