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Hello,

what is the recommended concrete mix ratio for?:

a) RCC (M15 or M20 or ?)
b) PCC (M15 or M20 or ?)
c) external plastering (1:4 or 1:6 or ?)
d) plinth beam (M15 or M20 ?)

Hi,
for your a, b, d question,
specification will depend on strength that needs to be achieved vs. size of structure . For e.g. same strength column or beam when made in M20 will be thinner as compared to M15 as in M20 cement ration will be more. It is decided by structural engg. Usually now a days it is always M20

c) external plastering (1:4 or 1:6 or ?) -> 1:4

I am pasting here my reply in another thread for the same question:

Use M15 grade concrete for Footings, Beams and Slab and
Use M20 grade concrete for Columns.

M15 in theory means, 1 part of cement, 2 parts of sand and 4 parts of stone. But in practice, it is advisable to go for 1 part of cement, 3 parts of sand and 4 parts of stone.
In terms of pots it would be 4 pots of cement, 12 pots of sand and 16 pots of stone.

M20 in theory means, 1 part of cement, 1.5 parts of sand and 3 parts of stone. But in practice, it is advisable to go for 1 part of cement, 2 parts of sand and 3 parts of stone.
In terms of pots it would be 4 pots of cement, 8 pots of sand and 12 pots of stone.

Increasing sand ratio helps in reducing honeycombs in concrete, which are very common if vibrator is not used.

While concreting, the best practice is to use water reducing agent like conplast(Fosroc) to maintain water:cement ratio, and using a pin vibrator for columns, footings, beams and pin as well as plate vibrator for slabs. Many contractors use only plate vibrator for slabs, as putting pin vibrator across the whole slab area is a cumbersome process. But if we can stand at the site, and insist on using pin for slab, it does make the RCC slab very compact and strong. One can easily see the amount of compaction a pin vibrator brings in a slab visually, as it reduces the height of the concrete poured on to the slab substantially, while a plate vibrator simply levels and gives a smoother finish to the roof. I am not saying plate vibrator is not effective - I am only saying both have their roles to play in the slab concreting.