When the ratio is in proportion, say 1:2:4 , 1:3:6, 1:4:8 etc.they are always in vol. When strength is specified by m10, 20,25 etc,. – it is a design mix. to deliver the required strength of conc.i.e 10 N/mm² etc.
It is not easy to easy to mix. the different ingredients always by wt. So for simplycity mixing by volume is standardised against the designated strength-to achieve a nearer value like:
M10: – 1:3:6
M15 – 1:2:4
M20 – 1:1.5:3
In small works like in our residential bldg. we use by vol. as it is easy to handle by some fixed vol. of form works. But in major works or precision works, where batching plant is used, it is bound to be by wt. not by vol.
When mixing is done by wt. more than 95% of design strength is achieved.