M40

Its depend upon mix design

M40 is the high strength concrete bcz it has 40 N/mm2 
strength.others are less

Its M40

M40......bcuase it have a high characteristic compressive 
strenth attained in 28 days.....

please give ratio for M25to M40. because the ratio 
available in IS code only M10-M20, most case M25,so more 
then M25 ratio is our own preparation. so my suggestion is 
M20 or M25.

M40
because its have more cement content and it also depends 
upon what is mix design if you are giving cement content in 
M20 more than M40,then M20 will give higher strength than 
M40

M40 becoz the ratio of cement:sand:aggregate  is higher 
then the other. which are the core factor for any mix. the 
property of any mix like compreesive strength, 
characterstic strength etc . largely depends on the ratio.

m40

M- 40 is strongest It depends upon the design mix, . M-40 
means its strengthis 40n/mm2