A concrete of Grade C20 means that the concrete will have a compression resistance of 20 N/mm2 per square millimeter in 28 days.

Proportion for grade C20.

C20 is 1:2:4(1 part cement, 2 parts fine aggregate, and 4 parts coarse aggregate.)

where as C20 symbolizes aCylindrical sample in which cement, fine aggregates, coarse aggregates are mixed in such a ratio that its characteristic compressive strength is 20 N/mm^2 after 28 days of curing.

Generally Strength of Cylinder sample= 0.8 x Strength of Cube. example:- M20 is equivalent to C25