What is the differences between C30 Concrete and C40?

The number relates to the compressive strength of a cube of concrete when tested (i.e. 30 N/mm2 or 40N/mm2) The higher the number the greater the strength of the concrete. The choice of concrete grade is usually made at the design stage for the strucutre and effects the overall strength and resistance of the strucutre to decay. You would use a higher strength concrete in a more exposed environment.

It is important to use the concrete grade specified for the proposed strucutre.