R28 = 160(C/W – 0,5)
where 'R28″ is the compressive strength of concrete after 28 days of hardening expressed in kg / cm2,
C - weight of cement in kg,
W - weight of water in kg.
As you can see, the compressive strength of concrete depends on the weight ratio of cement to water. The ratio may be 1.2-2.5. On larger concrete or reinforced concrete structures and in concrete product factories, it is necessary to maintain a constant, specific concrete strength, due to the safety of the structure and rational requirements, economical cement management.