How do you control the strength of concrete?

The most important factor is Water Cement ratio. then the quality of cement and aggregates, by adjusting the cement  proportion we can achieve the higher strength like adding fly ash for hoemoginity and similar methods plus adding the right quantity of super plasticizer we can modify the setting time to achieve higher strengths, proper curing is the most important factor, but one has to conduct trials and cube testing for 3 / 7 / 21 days respectively

It is ture that water to cement ratio control the concrete strenght but its not always true Its depends upon the mix matrix and water content per cubic meter concrete Admixture plays a very important role in the concrete ,

All the ingredients of concrete play a role in concrete strenght and its development its very complex chemistry

The easiest way to add strength is to add cement.

The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely effected by adding more water.