Strength of concrete ?

The strength obtained after keeping the concrete sample in water for28 days.

It may varies according to the concrete mix design.

Without lateral thinking the compressive strength of concrete tested in a laboratory is called strength of concrete. We consider compressive strength in the structural design. Due to pozzolanic activity, the56 days strength (some times)  more than1.5 to1.6 times the28 days strength. In near future, the columns, dams, mass concrete works, may define to use56 days strength in the design of structures. In one of the developed country's code,56 days can be used in the design of RCC members.

depends on the mixture, what strength do you want? in which environment? do you need it to strengthen quicker then normal?

Also concrete needs the expertise and design. All concrete mixtures are different depending on there duty en scope. Ask the responsible structural engineer, also it has to be on the shop drawings!

Strength of concrete is controlled by the batching methods according to specs.

water : cement ratio. I am present in school about this topic

concrete strenth is in fact  the maximum compression stress of concrete after7,11, or28 days after curing in lab conditions.