# Size of specimen and compressive strength of concrete

Hello everyone.
I have a question about compressive strength of concrete. It is said that (in my country) 150 mm x 150 mm x 150 mm concrete cube is used for concrete testing. Someone asks me why 100 mm x 100 mm x 100 mm is not used. In fact, I know that sometimes it is allowed or even prefered to use 100 mm one instead of 150 mm one. I have looked up some research papers and they mention that it depends of the designed compressive strength of concrete.

One explanation says the applied load of a compression machine is often a definite value, let's say 2000 kN. If the concrete produced is of a designed compressive strength 60 MPa, then 150 mm cube is sufficient for the test. If a higher strength concrete is produced, then a 100 mm cube is more suitable. (As stress = Force/Area, same force smaller area can give a higher stress)

My question is that what will happen if I make a 100 mm cube from the same concrete mixture and put it onto the compression machine? Is the resulting compressive strength higher or lower than that of a 150 mm cube? Some people say it should be slightly lower. What is the reasoning?