Concrete core sample

Are the samples the same area exposed to compression? Compression is, after all, measured in Pounds per Square Inch (PSI) or MPa. I get about 7 in² for the cylinder, and about 35 in² for the cube.

That is the nice thing about standardized tests... something is varied while several things are held constant. This gives the value being investigated.

This differents is effect of friction on contacts of press plate and concrete. This friction give a most strength for deviatoric stresses. Effect of this friction depend on cube size (and shape of sample, and high and width of sample, etc.)

If you can delete friction, the different of compressed strength will be vanished. According to National Standart of Russia, investigations is worked with friction on the contact of steel press plate and dry concrete for cube with 150mm edge. Theare is cofficents for convert "150mm" cube strength to "200mm" and "100mm" cube strength.
Please, sorry for my English...

As per IS standards the cube to cylinderical convertion ratio is 80% . For M80 you should get 64 N/mm2 minimum. At the same time the Dia to Depth ratio of cylinderical sample not more than 2.

The result also depends upon the compaction of pile and voids present in the sample.

I have done the same test several time. But it will not give a require strength.