concrete strength

The whole point of a sample is that you can test it to measure it's strength...

If depends on wht it is made from....
If I made a sample from the old bag of cement in my garage and the ballast I swept off my flat roof the strength would be very low.

Somewhere within the range from 0 to 6,000 lbs. per square inch. Usually measured at 28 days. Variables are the mix composition, curing conditions, and temperature. There was a recent thread where this issue was discussed in detail. I suggest you review the other thread.