An acid is termed weak or strong on the basis of it's disassociation constant (Ka). This is a function of it's chemical structure. Concentration is simply the number of molecules in a given volume.
Acid concentration is how acidic a substance is, and is measured on a pH scale. -7 being the most basic, 0 being water, and 7 being the most acidic (comparable to stomach acid).
There are many different concentrations of "concentrated sulphuric acid". It is possible to deduce the concentration of the sulphuric acid by titration.
I suggest you look up titrations. It's a type of chemical procedure meant to accurately measure the concentration of an unknown concentration of a specific chemical. They're mainly meant for acid...