In order to make a structure of a composite material including elongated hollow shapes, such as hollow shapes associated with stiffeners of a stiffened panel, the invention comprises making cores including a bladder made of an elastomer material. The invention relates to a method for making such extractable cores in order to obtain cores having a very important length, and allowing the use of said cores for the industrial production of composite parts. The bladder is a cylindrical bladder that is conformed in a core mould, wherein the core itself does not have to be cylindrical, before being filled with a granular material and submitted to a negative pressure in order to impart a stable shape to the core by compaction of the granular material.