Sorry, but I do not understand your problem. Please enlarge on it.

It sounds as if you are trying to follow a specific method, likely one already in use by some organization. Can you tell us which protocol (set of procedures) you are going to use? Does it have a reference number, pictures of the mold or the sample beam (or better yet, drawings of how to build the mold)? What organization specifies this method? Is it a standards group, an industry group, and individual company, or perhaps a college or university? How will the beams be tested: compression, bending, tension, shear, torsion . . .? I have done concrete beam sample tests of several kinds, but none used cast-in or carved-in "keys". Unless the cavities are cast by the mold, I cannot imagine getting valid results: "carving" will never result in repeatable results, IMO, and the time delay of doing the carving will further confuse the result.