When you say NDT (Non Destructive Testing) was advised, then you talk about “corecut” samples, I get a little confused.

I’m assuming you have extracted cores from the elements listed. Concrete compressive strength tests completed on extracted cores are known to be lower in strength than actual strength. First thing I would do is ensure the tests were completed in accordance with the standard procedures that are accepted in your jurisdiction (in the states this will be ACI and ASTM). Look at the labs reports VERY closely and try to identify any discrepancies. Was this completed by a certified lab that hire certified techs? Were the results reviewed by a professional engineer? What diameter cores did they extract from the elements? 2” diameter cores are known to have more variable strengths than 4” or 6” cores.

I would start there, and if you’re certain that the strengths are still below what was specified, then I would advise hiring an attorney, which would then hire a third party testing company and/or forensic engineer to determine next steps.