If you create the 2 items as separate multibodies within the same part you should be able to suppress the un-needed body and the mass properties would only show the un-suppressed body. You create the 2 multibodies by not "merging" them when modeling each portion.
You can create a multi-body part or you can create an assembly. If you make a multi-body part, then you will need to make a copy of the mesh. Use the Combine feature and subtract the copy of the mesh from the concrete. If you make an assembly, edit the concrete in the context of the assembly, then use the Cavity feature to subtract the mesh from the concrete.
Thanks for the replies guys,
I have been swamped at work and forgot to check up on this thread.
Because I have to create the concrete part within my assembly to fit certain components the cavity command works a treat, although I had to alter my mesh slightly so that there was a 0.1mm gap between the crossing mesh as it was creating "zero geometry" when I tried the cavity command, I will make this gap smaller in practice to reduce the extra material between the mesh bars though.