[jackjr-123]

15 MPa is not that impressive. Re solid sections, if you look at other video of concrete / polymer concrete bases being manufactured there are many that are not completely solid (but rather more like an old school cast iron sand cast machine base).

Why should we care about the flexural/tensile strength in a machine tool?

I also don't understand why you suggest making hollow sections. The 10kg saved won't change anything except making the mold more complex.