Physical is Physical, no problem there. The problem with Verb is that is just doesn't mean anything in the context. The verb here is obviously intended in the Italian as something to be "conjugated" interpreted in specific ways, played with according to specific ways (otherwise it is verbum as a substantive word a fixed absolute thing which is hardly the case here). Grammar would just about go here as indicating a rigid set of strict rules. Then there is poetics, not so close to verbo but still a set of rules connected with words or lastly you could put a weak "expression".
I have put grammar as the first option, because although it is unusual to connect the word with art, it is also unusual to connect the word "verbo" with art in Italian.

Finally, the word "representation" as illustrated in David's reference, has the specific meaning of a "likeness" as in a photograph, which is not the case of Richard Serra's sculptures. They are not physical likeness, physical representations of anything I can see.

http://images.google.com/images?ie=UTF-8&sa=N&sourceid=deskb...