i removed a (non load bearing) wall, and the floors were poured between these walls, so that left a gap.
the floors are insulated and i didn't want to create a cold bridge here, but the proper sheets (i guess polystyrene) are pricey and only come in big packs, so i improvised a bit. i used polyurethane foam, and then i added a bunch of rebar for good measure -- i was a bit worried the polyurethane foam might be too soft and it might crack? probably unfounded but w/e, that's like, 5 bucks of rebar.
probably need to sand it down a bit or something, maybe pour some levelling compound, idk. we'll see once it's properly dry