I can't remove the concrete floor.

probably known bug. place/build a building with flooring over it, after completion remove the building

after that it will be gone.

this occurs when removing manually placed concrete tiles during deconstruction of a building

I think that's what I did.

the game gets confused if you do that and forgets the force-placed flooring is still there and never removes it

I found that if you save the game and reload you can delete them

that is a way or just build something small on top of it and dismantel it after the construction is done