Even large walkins are pieced together with sectional walls, floors and ceiling parts. I've seen a 12x12 cooler brought in piece by piece through a regular door and assembled in the final room. Its pretty routine. Never have seen a walkin on a concrete floor. Not that they don't exist, I just have never seen one. Always needed to step up into the cooler, because the base is part of getting a good surrounding insulation and is vital to the door seal.