So a carbon nanotube rebar would improve the condition of concrete, perhaps prolonging the damage of decay. Would it be sensible for carbon nanotubes to be added into the recipe for steel?
While my answer to your last question said it's hard to produce nanotubes in abundance - and it's hard to even identify them - further research shows that craftsmen worked nanotubes into steel swords before the 1600s! The method they employed can be adapted, so you don't have to worry about scaling up complicated lab experiments to produce what you need.
Damascus swords, which have a reputation for strength and sharpness, supposedly incorporate small carbon and iron structures - including nanotubes. They could be crafted using far less advanced knowledge and techniques than we have today, and perhaps they are a scaleable technology that would work for your buildings.
Some sources caution that regular steel works practically as well: this may be nothing special, and the subject is up for debate, but this proves that yes, nanotubes can be incorporated in steel to some practical degree.