Modern concrete has steel rebar, which is very useful, but eventually corrodes. Stainless steel rebar could be used if longevity mattered, but usually it doesn’t because the building will likely become functionally obsolete and need replacing before then.
Ultimately most tall structures as a reinforced concrete skeleton with glass hung on the sides.
It seems to me that as you're going to replace it with another basically identical concrete skeleton it makes sense to have that skeleton last as long as possible. And then refit that skeleton.
It only corrodes if it's not completely encased in concrete. The concrete stops the reaction. And, if one portion of the rebar is not encased it will eventually corrode through overtime.
This is something I wasn't aware of that is fascinating. The ph balance of the rebar's environment determines the progression of oxidation. There was a guy in the 1920's who observed this and had the idea to put cement in paint; it was so successful he renamed the company to cement spelled backwards.
Inside the concrete, Ca(OH)₂, which is generated by the hydration of cement,
creates a strongly alkaline environment, with a pH value of more than 12.7.
In this environment, a passivation film forms on the surface of the rebar to
prevent corrosion. However, the pH of the concrete decreases as micro-cracks
develops and chloride from the outside infiltrates the rebar. Eventually, as
the environment inside the concrete becomes neutralized, the corrosion of the
rebar accelerates, which reduces the overall durability and stability of the
reinforced concrete structure.
A sighting of preserved steel reinforcing bars sticking out of old concrete
rubble piqued the curiosity of our company founder, Albert C. Bean, Sr. After
investigating this finding, his company, Armor Oil & Chemical Company, began
producing a patented cement-filled coating formulation that, in its updated
form, still protects structures from corrosion today.
Yup, every alternative rebar has drawbacks which causes increased costs or different failure modes which is why they tend to be used in specialised applications where they are necessary.
