I really don't intend to argue over this because it is far
outside my expertise, but I believe it may
be the issue is allowing salt - chloride - to continually penetrate the concrete and affect the rebar vs whether the rebar has a small amount of surface rust to start with.
This was around a dozen years ago when I looked into it and the Internet wasn't anything like it is today. But it was something like this that I found from a professional concrete group's advice (on paper, not online) years ago:
Some Rust on Rebar Is Acceptable The ASTM standard specification for deformed steel reinforcement and the Concrete Reinforcing Steel Institute (CRSI) Manual of Standard Practice both give the same information: Reinforcing bars with rust, mill scale, or a combination of both should be considered as satisfactory, provided the minimum dimensions, weight, and height of deformation of a hand-wire-brushed test specimen are not less than the applicable ASTM specification requirements. This inspection criteria recognizes studies that have shown mill scale and rust enhance the bond between concrete and steel.
Anyway, I'm fully prepared to be wrong. That's the professional advice I received by mail about a dozen years ago in the USA. Probably if you keep your roof impermibilizanted and your walls painted, the salt should not get in and should not help corrode the reinforcing further, provided the concrete is solid and not allowing air and water to reach the steel.
However, I am NOT
an engineer, metals specialist, chemist, or beach-front contractor. So, I'm fine if I'm wrong.
I only mean to suggest that there is varying information out there and maybe
we don't need to stress over it.
Cement and Concrete Research
This paper presents the results of a preliminary study in which the effect of the initial rusting on the corrosion behavior of rebars embedded in concrete has been investigated. Concrete specimens were made with pre-rusted and rust free rebars of different compositions. Two concrete mixes, one with a sodium chloride content of 2 kg/m3 of concrete and the other without any sodium chloride were used. The reinforced concrete specimens, immersed in potable water and then transferred to 5% NaCl solution, were subjected to corrosion monitoring for a period of 10 months. The test results indicate that the initial rusting does not have an adverse effect on the corrosion resistance of rebars embedded in concrete.