made you look, but close enough

Like Borg?

close enough by the description, the problem with today concrete is water getting into it, the roman concreted from what was described basically regen with water getting into it.

Yeah, water is a problem because rebar. Romans didn't use rebar... Water/salt and iron/steel don't mix too well over long periods. Rebar rusts from exposure, rust expands, concrete cracks and crumbles.

im no engineer and i know metal expands but i never understood why they just dont coat rebar with polymer, surely there is a poylmer that can expand with it.

They do on many (or use a coating) but it's not perfect. The ends for example, or nicks during install.

But also, cathodic protection which is commonly used for repairs will not work. Corrosion occurs underneath the coating and not conduct electricity.