Why do Disks become polygons in RegionPlot?

The problem is due to confusion in RegionPlot, since you are providing it a nested list of regions, which is not really standard syntax.

You could use Flatten@{RegionConcSect, RegionRebar} as its argument, or alternatively construct separate plots for each region, then combine them with Show. This also has the advantage of allowing you finer control on the formatting of the individual plots:

Show[
 RegionPlot[RegionConcSect, BoundaryStyle -> None],
 RegionPlot[RegionRebar, PlotStyle -> Black, BoundaryStyle -> Black],
 AspectRatio -> Automatic, PlotRangePadding -> None
]