The radius dimension tool should produce the radius of the arc and a center, e.g.:
R 0.50" with a leader to the outside of the arc, in the exterior radius mode, or a leader from the arc center to the inside of the arc in the interior radius mode. I have noticed the same effect which BAZZ describes when making a solid, combining elements, etc. The arcs then become linear elements arranged along the original arc, and apparently can't be directly dimensioned. Like BAZZ, I usually overlay the original with an arc using three points, then use the radius dimension tool to indicate both the radius and the center. Is there a better way to do this?