The builder has added bits
of scrap iron, steel, and probably anything else he could get
his hands on to add weight to the poured concrete ballast
(concrete is not very heavy, particularly as compared to lead).
Once water seeps into the ballast (from the bilge or possibly
via hull damage) the metal scraps begin to rust and expand,
causing the FRP of the hull to crack and rupture. The
voids in the concrete indicate where metal scraps were located.