Okay so are those floaters, or are they a part of the original mesh? It only makes sense to me that they are floaters; I don't understand how you would be able to map a piece so that you could non destructively place decals like that on assets.
The concrete material is a blend shader that uses vertex alpha to blend between two materials. For other wear and tear I did have an atlas with some grunge but we found it better to use the decal system in the editor because it allowed for more control.