Hello and thanks for the reply!
Well i try to replicate the damaged concrete in the image i posted above.But as i said im doing something very wrong and need some type of example how to do 
In the image you see it has sharp edges and flat in the bottom like a stairstep,Not sure how to archive that with zbrush 3.5
I have made a lot of custom alpha textures but i think im useing the wrong brushes,Also there are millions of different settings both for the stroke,alpha and brush so im kind of lost here,I cant even replicate it on a simple cube 
Its pretty annoying to see all character tutorials but almost nothing about environments,Anyway i cant find anything that explains this kind of subject!
Anyone may be know though if there is a tutorial on this or indepth explination,Not just use a brush and begin to mask.That doesnt say how or if there is a special alpha mask
Thanks guys and sorry for asking ,May be a dumb question if there isnt any tutorial on this subject!?