When is scarification needed for road work?

I assume you are talking about flexible (asphalt) pavements. Sacrification is needed when the cuts, potholes and all deformations are such that :

• The optimum moisture content required for proper binding can no longer be met.
• The filler material won’t bend properly if just put judging by the condition of the pavement.
• The peak daily traffic on the road, does not permit it to be safe without reconstruction.

The entire bituminous layer until the depth of the craters is scraped off, and a new layer with fresh material is then applied to reconstruct the road.