What is shotcrete or gunite?

Shotcrete (or Gunite) is concrete sprayed at high velocity onto a surface using compressed air. It compacts itself upon impact. It is used for large-scale repairs (e.g., retaining walls, tunnel linings) where setting up formwork is difficult. It allows us to build up thick layers of concrete quickly, overhead or vertically, without molds. The operator's skill is critical to avoid voids or 'shadows' behind rebar.