Both place concrete by pneumatically projecting the material from a hose. The difference is when water is added to the material. In a wet-mix application, all materials – including cement, aggregate, ad-mixture and water – are mixed together before being pumped through a hose and pneumatically projected. Typically, a concrete truck supplies the wet-mix and it is pumped through a shotcrete rig for placement. Generally, wet-mix is used for large volume placements with easy access for construction vehicles.
In dry-mix applications, all dry materials – including cement, aggregate, and ad-mixture – are mixed together, conveyed pneumatically through a hose and then, at the nozzle via a water ring, water is injected evenly throughout the mix as it is being projected. Generally, dry-mix is used for small to medium volume placements or for conditions with limited vehicular access.