Spayed concrete is the term used for the high pressure spraying of concrete or mortar onto existing surfaces. It is extremely versatile as it can be sprayed over any shape or profile and in vertical as well as overhead situations.
There are two methods, the dry mix method referred to as Shotcrete and the wet mix method known generally as Gunnite.
Because the material is pneumatically forced through the nozzle at high pressure when the concrete hits the surface it is both placed and compacted.
For strengthening works, new reinforcement anchor bars are fitted and steel mesh sections wire tied or welded into position. The steel gives the structure the additional reinforcement but also give the sprayed material something to grip and adhere to.
The dry mix method involves placing the dry ingredients into a hopper. The material is then sent pneumatically down hose to the nozzle. It is at this point that the water is added by the nozzleman controlling the nozzle. The mix is completed as it hits the surface.
A skilled nozzleman can adjust instantly the amount of water/material allowing very precise placement in overhead and vertical applications without the need to add accelerators.
The Wet-mix method involves a pre-mixed concrete, usually a ready mix. The mix is pumped through the hose where compressed air is introduced to force the material onto the surface. This method generally produces less rebound and therefore less wastage. The biggest advantage of this wet-mix process is that larger volumes can be placed in less time.