Reinforced Sprayed Concrete Methods

Shotcrete

• There are two basic types of methods or techniques for applying sprayed concrete. In the first, compressed air blasts the dry concrete mix through the hose and water is injected into the stream at the nozzle. Since the mixing at the nozzle is incomplete, it takes a lot of skill to apply this so-called "gunite" correctly. A more common method uses wet mixed concrete pumped through the hose and ejected from the nozzle by introduction of compressed air.

Added Ingredients

• Builders often add other ingredients to the shotcrete mixture in addition to the concrete mix. Air entraining chemicals, for example, improve the freeze-thaw resistance of the shotcrete, while plastic fibers help to keep cracking under control. Silica fume reduces the shotcrete's permeability while providing added strength and chemical resistance. In cold weather, using shotcrete is ordinarily inadvisable, but builders can sometimes apply it anyway by adding chemicals called accelerators that help it harden despite the cold.

Finishing

• Once the builder has applied the shotcrete to both sides of the surface, he may remove a little of the shotcrete to bring both layers down to the desired thickness. Afterwards the surface is textured and/or consolidated. The builder can also apply a finish even while the shotcrete is still fresh. Shotcrete surfaces can later be painted or have drywall attached to furring strips anchored onto the shotcrete.

Considerations

• The most readily apparent difference between shotcrete and traditional poured concrete is that the former does not require forms, although it must be applied to a surface. Moreover, the rebar that serves as reinforcement must already be in place so the shotcrete can harden around it. Shotcrete often makes use of a lower water to cement ratio and can in many cases make for more rapid construction.