Shotcrete refers to a process in which compressed air forces mortar or concrete through a hose and nozzle onto a surface at a high velocity and forms structural or non-structural components of buildings. The relatively dry mixture is consolidated by the force of impact and develops a compressive strength similar to normal- and high-strength concrete.

Materials used in the shotcrete process are generally the same as those used for conventional concrete-portland cement, lightweight aggregate, water, and admixtures. Shotcrete projects also call for the same types of reinforcement specified for conventional concrete, including deformed bars, welded wire fabric, and prestressing steel.

Wet or Dry

Shotcrete may be applied to surfaces using a dry- or wet-mix method. The wet-mix concrete method consists of portland cement and aggregate premixed with water before the pump pushes the mixture though the hose. Additional compressed air is added at the nozzle to increase the velocity of the mixture. In the dry-mix process, compressed air propels a premixed blend of portland cement and damp aggregate through the hose to the nozzle. In the nozzle, water is added from a separate hose and completely mixed with the dry mixture just as both streams are being projected onto the prepared surface.

Generally, the shotcrete gun nozzle is held at a right angle 2 to 6 feet (0.6 to 1.8 meter) from the surface. In most cases, shotcrete can be deposited in the required thickness in a single application. For some vertical and overhead applications and for some smooth finishes, shotcrete must be applied in 1 to 2-inch (2.5 to 5 cm) thick layers. Once shotcrete is placed, it can be finished in a variety of methods, including natural finish, broom finish, various rough trowel finishes, and smooth steel trowel finish. After finishing, the concrete must be cured for a period of at least seven days.

Since its invention in 1911, the shotcrete process has been used successfully for a wide variety of building projects, including all types of residential and non-residential buildings. Shotcrete, which can be applied to horizontal or vertical surfaces, is especially suited for curved or thin concrete structures and shallow repairs. Other applications include swimming pools, grain silos, fire-proofing structural steel, and many civil engineering structures such as bridges, tunnels, dams, tanks, and earth retention systems.

Although using shotcrete to form walls is new in the United States, the process has been used in other countries for years. A recent innovation that has made shotcrete more suitable for construction is the incorporation of insulation into the wall unit. The new system consists of 4-foot-by-8-foot panels (1.1x2.5 m) of a polystyrene core sandwiched between layers of wire mesh. Workers erect a frame of wire mesh or rebar in the shape of exterior walls, install conduit, pipe, and other utilities between the polystyrene and the wire mesh, and then spray the entire structure. The concrete covers the foam and hardens to form a reinforced wall with built-in insulation.

More information on shotcrete for buildings.

 
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