Shotcrete

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Shotcrete and gunite are two commonly used terms for sprayed concrete. Shotcrete is mortar or concrete conveyed through a hose and pneumatically projected at high velocity onto a surface. Shotcrete undergoes placement and compaction at the same time due to the force with which it is projected from the nozzle. It can be impacted onto any type or shape of surface, including vertical or overhead areas.

Shotcrete being applied to stabilize a tunnel.

1 History
2 Shotcrete vs. gunite
3 Dry mix vs. wet mix
4 Reinforcement
5 See also
6 Notes
7 External links

[edit] History

Shotcrete was invented in the early 1900s by Carl Akeley, a famous American taxidermist, to be used to fill his plaster models of animals. He used the simple method of blowing dry material out of a hose with compressed air and wetting it as it was released. This was later used to patch weak parts in old buildings. In 1911, he was granted a patent for his inventions, the "cement gun", the equipment used, and "gunite", the material that was produced. Until the 1950s when the wet-mix process was devised, only the dry-mix process was used. In the 1960s, the alternative rotary method was first devised.^[1]

[edit] Shotcrete vs. gunite

Shotcrete is today an all-inclusive term that describes spraying concrete or mortar with either a dry or wet mix process. However, it may also sometimes be used to distinguish from gunite as a wet-mix. The term shotcrete was first defined by the American Railway Engineers Association (AREA) in the early 1930s. By 1951, shotcrete had become the official generic name of the sprayed concrete process.

Gunite refers only to the dry-mix process, in which the dry cementitious mixture is blown through a hose to the nozzle, where the water is injected immediately prior to application. Gunite was the original term coined by Akeley, and trademarked in 1909, patented in North Carolina. The concrete is blasted by pneumatic pressure from a gun, hence "gun"-ite.

[edit] Dry mix vs. wet mix

The dry mix method involves placing the dry ingredients into a hopper and then conveying them pneumatically through a hose to the nozzle. The nozzleman who holds the nozzle then controls the addition of water at the nozzle. The water and the dry mixture is not completely mixed, but is completed as the mixture impinges on the receiving surface. This requires a highly skilled nozzleman, especially in the case of thick or heavily reinforced sections.

Wet-mix shotcrete involves pumping of a previously prepared concrete, typically ready-mixed concrete, to the nozzle. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The wet-gun procedure generally produces less rebound, waste (when material falls to the floor), and dusts compared to the dry-mix procedure.

[edit] Reinforcement

Sprayed concrete is reinforced by conventional steel rods, steel mesh, and/or fibers. Fiber reinforcement (steel or synthetic) is also used for stabilization in applications such as slopes or tunneling.