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Shotcrete is usually an all-inclusive term that can be used for both wet-mix and dry-mix versions. In the pool construction trade however, the term "shotcrete" refers to wet-mix and "gunite" refers to dry-mix; in this context, these two terms are not interchangeable (see "Shotcrete vs. gunite" discussion below).
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, then known as gunite (/ˈgənīt/), was invented in 1907 by American taxidermist Carl Akeley to repair the crumbling facade of the Field Columbian Museum in Chicago (the old Palace of Fine Arts from the World's Columbian Exposition). He used the method of blowing dry material out of a hose with compressed air, injecting water at the nozzle as it was released. In 1911, he was granted a patent for his inventions, the "cement gun", the equipment used, and "gunite", the material that was produced. There is no evidence that Akeley ever used sprayable concrete in his taxidermy work, as is sometimes suggested. F. Trubee Davison covered this and other Akeley inventions in a special issue of Natural History magazine
Until the 1950s when the wet-mix process was devised, only the dry-mix process was used. In the 1960s, the alternative method for gunning by the dry method was devised with the development of the rotary gun, with an open hopper that could be fed continuously. Shotcrete is also a viable means and method for placing structural concrete.
The nozzleman is the person controlling the nozzle that delivers the concrete to the surface. The nozzle is controlled by hand on small jobs, for example the construction of small swimming pools. On larger work the nozzle can sometimes be held by mechanical arms where the nozzleman controls the operation by a hand-held remote control.
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 controls the addition of water at the nozzle. The water and the dry mixture is not completely mixed, but is completed as the mixture hits the receiving surface. This requires a skilled nozzleman, especially in the case of thick or heavily reinforced sections. Advantages of the dry mix process are that the water content can be adjusted instantaneously by the nozzleman, allowing more effective placement in overhead and vertical applications without using accelerators. The dry mix process is useful in repair applications when it is necessary to stop frequently, as the dry material is easily discharged from the hose.
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 dust compared to the dry-mix procedure. The greatest advantage of the wet-mix process is that larger volumes can be placed in less time.
Shotcrete is an all-inclusive term that describes spraying concrete or mortar with either a dry or wet mix process. However, shotcrete may also sometimes be used (incorrectly) to distinguish wet-mix from the dry-mix method. 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 - whether it utilizes the wet or dry process.
Gunite was, at one time, a trademarked name that specifically refers to the dry-mix shotcrete process. In the dry-mix process, the dry sand and cement mixture is blown through a hose using compressed air, with water being injected at the nozzle to hydrate the mixture, immediately before it is discharged onto the receiving surface. Gunite was the original term coined by Akeley, trademarked in 1909 and patented in North Carolina. The concrete mixture is by pneumatic pressure from a gun, hence "gun"-ite.
The term "Gunite" became the registered trademark of Allentown Equipment, the oldest manufacturer of gunite equipment. Other manufacturers were thus compelled to use other terminology to describe the process such as shotcrete, pneumatic concrete, guncrete, etc.
Shotcrete has emerged as the all- inclusive industry term to correctly describe "pneumatically applied concrete" - either by the wet or dry process. The term "Gunite" is a noun (product name) and should not be used as a verb (as in, to "gunite" something). As Per the American Shotcrete Association (ASA) the correct terminology is "shotcrete - wet mix" or "shotcrete - dry mix."
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.
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