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Shotcrete, gunite // or sprayed concrete is concrete or mortar conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique. It is typically reinforced by conventional steel rods, steel mesh, or fibers.
Shotcrete is usually an all-inclusive term for both the wet-mix and dry-mix versions. In pool construction, however, shotcrete refers to wet mix and gunite to dry mix. In this context, these terms are not interchangeable.
Shotcrete is placed and compacted at the same time, due to the force with the nozzle. It can be sprayed onto any type or shape of surface, including vertical or overhead areas.
Shotcrete, then known as gunite, was invented in 1907 by American taxidermist Carl Akeley to repair the crumbling façade 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.
The dry-mix process was used until the wet-mix process was devised in the 1950s. In the 1960s, an alternative method for gunning dry material with a rotary gun appeared, using a continuously fed open hopper.
The nozzle is controlled by hand on small jobs, such as a modest swimming pool. On larger work it is attached to mechanical arms and operated by hand-held remote control.
Dry 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 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-process procedure generally produces less rebound, waste (when material falls to the floor), and dust compared to the dry-mix process. The greatest advantage of the wet-mix process is all the ingredients are mixed with the water and additives required, and also larger volumes can be placed in less time than the dry process concrete .
Shotcrete machines are available which control the complete process and make it very fast and easy. Manual and mechanical methods are used for the wet spraying process but wet sprayed concrete is traditionally applied by machine. The high spray outputs and large cross-sections require the work to be mechanised. Concrete spraying systems with duplex pumps are mainly used for working with wet mixes. Unlike conventional concrete pumps, these systems have to meet the additional requirement of delivering a concrete flow that is as constant as possible, and therefore continuous, to guarantee homogeneous spray application'.
Shotcrete vs. Gunite
Gunite was originally a trademarked name that specifically referred to the dry-mix pneumatic cement application 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 is an all-inclusive term for 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.
Shotcrete is a generic term for spray-application of concretes, referring to both the wet or the dry application method. Gunite is a specific term referring to dry concrete applied using pressurised air, where mixing with water occurs at the nozzle, as it is applied. The Gunite technique allows a skilled worker to add precisely the correct amount of water to obtain the required viscosity for application to both vertical or horizontal surfaces (overhead), and so it can develop the required strength for the final product.
Shotcrete is used to reinforce both temporary and permanent excavations. It may be employed, in concert with lagging and other forms of earth anchor, to stabilize an excavation for an underground parking structure or hi-rise during construction. This provides a large waterproof enclosure in which a structure can be erected. Once the structure is completed the area between its foundation and the shotcrete is backfilled and compacted.
Shotcrete is also a viable means and method for placing structural concrete.
Shotcrete is very useful in hard rock mining. Development of decline pathway to go underground is critical for movement of heavy machinery, miners, and material. Shotcrete helps make these paths safe from any ground fall.
In-ground swimming pools can use either the wet or dry application method. Shotcrete-wet is ideal for covering large areas quickly, whereas shotcrete-dry (Gunite) is more useful for frequent start-and-stop operations. Neither is superior for this application, and the technique should be selected based on individual project needs.
Costs vary depending on project needs.  Shotcrete swimming pools are typically more durable and longer lasting than poured concrete. Poured concrete can have a compressive strength of 2,500-3,000 psi, whereas shotcrete exceeds 4,000 psi. Additionally, the use of Shotcrete allows for customization in shape, depth, and styles that are not typically available in traditional pool kits. Shotcrete pools can also have many different types of finishes including tile, quartz, or glass beading.
- Reinforced concrete
- Slurry wall
- Structural engineering
- The Flintstone House – an example of shotcrete housing construction
- Teichert, Pietro (Summer 2002). "Carl Akeley--a tribute to the founder of Shotcrete" (PDF). Shotcrete: 10–12. Retrieved 13 April 2014.
- Davison, F. Trubee (March–April 1927). "Akeley, the inventor". Natural History. XXVII (2): 124–129.
- [permanent dead link], Shotcrete in Tunnel Construction, Introduction to the basic technology of sprayed concrete, By; Jürgen Höfler, Jürg Schlumpf, p.32, 6.1.2 ,machines[dead link]
- ACI Report 506R-05
- "Norwegian technology in the world's deepest immersed tunnel" (in Norwegian) Teknisk Ukeblad, 12 October 2013. Accessed: 13 October 2013. Technical report : Claus K. Larsen. "Testing of fireproofing for concrete[permanent dead link]" 'Norwegian Public Roads Administration, 2007.
- WSN 2014, B. Simser - Empirical experience with shotcrete in deep underground mines
- "Cost to install a Gunite concrete pool - Estimates and Prices at Howmuch". HowMuch. Retrieved 2017-10-04.
- Aquapool. "Gunite and inground gunite pools | Aqua pool & Patio | New Pool Construction, Renovation, Installation and Design". Aqua pool & Patio | New Pool Construction, Renovation, Installation and Design. Retrieved 2017-10-04.
- "4 Types Of Finishes For Your Gunite Pool - Ferrari Pools". Ferrari Pools. 2017-06-27. Retrieved 2017-10-04.
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