
Shotcrete, gunite (), or sprayed concrete is
concrete
Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...
or
mortar conveyed through a hose and
pneumatically
Pneumatics (from Greek 'wind, breath') is the use of gas or pressurized air in mechanical systems.
Pneumatic systems used in industry are commonly powered by compressed air or compressed inert gases. A centrally located and electrically- ...
projected at high velocity onto a surface. This construction technique was invented by
Carl Akeley and first used in 1907.
[ The concrete is typically reinforced by conventional steel rods, steel mesh, or fibers.
The concrete or mortar is formulated to be sticky and resist flowing when at rest to allow use on walls and ceilings, but exhibit sufficient shear thinning to be easily plumbable through hoses.
Shotcrete is usually an all-inclusive term for both the wet-mix and dry-mix versions invented by Akeley. In swimming 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/consolidated at the same time, due to the force with which it is ejected from the nozzle. It can be sprayed onto any type or shape of surface, including vertical or overhead areas.
Shotcrete has the characteristics of high compressive strength, good durability, water tightness and frost resistance.
]
History
Shotcrete, then known as gunite, was invented in 1907 by American taxidermist
Taxidermy is the art of preserving an animal's body by mounting (over an armature) or stuffing, for the purpose of display or study. Animals are often, but not always, portrayed in a lifelike state. The word ''taxidermy'' describes the process ...
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
The World's Columbian Exposition, also known as the Chicago World's Fair, was a world's fair held in Chicago from May 5 to October 31, 1893, to celebrate the 400th anniversary of Christopher Columbus's arrival in the New World in 1492. The ...
). He used the method of blowing dry material out of a hose with compressed air
Compressed air is air kept under a pressure that is greater than atmospheric pressure. Compressed air in vehicle tires and shock absorbers are commonly used for improved traction and reduced vibration. Compressed air is an important medium for t ...
, injecting water at the nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe (material), pipe.
A nozzle is often a pipe or tube of varying cross ...
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 nozzle operator 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 nozzle operator, 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 nozzle operator, 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. A disadvantage to the dry mix method is dust and higher rebound.
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
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.
Depending on the fineness of the filler, mortar shotcrete (fraction size up to 2.5 mm) is distinguished from shotcrete (up to 10 mm), and syringe concrete, or sprayed concrete (up to 25 mm).
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, trademark
A trademark (also written trade mark or trade-mark) is a form of intellectual property that consists of a word, phrase, symbol, design, or a combination that identifies a Good (economics and accounting), product or Service (economics), service f ...
ed in 1909 and patented in North Carolina
North Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, South Carolina to the south, Georgia (U.S. stat ...
. The concrete mixture is applied 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 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.[ACI Report 506R-05] By 1951, shotcrete had become the official generic name of the sprayed concrete process—whether it utilizes the wet or dry process.[
]
Applications
Shotcrete is commonly used to line tunnel walls, in mines, subways, and automobile tunnels. Fire-resistant shotcrete developed in Norway is used on the Marmaray tunnel in Istanbul
Istanbul is the List of largest cities and towns in Turkey, largest city in Turkey, constituting the country's economic, cultural, and historical heart. With Demographics of Istanbul, a population over , it is home to 18% of the Demographics ...
.
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 buildings 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.
Also, the shotcrete is carried out much faster than the repair mixtures usual non-mechanized application.
See also
* Cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
* Rebar
Rebar (short for reinforcement bar or reinforcing bar), known when massed as reinforcing steel or steel reinforcement, is a tension device added to concrete to form ''reinforced concrete'' and reinforced masonry structures to strengthen and aid ...
* Reinforced concrete
Reinforced concrete, also called ferroconcrete or ferro-concrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having higher tensile strength or ...
* Slurry wall
A slurry wall is a civil engineering technique used to build reinforced concrete walls in areas of soft earth close to open water, or with a high groundwater table.
This technique is typically used to build diaphragm (water-blocking) walls surro ...
* Structural engineering
Structural engineering is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and joints' that create the form and shape of human-made Structure#Load-bearing, structures. Structural engineers also ...
* Flintstone House – an example of shotcrete housing construction
Notes
External links
{{Authority control
Concrete
Tunnel construction