Construction Crew Sprays Concrete

Shotcrete is the generic term for cement, sand, and fine aggregate concrete or mortar that is applied pneumatically to a surface at a high velocity. Another well-known term for shotcrete is gunite, a dry-sprayed mortar^[1] . In the shotcrete industry, there is a clear distinction between the two. According to the American Shotcrete Association, shotcrete is more of an all-inclusive term to describe the spraying of concrete or mortar that is either part of a wet- or dry-mix process. Gunite, on the contrary, refers to a dry-mix process where a dry cement mixture is blown through the hose of a nozzle, where water is added just before application. The application of dry-mix is contingent on manipulating the nozzle, and this usually requires a highly skilled nozzleman.^[2]

Contents 1 History 2 Uses 3 Types/Process 3.1 Dry-mix Shotcrete 3.2 Wet-mix Shotcrete 3.3 Steel Fiber Reinforced Micro Silica Shotcrete 3.4 Mesh reinforced shotcrete 3.5 Support and Reinforcement 4 References

[edit] History

Shotcrete was first used in 1910 when a naturalist by the name of Charles Akeley invented a revolver shotcrete machine to recreate animal skeletons. The revolver gun was introduced for the first time at the Cement Show in New York in 1910 and became widely used as a tool in construction. Several improvements and modifications were made to the gun and by the 1950s it was being used around the world.^[3] Gunite was the principal application used in Akeley’s revolver gun.

After World War II, new types of “continuous feed” gun machines were being produced that permitted the shotcrete to be delivered and then sprayed on to surfaces in a continual stream. These developments were the first major changes made since Akeley’s original revolver gun. Wet-mix shotcrete had yet to be created.^[4]

[edit] Uses

Shotcrete is a common application used for providing underground support in projects or industries such as mining and tunnel excavation and construction. Ladislaus von Rabcewicz, one of the original founders of the New Austrian Tunneling Method, was responsible for introducing the use of shotcrete in weak ground tunnel construction during the 1930s.^[5] It has also become a widely used application in the mining industry in providing underground support to permanent opening such as ramps, haulages, shaft stations, and crusher chambers.^[6] Excavations are typically shotcreted immediately after being done, then reinforced with steel fiber or mesh.^[7]

[edit] Types/Process

[edit] Dry-mix Shotcrete

Dry shotcrete components are mixed in an agitating hopper. The mix may be pre-dampened with a little water to reduce the level of dust. Compressed air is then introduced through a rotating barrel or feed bowl that sprays the material out in a continuous stream through a delivery hose. As the mix reaches the nozzle, water is added. Dry-mix shotcrete is used more in mining because of the inability to use large transit mix trucks as dry-mix shotcrete uses smaller, compact equipment for application.^[8]

[edit] Wet-mix Shotcrete

With wet shotcrete, components and the water are premixed in a truck mounted mixer. The shotcrete is then delivered into a positive displacement, pumping unit. The mix is then hydraulically delivered to a nozzle where air is used to spray the material on a surface.

Wet shotcrete is better suited for high production applications in mining and civil engineering projects where a deep shaft or long tunnel is being driven and where delivery trucks have ample space to operate.^[9]

[edit] Steel Fiber Reinforced Micro Silica Shotcrete

Two of the latest developments in shotcrete in the past few years have been the silica fume, a cementitious admixture, and steel or polypropylene fiber reinforcement. When silica fume is added in quantities of eight to 13 percent by weight of cement it gives shotcrete the capacity to achieve a compressive strength that is up to three times that of a regular shotcrete mix. What is created is a very durable, permeable, and solid shotcrete mixture.Steel fiber shotcrete was first used during the 1970s and has since become  popular worldwide as a substitute for plain shotcrete reinforced with wire meshing.^[10]

[edit] Mesh reinforced shotcrete

Mesh reinforced concrete has been typically used in civil and mining engineering projects. In some cases, it is still preferred over other shotcrete applications. It is particularly useful in the stabilization of rock slopes or in the building of bulkheads for underground fill. The wire or weld meshing provides additional support and reinforcement.^[11]

[edit] Support and Reinforcement

As an application, shotcrete is seldom used in isolation, but is used instead in combination with other forms of support and reinforcement such as rock bolting, cable bolts, lattice girders, or steel sets.^[12]

[edit] References

1. ↑ Shotcrete Support. Rocscience. 2008-09-29.
2. ↑ ASA Frequently Asked Questions. Shotcrete. 2008-09-29.
3. ↑ The History of Shotcrete. Shotcrete. 2008-09-29.
4. ↑ History of Shotcrete. Shotcrete. 2008-09-29.
5. ↑ Shotcrete Support. Rocscience. 2008-09-29.
6. ↑ Shotcrete Support. Rocscience. 2008-09-29.
7. ↑ Shotcrete Support. Rocscience. 2008-09-29.
8. ↑ Shotcrete Support. Rocscience. 2008-09-29.
9. ↑ Shotcrete Support. Rocscience. 2008-09-29.
10. ↑ Shotcrete Support. Rocscience. 2008-09-29.
11. ↑ Shotcrete Support. Rocscience. 2008-09-29.
12. ↑ Shotcrete Support. Rocscience. 2008-09-29.