SHOTCRETING K.V.SUBBA RAO

Shotcreting Shotcreting is a method in concrete work in which the concrete mix is applied in layers under the pressure of compressed air . Shotcreting is done with a special unit, which consists of a mortar gun or concrete sprayer and a compressor Gunite is a trademarked name of Gunite.com, that is incorrectly used to describe the dry-mix shotcrete process, in which the dry cementitious mixture is blown through a hose to the nozzle, the water being injected at the nozzle immediately before it exits the nozzle. Gunite was the original term coined by Akeley, trademarked in 1909 and patented in North Carolina. The concrete is blasted 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 emerged as the only acceptable industry term to correctly describe "pneumatically applied concrete" - either the wet or dry process. Because the term "Gunite" is a noun (product name) and not a verb (action), it is impossible to "gunite" anything. Per the American Shotcrete Association (ASA) the correct terminology is "shotcrete - wet mix" or "shotcrete - dry mix."

Shotcreting A dry mix of cement and aggregate, usually sand, is prepared for shotcreting. The compressed air forces the mix through a hose to the nozzle, moistens it with water supplied through another hose, and then projects it at high velocity (130–170 m/sec) onto the surface being shotcreted. Shotcrete, then known as gunite, was invented in the early 1900s by American taxidermist Carl Akeley, used to fill plaster model of animals. He used the method of blowing dry material out of a hose with compressed air, injecting water at the nozzle 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 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.

Shotcreting A single shotcrete layer is 10–15 mm thick and has high mechanical strength (40–70 meganewtons per sq m), density, water tightness, and cold resistance Shotcreting is used in building thin-walled reinforced-concrete structural components, such as shells, vaults, and tanks, and in producing the lining in tunnels. It is also used for hydraulic insulation, for finishing the joints of reinforced-concrete structural components, and for repair and strengthening of concrete and reinforced-concrete components. Shotcrete, then known as gunite, was invented in the early 1900s by American taxidermist Carl Akeley, used to fill plaster model of animals. He used the method of blowing dry material out of a hose with compressed air, injecting water at the nozzle 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 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.

Shotcreting The shotcrete is placed layer-by-layer For shotcrete placing special equipment are used, where by means of "dry" technology the mixture of cement, sand and metal batched in specified proportions is squeezed out with pressure air by means of flexible hose to the nozzle atomizer, where the mixture is moisturized with water. When the "wet" technology is used, prepared concrete is delivered to the machines

Shotcreting Shotcreting has high strength properties; resistance of its extension is 10 per cent more in comparison with standard concrete. The cover of high strength, density and waterproofing capacity is formed because of impact laying Shotcrete application allows decreasing face thickness and out-turns; excluding the necessity of formwork and mechanizing the concrete work process totally.

Materials for shotcreting Cement Sand Coarse Aggregates Water Chemical additives Compressed Air

Materials for shotcreting Chemical admixtures are added for acceleration of setting and hardening. Sodium Carbonate = ½ Kg per 50 Kg bag of cement Sodium Aluminate = ½ Kg per 50 Kg bag of cement Calcium Carbonate = 1 Kg per 50 Kg bag of cement Alternatively, following additives can also be used: Super plasticizer @ 1 % by weight of cement Accelerator (say sodium Silicate) @ 5 % by weight of cement

Preparation of Surface The following precautions are necessary in preparation of the surface before the shotcrete is applied: The surface should be free from lose materials. It should be clean to receive the shotcrete layers. In case the surface is smooth it should be roughened with sand blasting. Otherwise the surface could be cleaned with air water jet. .

Preparation of Surface The thickness of the shotcrete layer being only 76 mm any undulations in the surface more than such thickness will have to be backfilled with either masonry or lean concrete depending on the site conditions and specifications prescribed.

Mix of shotcrete Cement 450 kg / m3 Sand 1100 kg / m3 (0.70 m3) Coarse aggregate (5 mm – 10 mm size) 500 Kg/M3 (0.30 M3) Proportion by weight 1:2.44:1.11

Application Methodology Spray pneumatically the first layer of shotcrete of 38 mm thickness Welded wire mesh of 100 mm x 100 mm x 5 mm is nailed with the help of nails. Binding wire is used to keep the mesh in place. The second and final layer of 38 mm of shotcrete is sprayed pneumatically over the first layer.

Precautions to be taken The finished shotcrete layer is to be kept wet continuously for at least a week after completion. There would be rebound during the operations which shall not be used for shotcrete. The time lag between the first layer and the second layer is to be kept as minimum as possible.-say less than 8 hours.

Precautions to be taken After completing the shotcrete and after the mix is set, the surface should be thoroughly sounded by a hammer for drumming areas resulting from rebound pockets or lack of bond etc., which should be cut out and replaced with fresh shotcrete. Where dry process is adopted the jetting of water should be synchronized with that of the concrete mix for a homogeneous shotcrete. Where wet mix is adopted the specified water cement ratio is to be strictly enforced.

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Shotcreting NSRC

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