PROCESSES USED FOR SHOTCRETE OR GUNITING.

Shotcrete or guniting is a mortar or a fine concrete that is pneumatically transported through a hose and projected on the surface at a high velocity.

Two different processes used for shotcrete are given below:
1. Dry-mix process
2. wet-mix process

1. Dry mix process:
Several forms of equipment are available for shotcreting by this technique. A common layout includes an air compressor, a material nose, air and water hoses, a nozzle gun and a pressure tank or pump for water supply. 

The various stages involved in the dry mix process are as follows:


  1. The cement and sand are thoroughly mixed. The mixture of cement-sand is fed into a special air pressured mechanical feeder termed as cement gun.
  2. The mixture is metered into the delivery hose by a feed wheel or distributor.
  3. .This material is carried by compressed air through the delivery hose to a special nozzle. The nozzle is fitted with a perforated manifold through which water is introduced under pressure and intimately mixed with other ingredients.
  4. The mortar is jetted from the nozzle at high velocity on to the surface to be shotcreted. Any alteration in the quantity of water can be easily accomplished by the nozzleman.

Advantages of Dry mix process :
  1. The dry mix process is preferred for light weight aggregate concrete.
  2. The lower w/c ratio obtained with the dry process probably accounts for the lesser creep, higher strength and greater durability of concrete
  3. The dry process equipment can convey the material to a distance of 300m to 500 m horizontally and 45 m to 100 m vertically.

2. Wet mix process :
In this process, cement, sand small sized coarse aggregate and water are mixed before entering the chamber of delivery equipment. The ready mix concrete (RMC) is received into feeding chamber from which the concrete is blown by compressed air at a pressure through rubber hose.
Considerable quantity of mortar or concrete, which is jetted on the surface will be bounce back due to high velocity of jet, this phenomena is called rebound. This value is depend upon the ratio of water to cement.

Advantages of wet mix process :
  1. The higher durability can be easily achieved by using air-entraining in the wet process.
  2. The water cement ratio can be accurately controlled in the wet process.
  3. Wet process does not cause dust problem.
  4. The higher rate of placing can be achieved in wet process.
  5. Work can be continued in the windly weather with the wet process easily than dry process.

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