What is Shotcrete

Shotcrete is spraying the concrete with dry or wet materials through a compressed air stream at high speed on the desired surfaces. Due to the high pressure and speed are dynamically compressed to create a dense, self-supporting, and load-bearing layer.

Shotcrete is widely used as a temporary storage system and as a permanent cover in some cases.
Shotcrete, or sprayed concrete, is a general name for mortar or concrete containing cement that is sprayed to the desired spots by the machine.

Types of shotcrete

Shotcrete is split into two general categories in terms of mixing method:

  • Wet Mix Shotcrete
  • Dry Mix Shotcrete

Shotcrete film

Shotcrete film

Shotcrete or sprayed concrete

Sprinkled concrete or gunite is not a new invention. The first experience of using dry matter spraying for new structures dates back to 1907 by an American taxidermist named Carl Ethan Akeley in Pennsylvania.

The use of shotcrete is common all over the world today. It is believed that the development of technology in the future will be more mixed with the spraying process, for instance, a new generation of concrete admixtures, concrete improvers, micro silicates, polypropylene, and steel fibers for sprayed concrete.

Shotcrete History

In the past, dry shotcrete was broadly employed. Because the tools needed were more accessible and cheaper, and dry matter could be transported for longer distances (an essential advantage in mineral applications).

Nevertheless, wet mixing methods have notable advantages, including reduced waste, no need for skillful manpower, and the need for less on-site equipment in underground mines.

Shotcrete quality

Shotcrete quality depends on several factors, including operator, mixed water control, nozzle speed, and spraying method. In any of these cases, the ability and practice of the operator are very critical.

Advantages of shotcrete admixture

  1. Reduced material loss.
  2. Makes it easy to build arched and curved roofs.
  3. Increased adhesion of concrete.
  4. Seals surfaces and closes seams quickly.
  5. Reduced thickness of the final lining and no need for molding in tunnels for designs using sprayed concrete with the help of shotcrete concrete additive (sprinkled concrete) without secondary lining.

Selection of the shotcrete type

Several factors influence the choice of shotcrete, but each has its benefits and disadvantages and can be changed to the other depending on the conditions and operations.

Factors influencing the choice of shotcrete operation type:

  • Economic factors
  • Access to materials and equipment
  • Location of operation
  • Ability and capability of the operator

Today, considerable attention is paid to the material return, the amount of waste, and their properties, such as stability resistance to weathering in choosing the method.

Wet Mix Shotcrete (WMS)

In this technique, the prepared concrete is poured into the shotcrete pump, and after transfer by pipe, it is sprinkled on the desired surface.

Wet mixers are employed in spots where high compressive strength is required.

Using this method, it is feasible to build a concrete wall with a thickness of 50 cm and a roof with a thickness of 20 cm in one step.

Advantages of Wet Mix Shotcrete (WMS)

Normally, no formatting is required. In this way, molding costs, manpower, equipment, and operation time are significantly decreased. There is no need to use the formwork to see the workspace and place the concrete properly between the rebar’s.

  1. The possibility of building concrete structures with curved, circular, and irregular shapes (such as pools and ponds)
  2. The possibility of steadying mountains and rocks by covering them with a mesh and spraying concrete on them
  3. Covering the base of bridges and tunnel lining
  4. Increased thickness of concrete pipes in corrosive and fire-hazardous surroundings
  5. Better mechanical strength than dry mix shotcrete
  6. Better adhesion between concrete and rebar
  7. Reduced permeability and proper sealing

One of the most popularly accepted applications of WMS, which does not need much expertise, is the building of swimming pools and reservoirs. In this case, the sealing is done well and the permeability is very low because of the high air pressure.

Another use of WMS is dam repair. This shotcrete can be employed in executing overflows, the lining of diversion tunnels, the dam body cover (for structural reinforcement), and stabilizing maintaining walls and mountains around the dam

Dry Mix Shotcrete (DMS)

In this method, the water required for shotcrete is added in the last step while the material leaves the nozzle. This method is used for repair work and coating with a thickness of less than 10 cm.

Since this method does not use coarse aggregates, the obtained concrete does not have high mechanical strength.

Disadvantages of dry mix shotcrete

  1. Due to the lack of water for all the seeds, some parts may remain unhydrated.
  2. Dust created by scattering of cement grains in the workshop is high.
  3. Most of it is wasted due to the mortar not sticking to the surface (because of not being hydrated).

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