Shotcrete | Preparation, Application, Advantages & Disadvantages of Shotcrete

Shotcrete is the process of application of mortar or very fine concrete by jetting at high velocity onto the vertical or overhead surface.

The shotcrete method needs less formwork and can be more economical than normal concreting.  It is reinforced by normal steel rods, steel mesh, and fibers.

It is the force of this showering action that results in the compaction of the concrete or mortar which then forms layers of concrete to the required thickness.
Shotcreting has been an allowable way of placing cementitious material in different applications.

This mechanism reduces the rebound waste that occurs through the shotcreting proc fa these fibers also resist plastic shrinkage and cracking through their ability to enhance the early-stage tensile strength of concrete.

  1. History of Shotcrete 

The history of Shotcrete is like a story and the development of cement guns and the gunite(Shotcrete) process in Allentown was not coincidental. Carl E. Akeley (taxidermist, sculptor, and biologist) innovated the rudimentary machine on June 24, 1907, known as the “Plastergun“, that forced dry plaster through a hose utilizing compressed air.

When it gets to the nozzle, the required water was given from another hose. With the emerging jet of plaster, water, and compressed air, Akeley applied a 1/4-inch layer to the outer wall of the museum.

The machine works on the double chamber principle:

The material entered the Conveying hose from two chambers placed one on top of the other, and the two chambers were pressurized alternately. The double-chamber gun was born.

  2. Processes of shotcrete  

a. Dry mix process

b. Wet mix process

a. Dry Mix process

• The binder and aggregates are drily mixed.

• Mix is fed into the delivery hose.

• Mix is transported by compressed air to the introduced under pressure.

• Material is spread at high velocity onto the surface, where water is to be shotcrete.

Step 1: Dry shotcrete components, which may be slightly pre-dampened to reduce dust, are fed into a hopper with continuous agitation.

Step 2: Compressed air is introduced through a rotating barrel or feed bowl to convey the materials in a continuous stream through the delivery hose. water is added to the mix at the nozzle.

Step 3: Then the material is consolidated on receiving surface by high impact velocity.

In this method, the mixture of Cement and Damp sand adds water to the mix at the nozzle and is conveyed through a delivery hose pipe to a mechanical gun termed as Delivery equipment.

Advantages of the Dry mix process

1. Easy start-up, shutdown, and cleanup.

2. Controls of material on site.

3. Widely used in mining.

4. Nozzle can be to 1000ft horizontally or 500ft vertically from the gun.

b. Wet mix process

In this method, all the ingredients such as cement small-size coarse aggregate, water, and sand, are prepared before moving the chamber of Delivery equipment.

• All ingredients (excluding the accelerator) are mixed well.

• Mix is fed into the delivery hose.

• Mix is transported by compressed air to the nozzle, where the accelerator is added.

• Extra air is entered to rise velocity.

• Material is jetted at high velocity onto the surface to be shotcrete.

Advantages of the Wet process:

a. Little or no formwork is required.

b. Cost-effective method for placing concrete.

c. Ideal for irregular surface applications.

d. Permits for easier material handling in areas with hard access.

a. Shotcrete Materials

1. Portland Cement

2. Water

3. Sand

4. Aggregate

5. Adimixtures/ Fibres

b. Equipment Required

1. Batching and Mixing Equipment.

2. Adimixtures dispensers

3. Air Compressor

4. Shotcrete accelerator

  3. Preparation of Shotcrete  

The preparation of Shotcrete consists of Surface preparation which is described below.

a. Surface Preparation

Where shotcrete is to be used for repairing deteriorated concrete, it is essential that all unsound material first be removed.

Chipping should go on unless there are no offsets in the cavity which would cause a sudden change in the thickness of the repair.

The perimeter of the repair area may be saw-cut or chipped with a slight taper to the center of the area.

Feather edging should be avoided. After it has been calculated that the surface (whether concrete, masonry, or steel) to which shotcrete is to be bonded is sound, it should be made by dry sandblasting, wet sandblasting, grinding, or high-pressure water jetting.

Just prior to collecting shotcrete, all such surfaces should be thoroughly cleaned. Porous surfaces should be kept damp, but not wet, before placing shotcrete.

  4. Properties  

a. Hardened features are close enough to conventional cast-in-place concrete.

b. Shotcrete usually has more compressive strength than cast-in-place concrete due to the less water-to-cement ratio.

Shotcrete usually has:

• Contains higher water ( cement ratio )

• Contains higher sand content

• Includes smaller maximum aggregate size (10 or 12 mm)

• Accelerator to increase the hydration reactions

• Plasticizers, stabilizers, and mineral admixtures

• High ultimately age (e.g., 1 day) strength.

  5. Applications  

a. Shotcrete can lead to cost savings.

b. Where formwork is impractical or can be reduced or eliminated.

c. Where access to the work area is difficult.

d. Where thin layers or layers with variable thickness are required.

e. Where normal casting procedures are impractical.

  6. Advantages  

1. Vertical or overhead repairs

2. Fast and economical

3. Well suited for curved and irregularly shaped surfaces

4. Requires little or no formwork

  7. Disadvantages  

a. A strong wind will separate the material between the nozzle and the point of deposit, reducing the strength

b. It requires skilled and experienced laborers.

c. A greater degree of geotechnical knowledge is required

  8. Difference Between Shotcrete and Gunite  

Read More:  Reinforced Cement Concrete