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What is Shotcrete?

Updated: Feb 6

Shotcrete is a method when we spray concrete or mortar with a high velocity (130–170 m/sec) on a vertical surface. In the shotcrete method, a nozzleman sprays concrete or mortar on a vertical surface to compact it with concrete.


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.


The hardened property of shotcrete is the same as conventional concrete but the process through which we apply it helps it in making an excellent bond with the substrates. This method is applied specifically in new construction & repairs suitable for curved and thin elements. Shotcrete is also known as sprayed concrete.

Applications of Shotcrete.



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The shotcrete method is used in the construction of the thin overhead vertical and horizontal surfaces.


It’s especially used in curved and folded surfaces such as tunnel lining, domes, canals, or reservoirs.


Other structures for which it’s used are water-retaining structures, swimming pools, and pre-stressed tanks.


It’s applied for the stabilization of rock slopes and temporary protection of freshly excavated rock surfaces.


It’s utilized for protection against long-term corrosion of piling, coal bunker, oil tanks, steel-frame structures, and other structures. Moreover, it’s also used for encasing structural steel for fireproofing.


What Material is Used in Shotcrete?


The following materials can be used in shotcrete

  • Cement

  • Sand

  • Coarse Aggregates

  • Water

  • Chemical additives

  • Compressed Air

Materials, Proportioning of Shotcrete


The material composition of shotcrete is an important part, the following proportion can be used while making shotcrete:

  • Chemical admixtures are added for the 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, the following additives can also be used:

  • Superplasticizer @ 1 % by weight of cement

  • Accelerator (say sodium Silicate) @ 5 % by weight of cement

Materials

Quantity

​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

Shotcrete Mix Design

The following grades of concrete and ratios of concrete mix design used in shotcrete:

Grades of Concrete

Ratios of Concrete mix design

​(Cement: Sand: Aggregate)

M5

​1:5:10

​M7.5

1:4:8

​M10

1:3:6

​M15

1:2:4

​M20

​1:1.5:3

​M25

​1:1:2

​M30

​1:0.75:1.5

M35

1:0.5:1

M40

​1:0.25:0.5

Properties of Shotcrete

  1. In this method, cement content is high and a small maximum size aggregate is used.

  2. It increases durability in most cases.

  3. The conventional concrete is compacted by vibration. While the shotcrete is compacted by high-velocity jet impinging on the surface.

  4. This process increases the cement content and also brings about the different air-void systems that will affect the durability of the concrete.

  5. There is a greater effect on the properties of in-place concrete by the application procedures than the mix proportions.

  6. Shotcrete specimen is sewed from test panels of about one meter square and 75mm thick made by gunning out the plywood form, which is known as shotcrete.






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