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What is Custom Concrete? Where is Shotcrete Used?

Custom concrete is the name given to concretes manufactured to satisfy customer expectations that differ from the properties of normal concrete based on the area of application. In our series on custom concrete, the definition and importance of certain types of custom concrete as well as the materials, the blend ratios, their characteristics and areas of application will be discussed. This article will discuss a type of concrete called shotcrete, which is ideal for concrete applications such as tunnel lining, folded-plate roofs and shells. Shotcrete used for slope stabilization and repair offers an application advantage because the cost of forms is reduced.

Types of Concrete Why Use Custom Concrete?

As technology continues to improve, there are also advances in the concrete industry, and these innovations in concrete production are referred to as custom concrete.

Custom concretes are different from normal concrete in terms of their properties and production techniques. These differences are achieved in the following ways:

  • By using aggregate with different properties
  • By including special additives
  • By using normal aggregate and cement but changing the component ratios
  • By changing the production and/or application technique

What is Shotcrete? Areas of Application

Shotcrete is the name given to sprayed concrete where the slurry is sprayed on a solid surface at high speed from a hose under high pressure. The fresh concrete mixture sprayed on the surface at high speed gets compacted and adheres extremely well. In fact, it can even stick to vertical or overhead surfaces without sagging.

Areas of application for shotcrete include:

  • Thin, concrete sections with little reinforcement, such as shells, folded-plate roofs, and tunnel linings.
  • Stabilization of rock or soil on cleavage slopes and deep excavation surfaces
  • Places where making forms is challenging
  • Repairing damaged concrete
  • Protecting steel elements from fire
  • As a thin coating over concrete or steel surfaces
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Material Components of Shotcrete and Blend Proportions?

The materials used in the manufacture of shotcrete are similar to those in normal concrete. Cement, small and large aggregate, water, and when necessary, chemical and mineral additives and fibers (polypropylene, steel) are used.

The most important problem in the application of shotcrete is rebound of the sprayed concrete which prevents adhesion to the intended surface or the concrete adhering to the surface when applied but later coming loose and falling off. In order to prevent this, the mixture must not be too wet or too dry.

The amount of rebound in the shotcrete depends on a number of factors, including blend design, the adhesion capacity of the mixture, the application technique (application equipment, the speed at which it is ejected from the spray nozzle, the diameter of the hose) and construction site conditions (experience of the operator, the type of surface it is being applied to, the thickness of the layer, ambient temperature, etc.).

The best water-cement ratio for shotcrete blends is between 0.35-0.50.

Large aggregate in the blend is more likely to rebound. The amount of rebound is also higher in the initial layers and in applications to ceilings. The likelihood of shrinkage also increases as the amount of aggregate decreases in the parts that fall.

In order to prevent clogging during spray application and an increase in the diameter of the hoses carrying the mixture (and the resulting increase in pressure), the amount of large aggregate used and the largest particle size of said aggregate (Dmax) should be kept to a minimum.

With shotcrete that uses fibers, the amount of reinforced steel mesh can be reduced depending on the amount of fiber. In shotcrete with fiber, the amount of fiber rebound and separation from the surface increases in proportion to the aggregate/binding material ratio.

How is Shotcrete Manufactured?

There are two different methods used to produce and apply shotcrete:

  1. In the wet system, the cement, aggregate and water are mixed ahead of time. This wet mixture is transferred to the spray nozzle inside a hose using compressed air or a pump, and then sprayed at high speed on the surface to be covered.
  2. In the dry system, the cement and aggregate are mixed dry, and transferred to the end of the hose with compressed air. Pressurized water is then added to the dry mixture at the end of the hose, and the resulting wet mixture is then sprayed on the surface to be concreted at high speed.

Although both systems can be used successfully, the amount of water added to the mixture can be better controlled in the wet system, which means the quality of the concrete can also be better controlled. There is also less dust created in the wet system, which results in healthier working conditions. On the other hand, the dry system is better suited for concreting surfaces that have flowing water.

How is Shotcrete Applied?

The quality that will be achieved with shotcrete applications depends largely on how experienced the operator is at using the nozzle.

In order to ensure the best shotcrete application, you must:

  • Remove all dust, dirt and loose materials from the surface where the shotcrete will be applied.
  • Clean all corrosion from the steel reinforcements, if applicable, on concrete surfaces that will be repaired.
  • Avoid spraying the intended surface until a homogeneous mixture comes out of the hose at the beginning of the process.
  • Keep the vertical distance from the end of the hose to the surface between 0.5-1 m.
  • Ensure that the steel reinforcements on the surface to be sprayed have at least 20-30 mm of clear space behind them, are clean and then completely filled and covered with concrete.
  • On vertical and sloped surfaces, apply shotcrete beginning at the bottom.
  • Continue applying in layers working towards the top.
  • Ensure the maximum thickness of the layers is 30 mm on ceilings and 50 mm on vertical elements, bearing in mind the experience of the operator.
  • Wait for previously applied layers to achieve some degree of hardening (this time period can be shortened by using additives that accelerate setting time).
  • Immediately clean up and remove rebound material.
  • Cure the concrete (the risk of rapid drying is high because the surface area/volume ratio is high).
  • Protect the concrete from freezing until compressive strength reaches 5 MPa. 

What are the Advantages of Using Shotcrete?

Shotcrete is a custom concrete that can be used to prevent the risk of collapse in structures such as tunnels and slopes, or for decorative purposes in applications such as pools and parks. What gives shotcrete an advantage over normal concrete in the areas of application covered in this article is the technique used to place the concrete. There is less of a need for forms and details, which saves on building costs. You can read about other types of custom concrete in our series of articles on the subject.

References Used:

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