What Is Shotcrete? – Everything You Need to Know About Shotcrete

The use of concrete has been an integral part of constructing various structures for many years. However, the method of application, the materials mixed with it, and the consistency of the concrete used vary depending on the type of structure. For this reason, shotcrete is often used instead of conventional concrete in many applications. You may have heard the name of this widely used construction material many times before, but have you ever wondered what shotcrete actually is and how it is defined? Is it a product or a process?

What Is Shotcrete?

Shotcrete refers to the process of spraying concrete and dry or wet materials onto surfaces. This process is usually carried out at high speed; this high velocity causes the materials to become compacted and adhere to the surface.

However, if shotcrete is considered as a material, it refers to a mixture of cement, aggregates, water, and shotcrete admixtures. In this definition as well, the high velocity of spraying concrete onto the surface is of great importance.

 

Types of shotcrete

Based on the application method, shotcrete is divided into two main types: wet and dry. This difference in the application method can lead to variations in the properties of the material. Therefore, before using shotcrete for different applications, you should understand the differences between these two methods. To purchase a shotcrete machine, visit Kalar shotcrete machine products.

Dry shotcrete

In dry shotcrete, mixing takes place inside the machine; this means that sand and cement are fed into the machine’s pipeline using a pump. Then water is added to the mixture at the nozzle exit. This can cause some cement particles not to fully hydrate. Dry shotcrete is used for creating coatings with a thickness of less than 10 cm, in confined spaces, and in small-scale projects.

Advantages of dry shotcrete:

• Relatively high strength

• Low cleaning and maintenance cost of the equipment

• Ability to be applied in confined spaces

• Requires less equipment for operation

• Low water-to-cement ratio

Disadvantages of dry shotcrete:

• Produces a large amount of dust

• Increased likelihood of errors in shotcrete mix design due to human factors

wet shotcrete

In wet shotcrete, the mixture is prepared in advance and then poured into the shotcrete machine. For this reason, it is possible to apply thicker layers using the wet shotcrete method. In the wet process, wall shotcrete with a thickness of 50 cm and ceiling shotcrete with a thickness of 20 cm can be applied. This method is used for large-scale projects.

Advantages of wet shotcrete:

• High accuracy during mixing due to mechanization

• Faster execution compared to the dry process

• Lower dust generation

• Less material rebound during application

• Ability to construct curved or irregular structures

• Increased thickness of concrete linings in corrosive environments

• Better adhesion to reinforcement bars

• Good waterproofing properties

Disadvantages of wet shotcrete:

• High cost of equipment and labor

• Relatively low initial strength

• Limited distance between mixing site and application site

• Short setting time of shotcrete, which restricts project execution time

The table below shows the differences between wet shotcrete and dry shotcrete from various aspects:

	Wet shotcrete	Dry shotcrete
Maintenance cost of required equipment	Low	High
Cost of purchasing the required equipment	High	Low
Mix design	Precise control of the mix design without human intervention **Ability to adjust aggregate moisture content	Possibility of mixing cement and aggregates outside the construction site **High error in adjusting aggregate moisture content
Execution method	Possibility of manual shotcrete application at a rate of 3 to 10 cubic meters per hour Capability of up to 25 cubic meters per hour using remote-controlled equipment	Application rate of 1 to 5 cubic meters per hour
Waste (material loss)	Between 5% and 15%	Up to 30%
Dust generation	Low	High
In-place quality	Consistent	High variability
Transport distance through hose	Up to 200 meters	Up to 500 meters
Application	High-volume work	Low-volume work or intermittent operations
Operation (or Operator control)	Relatively easy	Requires high skill and precision

 

Factors affecting shotcrete quality

Various factors play a role in determining the technical properties and quality of shotcrete, the most important of which include the following:

• Shotcrete mix design

The technical specifications of shotcrete depend on its mix design, meaning the method and proportions of mixing sand, cement, and aggregates. Aggregates vary in size depending on the intended application. The more closely the technical properties of shotcrete comply with standards, the higher the strength and dynamic performance of the resulting layer will be.

• Method of execution

Operator, spray method, and nozzle speed are among the most important factors affecting shotcrete quality. The operator’s experience and skill not only influence concrete quality, but can also affect material consumption and the durability of the cement spraying machine. For example, if the operator is not careful during concrete mixing, the concrete will not have proper adhesion and its physical properties will be poor.

If you are interested, you can read the article “What Is Shotcrete Concrete?”.

Applications of shotcrete

Today, shotcrete is widely used as both a material and a method in various civil engineering operations. Almost anywhere a strong concrete layer is required, either dry or wet shotcrete can be used. In the following, some of the most important applications are mentioned:

• tunnels
• Dams
• Various ground surfaces such as sidewalks
• Swimming pools
• Different parks
• Fire-resistant surfaces
• Domes
• Areas of large industrial structures
• Wall coatings in zoos, pools, and dams

Shotcrete mix design

If you want to know the importance of shotcrete mix design, it should be said that its importance is related to the differences between wet and dry shotcrete. In other words, depending on the type of project you intend to carry out and the surface you want to cover, the mix design will be affected. The general important points in the design are as follows:

• Using sharp-edged sand or rounded sand
• Maximum size of used aggregates up to 20 mm
• Maintaining a water-to-cement ratio in the range of 0.35 to 0.5
• Maintaining a cement-to-aggregate ratio of 1 to 4 to 4.5

Which type of shotcrete is suitable for you?

The operator, spraying method, and nozzle speed are among the most important factors affecting shotcrete quality. The operator’s experience and skill not only influence concrete quality but can also affect material consumption and the durability of the cement spraying machine. For example, if the operator is not careful during concrete mixing, the concrete will not have proper adhesion and its physical properties will not be desirable.

• Project size and volume
• Project cost and budget
• Required project execution time
• Location of the operation
• Operator capability
• Available equipment
• Resistance of materials to weathering
• Desired coating thickness
  
  

Advantages of using shotcrete

The use of shotcrete has many benefits. Here we will discuss some of the key advantages of this popular material:

• Fluidity

Shotcrete is widely used in the construction of swimming pools and other hydraulic structures and is ideal for this purpose. Not only is it easier to spray this material instead of forming a solid structure, but the materials used in it also make it lightweight.

• High durability

Since this material has been used for several decades in various applications, it has proven its reliability. You only need to spray it onto the surface and you can be confident in its long-term durability. These surfaces remain resistant for decades under different conditions, including heat, pressure, water, and impact.

• No need for formwork

You may wonder what the reason behind this advantage of shotcrete is. Normally, a structure requires formwork to contain and shape poured concrete under internal pressure; however, this is not the case with shotcrete, as the material can be easily applied to surfaces by spraying without the need for conventional formwork.

• Time and cost savings

When you use shotcrete for your project, you achieve significant cost savings because materials such as conventional concrete are expensive and heavy.

• Admixtures

Shotcrete can also be combined with other materials to improve its properties. For example, you can slightly increase the amount of water used in the mix to make it lighter.

• Ease of use

Since shotcrete can be sprayed onto any surface, it can even be used in areas that are difficult to access. Whether the target surface is underground or above ground, shotcrete performs effectively. It can be shaped into different forms regardless of whether the surface is flat or has cavities, slopes, or irregularities. Shotcrete can be used effectively in almost any location.

Method of applying shotcrete using a shotcrete machine

The components of shotcrete (aggregates, cement, water, fines, chemical admixtures, etc.) form a complex system that ultimately determines the final quality. One of the key aspects in determining shotcrete quality is the correct application method, which is essential to ensure proper adhesion of shotcrete to the surface and to minimize rebound and material loss. Following the rules below during execution helps achieve the best possible results and maximum operator safety.

• Cleaning the surface before spraying

For preparing the substrate in underground applications, the first step is to assess the risk of rock falls and loose material. All loose materials and debris must be removed, and the substrate must be cleaned of dust using water, air, or a combination of both.

• Maintaining a distance of 1 to 2 meters between the nozzle and the target surface

The force that ensures the adhesion of shotcrete to the surface is determined by the distance between the nozzle of the spraying machine and the target surface. If the distance is too short, it leads to a high amount of rebound (material bounce-back). In addition, if the distance exceeds the standard range, the adhesion and compaction of the shotcrete will be significantly weak.

• Positioning the nozzle at a 90-degree angle relative to the surface

The angle between the nozzle and the target surface affects the proper compaction of shotcrete and the amount of rebound. In addition, spraying should be carried out in circular motions evenly over the entire surface to achieve a uniform result.

• Filling holes and cracks on the surface

Before applying the first layers, it is necessary to level the surface by filling holes and cracks in the ground.

• Layer-by-layer application

The layers should be applied starting from the bottom of the surface and progressing upward. The first layer acts as a support and bonding layer for the subsequent layers. The number and thickness of the remaining layers depend on the intended application of the structure.

• Clean the surface after longer intervals between layer applications.

If a long time passes between applying one layer and the next, it is recommended to clean the surface with water, air, or both to remove dust and facilitate adhesion to the surface.

• Cleaning the concrete spraying machine immediately after use

A key part of proper maintenance and cleaning of equipment is preventing material buildup in hoses, nozzles, concrete pumps, and additive pumps. This helps avoid irregular flow and ensures smooth operation. Operations can only be resumed with equipment that has been fully and properly cleaned.

• Hydrating the surface

To ensure that sprayed concrete does not lose its mechanical strength and to prevent cracks caused by rapid hardening, it is recommended to keep the surface moist for at least 7 days. To maintain moisture, methods such as spraying water on the surface, using coverings like paper or plastic sheets, or adding special admixtures to the concrete mix can be used.

• Continuous operator training

The operator, by keeping all shotcrete-related details up to date, achieves the best possible results in the project. The operator’s responsibilities include effective use of concrete spraying equipment, correct material mixing (proportioning, preparation, and application), and proper spraying techniques.

What are the execution (application) guidelines for shotcrete?

Knowing what the execution guidelines of shotcrete are helps you achieve a higher-quality material. Some of these guidelines are listed below:

• Maximum aggregate size in National Building Regulations of Iran (Chapter 9) is limited to 10–20 mm.
• Suitable aggregate-to-cement weight ratio is 3.5 to 4.
• Water-to-cement ratio is limited to 0.4 in wet-mix shotcrete.
• Use of cement with a dosage of 350 to 450 kg per cubic meter in the wet method.
• Designing for a 28-day characteristic strength between 35 and 55 MPa.

Why should admixtures be added to shotcrete concrete?

Due to the high speed of the spraying process, the aggregates in the mixture may rebound and be lost. To prevent or reduce this material loss, additives such as silica fume should be added to shotcrete. This rebound during application can cause the properties of the sprayed concrete on the surface to differ somewhat from those of the mixed concrete. Therefore, this difference should be considered during mix design, and it should be minimized by using additives.

In addition to reducing material loss and preventing the rebound of aggregates, especially coarse aggregates, admixtures also make the construction of curved and arched ceilings easier. Furthermore, admixtures increase the adhesion of concrete and can improve the speed of sealing joints and waterproofing the surface.

What are the safety precautions when working with shotcrete?

In addition to the execution guidelines that improve the quality of this material and the coating produced by it, a series of safety precautions must also be considered during work. If you want to know what the main safety precautions during shotcrete application are, do not miss this section:

• When using the dry method in environments with potential explosion hazards, precautions must be taken; because the dry process involves static electricity, which may lead to accidents.
• Rebounded aggregates from the work surface travel at very high speed; therefore, you must protect your eyes and body during operation.
  Wear full-body protective clothing to prevent sprayed cement from damaging your skin.
  The use of masks is essential due to the alkaline environment created in shotcrete work sites.
• Use goggles equipped with water spray and wipers.
• Mixing shotcrete slurry runoff with lime water and white alum to prevent environmental damage.

Shotcrete price

The price of shotcrete, as well as its machinery and equipment, depends on several factors. These include the type of shotcrete machine (dry-mix or wet-mix), mesh thickness, and… . …can be mentioned. For up-to-date and cost-effective pricing, you can contact the pricing consultants for this material on the Kalar Machine Manufacturing website.

Conclusion

Shotcrete is cost-effective both in terms of time and economy and plays a highly effective role in the construction process. When compared with traditional concrete placement methods, it is an excellent method for flooring and structural repairs. It provides high bond strength, increased compressive and flexural strength, and improved resistance against chemical and alkali attacks.

Both wet and dry shotcrete can be an appropriate, cost-effective, and durable solution for many design requirements. Using this material or spraying method for flooring and applying fireproofing to structures—in other words, using a fire-resistant coating—can significantly enhance a building’s safety against natural disasters such as fire or seismic loads caused by earthquakes.