Dry-mix and wet-mix shotcreting: comparison and applications

Shotcreting methods and their distinctive features

Shotcreting is the process of applying concrete mixture to the surface conveyed by compressed air using specialized equipment. There  are two methods of shotcreting: dry and wet-mix process shotcreting. These methods are distinguished by how the concrete mixture is fed to the nozzle. Both dry and wet methods have their advantages and limitations. But most shotcreting professionals note that, when properly applied, any of these methods allows for a top-quality result.

The scope of work, availability of materials and equipment, the time frame for the work, access to the construction site, the tasks and preferences of the contractor will be the deciding factors in selecting the most appropriate shotcreting method.

The final result when using dry and wet-mix process shotcreting most often will not be noticeably different because apart from some minor technical differences, the material itself is the same after hitting the base, irrespective of the application method.

Dry-mix shotcreting

During dry shotcreting into the machine, compressed air pressure is used to feed the dry mixture from one hose and water from the other. Dry mixture is pre-mixed from cement, fillers (sand, gravel) and additives (if necessary). The components are mixed to form a working solution.

To better understand the specifics of the dry method of applying, it is important to understand the process of building up a layer of shotcrete in this case.

The process of building up the shotcrete layer is performed in the following way:

• in the first stage (measured in milliseconds) almost all the mixture bounces off the surface; only a very thin layer of the cement clings on the surface, creating the adhesive layer;
• in the next stage, the smallest aggregate sticks to the surface, and everything else bounces off;
• progressively, larger and larger aggregate pieces remain on the surface until the rebound becomes stable.

The importance of shotcreting professionals for  dry-mix process shotcreting

The water-to-dry mixture ratio is adjusted directly during surface application, so the importance of an experienced shotcreting professional in this process is difficult to overestimate. If the nozzleman does not feed enough water, the surface will become dry and wobbly; and consequently, the amount of rebound will increase. Conversely, if the nozzleman  feeds too much water, the material will become liquid and drip down the surface instead of clinging to it.

Advantages of dry-mix process shotcreting

• High mixture flow rate – 130-170 m/s
• Strong adhesion
• Dry shotcreting provides an excellent adhesion to the surface, as layers are built up evenly one after the other, starting with an adhesive layer.
• High density and cohesion
• Good adhesion of grains is achieved as they are actually hammered into each other with great force. As a result, additional mixing and compaction of the concrete mixture occurs when it collides with the surface to be shotcreted.
• Optimal cement-to-water ratio
• Dry shotcreting method allows you to adapt to the construction site requirements and find the optimal cement-to-water ratio thanks to the ability to adjust the mixing of water and the consistency of the spraying mixture.
• Excellent maneuverability

Dry-mix shotcreting method allows the mixture to be fed over long distances. Noteworthy is that the transport hoses of dry shotcreting equipment are lighter than those used for the wet method, which improves maneuverability in the work.

Ability to switch equipment on and off during work

The equipment can be switched off during operation without significant loss of material since no water has yet been added. For this reason, the dry method is in greater demand, among other things, for small repairs and intermittent work.

Easy cleaning of equipment

As dry mixture is mixed with water directly during surface application rather than in the equipment, the equipment gets much less dirty during operation as compared to the wet shotcreting process. The dry mixture feeding hose can be cleaned by blowing it with air.

Disadvantages of dry-mix process shotcreting

The disadvantages of the method are the high rebound rate of up to 25%, forming of the large amount of dust and dirt, and the fact that the work must be supervised by an experienced nozzleman.

What is the area where dry shotcreting is most often used?

This method is used mainly for large renovation projects where dust protection and rebound removal can be effectively organized, in cases where a high-quality surface finish is not required, and the visual appearance is not decisive. This method is optimal for covering large surfaces such as bridges, fences, walls, and so on.

Wet-mix process shotcreting

Wet-mix shotcreting was introduced half a century later. It was primarily due to the development of construction chemistry. 

The wet-mix process is subdivided into continuous flow process and discontinuous flow process. 

Wet-mix continuous flow process shotcreting

Wet-mix shotcreting was introduced half a century later. It was primarily due to the development of construction chemistry. According to this method, the ready concrete mixture is fed into the wet shotcreting machine (concrete pump) and hydraulically (in a continuous flow) is fed through a hose to the nozzle. Another hose supplies the nozzle with compressed air, which breaks the continuous flow of mixture and delivers it to the shotcreted surface. 

Advantages of wet-mix continuous flow process shotcreting

Decreased rebound

The speed at which spraying is performed is significantly lower than in dry shotcreting, which reduces the amount of rebound by up to 10% and consequently reduces the loss of cement during application.

High quality shotcrete

The high quality of shotcrete is achieved by using additives:

• adhesion with the surface is achieved by means of forced application of the adhesive layer;
• application of a thick layer is reached by adding a solution of setting accelerants that is supplied to the nozzle by a separate hose;
• optimal cement-to-water ratio is possible due to using plasticizers.

High working speed

Modern wet shotcreting equipment demonstrates a higher productivity than dry shotcreting machines. For this reason, the wet method was preferred when working on large-scale sites.

Use of ready-mixed concrete

Since the mixture is delivered from the concrete plant in a concrete mixer truck, the skills of a nozzleman are less critical in the working process, as a solution of homogeneous composition and consistency is used. This factor also reduces the amount of dirt and dust formation when compared with the dry method.

Disadvantages of wet-mix continuous flow process shotcreting

The disadvantages of wet shotcreting include the fact that owing to the use of additives, the mixture is significantly more expensive. The second, but no less significant point is the inability to pause shotcreting works. Even if the work is stopped for a brief time, the grouted mixture, which is transported by hose, instantly sets inside it, and the hose cleaning is a long and time-consuming process.

What is the area where wet shotcreting is most often used?

Wet-mix process shotcreting is more in line with the industrial method of construction. It is also suitable for working in enclosed spaces and small sites such as rooms, swimming pools, and so on.

WET-MIX DISCONTINUOUS FLOW PROCESS SHOTCRETING

In its core, this method is in between dry and wet shotcreting. It involves loading the ready concrete mixture into a pneumatic concrete pump and using compressed air to feed it to the nozzle, from where it is sprayed at high speed to the surface in a discontinuous flow.

This method falls in between the gunite and wet-mix continuous flow methods.

The advantages of the method are that the mixture is already mixed at the concrete plant and does not need to be mixed with plasticizers. However, the method requires the application of an adhesive layer and the use of setting accelerants. Another disadvantage is the high rebound rate (25%). Also, time-consuming flushing of the hose is required if the work is paused.

So, what shotcreting method to choose?

As we already mentioned, no matter what method you choose, you can achieve excellent high-quality results, but both dry and wet shotcrete spraying have their function and use characteristics. To choose the method appropriate specifically for your needs, you should take into account different factors such as spraying team experience, scope and timing of shotcrete application, accessibility of the building site in terms of machinery, and so on. Unchanging conditions are properly chosen equipment, high-quality mixture, and careful preparation for the work. We will be happy to help you choose the shotcreting method and the needed equipment.

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