Introduction to Sprayed Concrete

 

Sprayed concrete is an excellent tool for stabilization and support of structures in a very short time and for concrete application without using any molds. Sprayed concrete is also the interaction of man, machine and concrete. Sprayed concrete is a high-performance material which functions only as well as these “three components of success”. Man, personified in the work of the nozzle man, requires great technical skill and dedication to the job. The operator must be able to rely fully on the machine and the sprayed concrete material. It is the interaction and quality of these components that finally determines the success of the sprayed concrete application.

In times of rapidly increasing mobility and limited space, the need for underground infrastructure continues to grow. Sprayed concrete has an important role in this requirement. This method is economically outstanding and almost unlimited technically, making it obvious answer.

Sprayed concrete (or shotcrete)  is a single technical term that covers different components of a complete technology:

1. The sprayed concreting process
2. The material sprayed concrete
3. The sprayed concrete system

These three components define a complete technology which has a long tradition, huge potential for innovation and a great future. The material sprayed concrete is a concrete mix design that is determined by the the requirements of the application and the specified parameters. As a rule, this means a reduction in the maximum particle grading to 8mm or  maximum 16mm, an increase in the binder content and the use of special sprayed concrete admixtures to control the properties of the material. Sprayed concrete was used for the first time in 1914 and has been permanently developed and improved over recent decades.

There are now two different sprayed concrete processes:

• dry process sprayed concrete
• wet process sprayed concrete

The main mix requirements focus on the workability (pumping, spraying application) and durability; they are:

• high early strength
• the correct set concrete characteristics
• user-friendly workability (long open times)
• good pumpability (dense-flow delivery)
• good sprayability (pliability)
• minimum rebound

The sprayed concreting process designates its installation. After production, the concrete is transported by conventional means to the process equipment. Sprayed concrete or sprayed mortar is fed to the point of use via excess-pressure-resistant sealed tubes of hoses and is sprayed on and compacted. The following methods are available for this stage of the process:

• the dense-flow process for wet sprayed concrete
• the thin-flow process for dry sprayed concrete
• the thin-flow process for wet sprayed concrete

Before being sprayed, the concrete passes through the nozzle at high speed. The jet is formed and the other relevant constituents of the mix are added, such as water for dry sprayed concrete, compressed air for the dense-flow process and setting accelerators when required. The prepared sprayed concrete mix is the projected onto the substrate at high pressure which compacts so powerfully that a fully-compacted concrete structure is formed instantaneously.  Depend on the setting acceleration, it can be applied to any elevation, including  vertically overhead.

The sprayed concrete process can be used for many different applications. Sprayed concrete and mortar is used for concrete repairs, tunnelling and mining, slope stabilisation and even artistic design of buildings. Sprayed concrete construction has various advantages:

• application to any elevations because sprayed concrete adheres immediately and bears its own weight
• can be applied on uneven substrates
• good adhesion to the substrate
• totally flexible configuration of the layer thickness on site
• reinforced sprayed concrete is also possible (mesh/fibre reinforcement)
• rapid load-bearing skin can be achieved without forms (shuttering) or long waiting times

Sprayed concrete is a flexible, economic and rapid construction method, but it requires a high degree of mechanization and specialist workers are essential.