Over the past century, sprayed concrete has replaced the traditional methods of lining tunnel profiles and has become very important in stabilizing the excavated tunnel section. Modern tunneling without sprayed concrete is inconceivable. Sprayed concrete is a single term that describes various components of a complete technology:

  • shotcrete as material
  • shotcreting as placing process
  • shotcrete as construction method

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 requirements of the application and the specified parameters. As a rule, this means a reduction in the maximum particle grading to 8 mm, 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 spraying
  • wet spraying

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 and dust

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 or 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 shotcrete accelerators when required. The prepared sprayed concrete mix is then projected onto the substrate at high pressure which compacts so powerfully that a fully-compacted concrete structure is formed instantaneously.

Depending on the shotcrete 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 stabilization 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
  • concreting without formwork
  • reinforced sprayed concrete is also possible (mesh/fiber 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.