Shotcrete

Introduction
Over the past century, sprayed concrete has replaced the traditional methods of lining
tunnel profiles and has become very important in stabilising the excavated tunnel section.
Modern tunnelling without sprayed concrete is inconceivable. Sprayed concrete is a single
term that describes various components of a complete technology:
• the material sprayed concrete
• the sprayed concreting process
• 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 requirements of the application and the specified
parameters. As a rule, this means a reduction in the maximum particle grading to 8 mm
or max. 16 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 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
• good pumpability (dense-flow delivery)
• the correct set concrete characteristics
• good sprayability (pliability)
• user-friendly workability (long open times)
• 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 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 setting 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 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 mechanisation and specialist workers are essential