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:
- The sprayed concreting process
- The material sprayed concrete
- 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.
