What is shotcrete?

Introduction to shotcrete or sprayed concrete

Shotcrete or gunite was invented by Mr. Carl Ethan Akeley (1864-1926) in 1910. For attractions of a park, this American Architect was mandated to realize in concrete the reproduction of a dinosaur. Considering the sizes of the structure, he had the idea to develop a “cement gun” machine allowing the spraying of a cementitious mortar. Shotcrete was created!

Probably a symbolic coincidence, but the same year, Mr. Kaspar Winkler founded Sika. Since that time Sika has greatly contributed to the development of the shotcrete technology. By shotcrete technology development, we mean the continuous development of chemical additives and admixtures for shotcrete and as well the development of spraying equipments.

Fields of shotcrete application

Shotcrete is mainly used in Underground construction projects as preliminary or permanent structural support. By Underground constructions, we mean the construction of structures like road-rail tunnel, hydropower plant, mines, parking, subway, metro, storage area etc.
However shotcrete is as well as an economical tool to realize stabilization work (slope), swimming pools, waterways, concrete repairs, inner lining and architectural structures. About 90% of the shotcrete applied goes into Underground construction projects. Total volume of shotcrete worldwide applied yearly is more than 12 millions cubic meters

What is shotcrete?

As per the American Concrete Institute (ACI), shotcrete can be defined as a mortar or concrete, pneumatically projected at high velocity through a pressure resistant conveying line onto a surface, where it is compacted on impact.

Cement, sand, aggregate, water, additives and admixtures are the components entering in the production of the shotcrete mix.

Compared to normal concrete, shotcrete differs mainly from three points:

  • The maximal size of the aggregate used.
  • The way to place it.
  • The mixture of shotcrete can be dry or wet.

Regarding terminology we can describe Gunite as sprayed mortar while Shotcrete as a sprayed concrete.
By gunite we means a cementitious mixture of which the particles size is limited to 8 mm.
For shotcrete we consider the use of aggregates of which the maximal size is 16 mm. However, in the last 10 years there is a tendency to limit the maximal aggregate size to 12 mm.

Shotcrete development can be summarized from its start to nowadays as follows:
Dry process –> dry process with powder accelerator –> wet process with Alkali liquid accelerator –> wet process with Alkali free accelerator.