Concrete

History

Shotcrete was invented in the early 1900s by Carl Akeley, a famous American taxidermist, to be used to fill his plaster models of animals. He used the simple method of blowing dry material out of a hose with compressed air and wetting it as it was released. This was later used to patch weak parts in old buildings. In 1911, he was granted a patent for his inventions, the “cement gun”, the equipment used, and “gunite”, the material that was produced. Until the 1950s when the wet-mix process was devised, only the dry-mix process was used. In the 1960s, the alternative method for gunning by the dry method was devised with the development of the rotary gun, with an open hopper that could be fed continuously. An example of Gunite application can be seen below

The term nozzleman is used for the person who controls the nozzle that delivers the concrete to the surface. The nozzle is still controlled by hand on small jobs, for example the construction of small swimming pools. On larger civil construction work the nozzle is held by mechanical arms and the nozzleman controls the operation by a hand held remote control unit.

 Shotcrete vs. gunite

Shotcrete is today an all-inclusive term that describes spraying concrete or mortar with either a dry or wet mix process. However, it may also sometimes be used to distinguish from gunite as a wet-mix. The term shotcrete was first defined by the American Railway Engineers Association (AREA) in the early 1930s.By 1951, shotcrete had become the official generic name of the sprayed concrete process.
Gunite refers only to the dry-mix process, in which the dry cementitious mixture is blown through a hose to the nozzle, where the water is injected immediately prior to application. Gunite was the original term coined by Akeley, and trademarked in 1909, patented in North Carolina. The concrete is blasted by pneumatic pressure from a gun, hence “gun”-ite.

The term “Gunite” became the registered trade mark of Allentown the oldest manufacturer of gunite equipment. Other manufacturers were thus compelled to use other terminology to describe the process such as shotcrete, pneumatic concrete, guncrete, etc. Shotcrete emerged as the most commonly used term after gunite and after the later development of the wet process came to used be for both methods.

Dry mix vs. wet mix

The dry mix method involves placing the dry ingredients into a hopper and then conveying them pneumatically through a hose to the nozzle. The nozzleman who holds the nozzle then controls the addition of water at the nozzle. The water and the dry mixture is not completely mixed, but is completed as the mixture impinges on the receiving surface. This requires a highly skilled nozzleman, especially in the case of thick or heavily reinforced sections. Advantages of the dry mix process are that the water content can be controlled and adjusted instaneously by the nozzleman which allows the material to placed more effectively in overhead and vertical applications without the use of accelerators. The dry mix process also has advantages in repair applications when it is necessary to stop frequently, as the dry material is easily discharged from the hose.

Wet-mix shotcrete involves pumping of a previously prepared concrete, typically ready-mixed concrete, to the nozzle. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The wet-gun procedure generally produces less rebound, waste (when material falls to the floor), and dusts compared to the dry-mix procedure. The greatest advantage of the wet-mix process is that larger volumes can be placed in less time. The following video is a wet-mix shotcrete being sprayed onto pre-prepared form and steel work.

While in theory the video makes the concrete cutting look easy the ‘fine art’, as I call it, is a skill learned through experience rather than a skill you are born with. I would not recommend that a DIYer attempt to take the task upon themselves. You have little time to waste and your mistakes are literally set in stone. The Rapid Shotcrete team located here in Perth are one of those companies with years of experience. While the team may not exactly look like a team of artist, the finished product is always completed with a sense of pride and a monument of their skill, much like an artist.