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.
Dry mix vs. wet mix
The dry mix method (gunite) 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. The key is adding only enough water for proper “hydration”, the process which causes the cement to bond to the surrounding aggregate, and not a bit more. However, if the nozzleman is less skilled, he may add either too much or too little water, either extreme greatly weakens the future concrete.
Wet-mix shotcrete involves pumping of a previously prepared concrete, typically ready-mixed concrete, to the nozzle. In the “wet-mix” method, the moisture content can be more closely controlled. The possibilities of too little or too much water, have been eliminated. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The wet-gun procedure also 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.
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