Shotcrete
Shotcrete is a wet mix design that is applied pneumatically using a pump and hose. Shotcrete gets compacted as it is placed due to the velocity it is being delivered. It is reinforced by steel—as are all three methods—and is the proper term to be used when referring to pneumatically placing concrete. Although the term shotcrete is a verb, it is commonly used to define both wet-mix and dry-mix methods.
Shotcrete concrete is mixed in plant (similar to the cast-in-place method) where all ingredients, including water, are combined and put in a ready-mix truck to be delivered to the site. The mix is then placed in the hopper of a concrete pump, and with the use of air the concrete is pumped, via high velocity, through hoses and a shotcrete nozzle. The concrete is placed in layers in a back and forth motion to attain the desired shape and thickness. The shotcrete method is used in many diverse areas such as tunnels, bridges, and ground stabilization. Although not required, a certified-nozzle person is recommended as proper technique is critical in constructing a structurally sound and watertight vessel.
Gunite
Gunite is a form of concrete that is applied to a surface pneumatically, similar to shotcrete. The mix design for the gunite application is different; however, as it has a higher percentage of cement and a lower amount of water than the two previous methods. Like shotcrete, the formula results in a concrete mix that is dry enough to be applied to vertical surfaces such as pool walls.
Gunite concrete is often referred to as dry mix. The reason for this label is the dry nature of its application. Unlike cast-in-place or shotcrete, with gunite all the dry ingredients are mixed at a concrete plant, or at the jobsite, before it is applied. There is no water added to the dry concrete mix until it is applied via the gunite nozzle.
Gunite equipment forces the dry concrete mixture into a hose under air pressure. The dry mixture passes through the hose to a specialized gunite nozzle that is connected to a water source under pressure. The dry-mix design of all the ingredients is combined with the water and mixed through channels in the nozzle before being placed. The result is a concrete mixture that can be applied extremely dry with a high cement content to bind the aggregate with a very high comprehensive strength.
