Differences Between Gunite and Shotcrete

Differences Between Gunite and  Shotcrete

Generally, when concrete is sprayed at a high pressure, it is called shotcrete, due to the fact that it is 'shot' on the surface. There are actually two types of shotcrete―wet mix and dry mix shotcrete. It is this dry mix shotcrete that is commonly called gunite, just because the applicator is gun-shaped. The other type, wet mix, is simply called shotcrete. 

Let's dive right into the gunite and shotcrete comparison.

Items	Gunite	Shotcrete
Basic difference	The cement& sand mixture is sprayed at a high velocity and pressure, and then mixed with water by the operator at the nozzle end.	Concrete mixture of cement and water is ready-made previously, and then sprayed at a high velocity and pressure.
Equipment	Gunite equipment is made up of a dry mixture container, a large air compressor, a hose, and spray nozzle. The air compressor is much larger than that for shotcrete, since more air pressure is required for spraying gunite than for shotcrete. The spray nozzle is connected to a pressurized water source. The water-cement ratio (W:C) has to be controlled by the operator. The hose of the equipment is much lighter, since it contains mostly air, carrying a mixture of sand and cement. In general, this equipment is more complicated than that in case of shotcrete.	Shotcrete equipment consists of a hopper containing the wet mixture, an air compressor, a hose, and a nozzle. The air compressor is much smaller. The spray nozzle is not connected to a separate water source, since the mixture already contains water. The operator can control the rate of concrete spray. The hose is slightly heavier than that in the gunite process, since it contains the much heavier water-cement-gravel mixture. Shotcrete equipment is much simpler, and requires less individuals to operate it.
Material	This is a mixture of fine-grained cement and sand. Water is mixed only at the nozzle.	This is a mixture of cement, sand, and aggregate such as pea gravel with water. The mixture has large-grained particles.
Application process	1.Pressurized air from the compressor enters the container and carries the cement dry mixture through the hose. 2.As the mixture exits the hose, the operator mixes the cement with water at the nozzle. 3.The complete mixing of the cement and water only occurs on impact with the surface.	1.Pressurized air from the compressor drives the water-cement mixture out into the hose. 2.The mixture exits the nozzle at a high velocity. The operator has to continuously vary the rate of spray to ensure a consistent concrete layer formation.
Advantages	1.The material for gunite being dryer, becomes harder on curing. 2.Higher compressive strength than shotcrete (7,000 to 9,500 psi). 3.The cement mixture being mixed on-site, provides more work time. 4.Smoother surface finish. 5.No shrinkage cracks formed. 6.In terms of cost, this process is comparatively cheaper.	1.In shotcrete, a skilled worker is required, but not as technically trained as a gunite operator, since shotcrete errors are less critical. 2.Less rebound materials formed, which can be compacted over the floor towards the end. 3.Because shotcrete is a premixed mixture, it forms a strong and consistent coating. 4.Less time requirement.
Disadvantages	1.Requirement of a highly skilled operator. 2.More room for error due to manual control of the water-cement ratio. 3.Generation of more rebound material, requiring excessive cleanup, leading to time loss. 4.Use of rebound material in pool construction may lead to weak spots, causing cracking and waterproofing loss. 5.Gunite application results in a lot of dust generation, the level of which depends on the skill of the operator. 6.Gunite application may cause inconsistent coating, leading to formation of weak spots. 7.Gunite being a dry, stoneless mixture, may clog the hose pipe. 8.Limited structural applications due to finer grade of cement. 9.Requirement of special paints for gunite finishing.	1.Lower compressive strength than gunite (6,500 to 7,500 psi). 2.When shotcrete mix is prepared in the factory, it has to be applied within a short span of time (generally within 90 minutes). 3.To prevent cement from hardening in the hopper, the operator may have to add more water to the cement mixture, which greatly compromises the rigidity of the final product. 4.Formation of shrinkage cracks. 5.In terms of cost, this process is comparatively expensive.

Ref.:

1. Website: Henan Coal Scientific Research Institute Co., Ltd.

2. Google Plus: Henan Coal Scientific Research Institute

3. Main Products: shotcrete machine / gunite machine, concrete pump, grouting pump, plaster machine, concrete mixing machine, concrete foaming machine, etc.