The main difference between gunite and shotcrete mixes is the stage at which water is added. But there are many other variations too. Gardenerdy will clear all your misconceptions about gunite and shotcrete pools.
Did You Know?
NASA found gunite suitable for use in space shuttle launchpads, while shotcrete was rejected because of the problem of melting.
Swimming pools are of two main types―above ground and inground. Different materials are used to make inground pools, like vinyl, fiberglass, and concrete. Obviously, concrete gives a strong and long-lasting pool. But while preparing the shell of the pool, there are different types of concrete mixes that can be used, leading to a lot of confusion.
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 vs. shotcrete comparison.
A gunite pool is a pool shell formed by spraying at a high velocity and pressure, a dry cement mixture that is mixed with water manually by the operator at the nozzle end.
A shotcrete pool is a pool shell formed by spraying at a high velocity and pressure, a ready-made mixture of cement and water that has been prepared by the manufacturer.
The construction process of both gunite and shotcrete pools is very similar, with the only difference being the concrete spraying stage. Given below are the general steps:
- Measurement and excavation of the ground.
- Installation of adequate plumbing.
- Installing a reinforced cage of ⅜” steel rebars, placing them at a distance of 10″, and securing the reinforcement with a wire.
- This is the stage of spraying concrete or mortar―either gunite or shotcrete―on the reinforcement. Both are applied in a similar fashion, using the compressed air and spray mechanism. In gunite application, the cement and water do not mix completely until the spray hits the surface. How well this happens depends on the skill of the operator. After application of both gunite and shotcrete, compressed air is blasted over the surface for compaction of the material. After this, the operator manually levels the surface using a trowel.
- A finish is applied on the concrete after about a week’s curing. It may be either plaster, which is just cement mixed with marble sand, paint, or fixing tiles. The pool is now complete.
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 gun differs too. 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.
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 an aggregate such as pea gravel with water. The mixture has large-grained particles.
- Pressurized air from the compressor enters the container and carries the cement dry mixture through the hose.
- As the mixture exits the hose, the operator mixes the cement with water at the nozzle.
- The complete mixing of the cement and water only occurs on impact with the surface.
- Pressurized air from the compressor drives the water-cement mixture out into the hose.
- 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.
WHICH ONE TO USE?
- When deciding whether to use shotcrete or gunite, the compressive strength should be kept in mind. Experts recommend a minimum strength of 4,000 psi (pounds per square inch). In general, a gunite pool with a 3,000 psi capacity will be equally strong as a shotcrete pool with a 3,000 psi capacity. When the pressure-bearing capacity is the same, whether gunite or shotcrete is used, does not matter.
- Another important consideration is the soil composition and strength. Consult a third-party soil engineer for the appropriate concrete suitable for that particular soil.
- The material for gunite being dryer, becomes harder on curing.
- Higher compressive strength than shotcrete (7,000 to 9,500 psi).
- The cement mixture being mixed on-site, provides more work time.
- Smoother surface finish.
- No shrinkage cracks formed.
- In terms of cost, this process is comparatively cheaper.
- In shotcrete, a skilled worker is required, but not as technically trained as a gunite operator, since shotcrete errors are less critical.
- Less rebound materials formed, which can be compacted over the floor towards the end.
- Because shotcrete is a premixed mixture, it forms a strong and consistent coating.
- Less time requirement.
- Requirement of a highly skilled operator.
- More room for error due to manual control of the water-cement ratio.
- Generation of more rebound material, requiring excessive cleanup, leading to time loss.
- Use of rebound material in pool construction may lead to weak spots, causing cracking and waterproofing loss.
- Gunite application results in a lot of dust generation, the level of which depends on the skill of the operator.
- Gunite application may cause inconsistent coating, leading to formation of weak spots.
- Gunite being a dry, stoneless mixture, may clog the hose pipe.
- Limited structural applications due to finer grade of cement.
- Requirement of special paints for gunite finishing.
- Lower compressive strength than gunite (6,500 to 7,500 psi).
- When shotcrete mix is prepared in the factory, it has to be applied within a short span of time (generally within 90 minutes).
- 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.
- Formation of shrinkage cracks.
- In terms of cost, this process is comparatively expensive.
Experts opine that all types of concrete, whether cast-in-place, gunite, or shotcrete, will give good long-lasting pool shells when done properly. It has been widely accepted that using only one technique at the total exclusion of the others may rob you of the benefits of all three. The bottom line is, there is a place for all the above methods at some stage in the construction of a pool.