Sprayed on concrete is a fast, easy method of installing cement for specific projects. The technique has been in use for decades but became popular in the 1960s. While many workers use the terms Gunite and Shotcrete interchangeably, there are subtle differences between the two.
What is gunite?
Gunite originated in the early 1900s. At the time Carl Akeley, a hunter and naturalist, was looking for a method of adhering plaster to animal-shaped wireframes. He designed a technique where he could spray the plaster into the structure, creating the gunite method.
In the early 1910s, the cement gun was invented based on Akeley’s design and the process changed from using plaster to using a concrete mixture. While it has been in use since the early 1910s, this method of installing concrete coating was not made famous until the 1950s and 60s.
The method of distributing gunite is what sets it apart from other sprayed concrete applications. Gunite is a dry gunned method meaning that the cement and sand are injected into the airstream of the sprayer. This stream of dry cement mix and sand is forced through the nozzle where it is mixed with the water. Before passing through the nose, the mixture remains dry.
The gun operator controls the water ratio at the nozzle. Due to this control, the operator must be extensively trained on how to manage the machine. Poor or little training can result in an inadequate mixture.
What is Shotcrete
Until the 1950s, all sprayed concrete projects used the gunite method, when the development of a wet mixture for cement guns occurred. Like it is predecessor, shotcrete is a mix of cement and sand blown through an airstream. The main difference is when the water is added.
Unlike gunite, shotcrete is injected into the stream wet. The appropriate water ratio occurs before delivery to site. The operator does not control the addition of water into the stream at the nozzle. While contractors still need significant training on proper use of the cement gun, it is not as technical with the shotcrete method.
Advantages of Using Sprayed Concrete
Many contractors in the trenchless technology industry use the names gunite and shotcrete interchangeably, arguing that Gunite is a patented name for the method. However, both delivery methods have advantages in certain situations, which may make one a better choice, depending on the project.
Both methods offer the advantage of being placed and consolidated at once. Additionally, the methods require a lower water-cement ratio than traditional cementing methods. Using shotcrete or gunite instead of conventional concrete is generally cheaper and offers less shrinkage over time. Sprayed concrete projects also have lower permeability. Both can be used in trenchless rehabilitation, trenchless construction and other trenchless projects.
Gunite’s most significant advantage is the ability of the operator to control the water content. Workers can do this instantly if they see that the mixture needs more or less water during application.
Additionally, due to the ability to adjust the water mixture, gunite is most effective on projects where the concrete is sprayed overhead. With a maximum aggregate size of 3/8”, dry gunning has improved quality and manageability when compared to traditional cementing methods.
In projects that require frequent stops during application, gunite is the better choice when compared to shotcrete, as the mixture remains dry until use.
Shotcrete works well with steel fiber and can replace welded wire mesh for many projects. Using steel fiber and shotcrete allows for better ductility, toughness, and flexural strength in the completed structure.
Since shotcrete comes premixed at the appropriate water to cement ratio, it is not as technical, although operator training is still essential. However, when a mistake is made during application, it is less critical than if a mistake is made using gunite.
Shotcrete produces less dust during application. For overhead sprays, the wet mixture produces less rebound, which occurs when material falls to the ground during the spray.
Finally, when compared to gunite shotcrete allows for a more substantial volume to be sprayed in a shorter amount of time. This time saving is due to the premixture of water and cement before application.
Uses for Gunite and Shotcrete
Spray concrete is not always appropriate for every job, despite its advantages. However, there are several projects where sprayed application, either gunite or shotcrete are the preferred method.
Construction workers find that sprayed concrete works best for slope stabilization and retention walls. Once the framework is in place, using shotcrete helps to erect the stabilizer faster and more cost-effective than traditional methods.
Gunite and shotcrete also work well for ditches, dikes, and dams. Additionally, workers building tunnels find sprayed concrete to offer the reinforcement necessary to keep the shaft from collapsing in on itself.
Domed structures with concrete ceilings benefit from using both gunite and shotcrete. However, if the dome is 50-feet or larger in diameter, shotcrete is the best choice.
Most commonly, inground pool construction uses both gunite and shotcrete. The sprayed on method stabilizes the pool structure and allows for completion in less time.
Shotcrete and gunite are both useful methods of applying concrete to a structure quickly. Gunite’s dry application makes it more versatile than shotcrete. However, the ease of not having to adjust the water ratio during use makes shotcrete the go-to choice for many contractors.