Gunite can be defined as a mortar made up of cement and sand that is transmitted by a gun. On a backing surface, it is pneumatically driven through a nozzle where high-velocity water is supplied.
Guniting is the process of applying mortar or concrete under pneumatic pressure with a cement gun, resulting in highly strong concrete with a high bond.
The high-velocity mix that exits the nozzle strikes the surface that needs to be fixed or protected. The coarser particles bounce from the surface during the process, leaving a superb bond coat of fine grout in close proximity to the backing surface.
A thin layer of grout forms during the process, acting as a cushion, minimizing the percentage rebound in subsequent layers. Because the rebound material contains more coarse components than the mix leaving the gun, the composition of the material deposited on the backing surface differs from that of the mix leaving the gun.
Application Procedure of Guniting
Rebound becomes the most important consideration in the application of gunite because it affects the economy. The following are approximate rebound values for various working conditions:
| Basements | 30% |
| Vertical Walls | 40% |
| Overhead Slabs | 50% |
| Beam Sides and Bottoms | 55% |
| Columns | 65% |
The material is compacted as a result of the impact caused by the jet force. For guniting, a relatively dry mix is preferred because the material will support itself without sagging even in vertical and overhead applications.
The guniting process is done in 40-50 mm layers. Following the application of the first layer and its drying, all loose material and laitance is removed with a hammer in order to find fake regions caused by a lack of bond or rebound pockets.
During the second layer’s placement, these pockets are cut and refilled.
The compressive strength of a good properly compacted gunite cured for 28 days reaches 42 N/mm2. Gunite weighs 2300 kg/m3 in average.
The curing period for guniting can last for 7 days.
Also Read : Different Types of Mortar Used in Construction
Also Read : Concrete Expansion Joint Filler Materials
Uses of Gunite
- Gunite can be used to make thin-section structures such as folding plates, shells, and thin walls, as well as tunnel and swimming pool linings.
- It’s also utilised to restore concrete that’s been damaged by fire, earthquakes, or chemicals.
- It can also cover prestressing wires and steel pipelines with protective coatings.
- From an architectural standpoint, guniting is best suited for providing rough surface texture.
- For refractory castables, pneumatic guniting is also used.
Standard Gunite Mixes Proportion
The proportions in place for various mixes for optimum nozzle velocity are shown in the table below.
| Nominal Mix Place in Gun (Cement: Sand) | Mix in Place (Cement: Sand) |
| 1 : 3 | 1 : 2 |
| 1 : 3.5 | 1 : 2.8 |
| 1 : 4 | 1 : 3.1 |
| 1 : 4.5 | 1 : 3.3 |
| 1 : 5 | 1 : 3.6 |
| 1 : 6 | 1 : 4.1 |
Precautions To Be Taken While Guniting
- For walls, guniting should begin at the bottom. The first gunite coating should completely surround the reinforcement. The distance between the nozzle and the backing surface should be between 0.6 and 15 metres.
- It is necessary to thoroughly clean the backing surface. Cleaning is followed by wetting and damp drying on concrete and masonry surfaces.
- The nozzle velocity must not exceed 140 m/s. The material granules in the nozzle interact with the rebound material at higher velocities, resulting in a porous mass with lower strength.
- Water pressure should be 0.45-0.675 N/mm2 greater than air pressure. If the air or water pressure changes, a certain amount of a too dry or too wet mix is applied, resulting in a splotchy appearance.
- For structure repair, the thickness of gunite should not be less than 40 mm.
- Reusing the rebound material is not recommended.