What is Guniting?
Guniting is a process used in construction for the application of slope stabilization and certain rehabilitation purpose mainly in the construction of retaining walls, swimming pool construction, tunnel construction, in fluid tank construction and some of the concrete repair works.
Clear definition of guniting is understood properly by knowing what is shotcrete.
Guniting was a method of early origin in the US, where the method is defined as the process of spraying a mix or mortar or concrete to a surface of application with the help of a spray gun. This method makes use of a spray gun and hence the process was named as Guniting.
Later, in the time of 1930s a method of spraying concrete or mortar mix with the help of a nozzle spray under compressed pressure and high velocity was followed by the American Railway Engineers Association(AREA).
The American concrete Institute too adopted this method in the 1950s and was named as shotcrete. These institutes never used the word ‘Gunite’ or ‘guniting’. The shotcrete can be carried out with a dry mix or a wet mix.
Now later it came to know that the dry Shotcreting process is called as the guniting. This is how shotcrete and guniting differs. Gunite is merely a trademark where some countries never use the word Gunite, instead uses dry Shotcreting process.
Procedure and Guniting Requirements
The guniting is hence called as the dry-mix shotcrete process which convey dry material from a machine to surface of application through a nozzle by means of compressed pressure and high velocity.
The application is facilitated by the addition of water at the nozzle area. The mix that finally comes out is a combination of dry material and water. The operator has the control on the addition of water and the combination water content.
The mix used in guniting is cement mortar mix. There may be variations based on the application and requirements of the area. The figure-1 below shows a typical arrangement of a guniting or dry-mix shotcrete system.
Fig.1: Arrangement of a Guniting System
As shown in the figure above, the dry ingredients are mixed in a bin and stored in a cement gun. During the time of application, under the action of pressure and high velocity, the dry mix is injected, and the water is introduced at the nozzle, just before the mix blast off.
There is a water supply pressure tank that will supply water with adequate pressure to the nozzle as shown in figure-1.
The general requirements for undergoing a good quality guniting process are:
1. A high quality finished work through guniting is obtained by having careful and skillful operation and control of the nozzle.
2. The surface of application must be free from dirt, grease or any other defective materials. This must be cleaned by high pressure water jet or by air blast.
3. Sand blasting can be carried out the surface of application before undergoing guniting. This helps in removing rust on the reinforcement.
4. If the surface of treatment has high adsorption, then it must be kept wet for a time period of 6 hours before undergoing guniting.
5. The mix proportion used for guniting are 1:3 and 1:4:5. This will generally have a water cement ratio of 0.30. A 1:3 mix will attain a strength of 70MPa in the 7th day after application.
6. The size of fine aggregate used in the mix is 10mm.
7. In order to reduce permeability, increase the resistance to weather and chemical attack, it is necessary to have high quality mortar.
Advantages of Guniting or Dry Shotcreting Process
The dry- mix shotcrete process is a method that is used widely and have the following advantages:
1. The process of guniting is highly versatile in nature. This method can be applied for structures to occupy any shape say undulating, spherical or curves. This versatility is the major reason why it is used in swimming pools, artificial caves and waterfall etc. so that special shapes and features can be provided. This feature made it employ in application of tunnel lining, slope protection, refractory works and many of the repair works. These also have a diverse use in the construction of dams, reservoirs, bridges, pipelines, and canals.
2. As the water added to the mix is carried out at the nozzle, the control of water is possible. This adjustment can be controlled instantly by the crew while carrying out the spraying procedure. This will hence make it possible to give a mix either wet or dry based on the demand. This is controlled by the addition of water at the nozzle.
3. When compared with the wet shotcrete process, the dry-mix shotcrete or the guniting mix can be prepared quickly. This is more useful in overhead application because if it is wet mix heights would bring problems like sagging or sticking or segregation.
4. For small works like filling the cracks for a smaller region or for very thin lining or coatings or for very fine treatments, it is recommended to opt dry-mix shotcrete process (guniting) when compared with the wet-mix shotcrete process.
5. Guniting is a best choice in those work types were unexpected stoppage of work is possible. The use of wet-mix shotcrete process will be not useful in such situations, resulting in wastage of mix.
Disadvantages of Guniting or Dry Shotcrete Process
1. The process of guniting will require highly skilled and experienced labor so that there is proper check on the pressure, the amount of water added at the nozzle and finally apply a good quality mix on the surface of application. Here the majority of quality is concerned with the operator who performs the guniting operation.
2. The guniting process will face the issue of rebound of the mix. As the mix is sprayed on the surface of the material there is chances that the ingredients get bounce off and then fall on the ground. This occurrence is called as rebound. This phenomenon is more common in the case of guniting of dry-mix process when compared with the wet-mix shotcrete process.