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.

The guniting is the most effective process of repairing concrete work which has been damaged due to inferior work or other reasons. It is also used for providing an impervious layer.

The gunite is a mixture of cement and sand, the usual proportion being 1:3. A cement gun is used to deposit this mixture on the concrete surface under a pressure of about 20 to 30 $N/cm^2$.

PROCEDURE OF Guniting

The cement is mixed with slightly moist sand and then necessary water is added as the mixture comes out from the cement gun. A regulating valve is provided to regulate the quantity of water.

The surface to be treated is cleaned and washed. The nozzle of gun is generally kept at a distance of about 750 mm to 850 mm from the surface to be treated and the velocity of nozzle varies from 120 to 160 m/sec.

Advantages of Guniting

Following are the advantages of guniting

• The high compressive strength is obtained. Strength of about 56 to 70 $n/mm^2$ at 28 days is generally obtained.
• The high impermeability is achieved.
• The repairs are carried out in any situation in a short time.

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.

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.

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.