Guniting is known as shotcreting sometimes, the nomenclature is generally adopted when coarse aggregate is also used. Basically the technique comprises applying cement mortar under high pressure directly on a surface. The high pressure application ensures dense surface of high strength and low permeability. Besides, the technique ensures good bond between old surface and low permeability, better bond between old concrete and fresh concrete, when adequate care is taken in surface preparation, mix design and application.
Guniting is a particularly useful method for repairing R.C.C. columns and beams which have cracked or where reinforcement has deteriorated. Formwork is not usually necessary, and even intricate shapes can be successfully constructed.
The total thickness of shotcrete should not be less than about 75 mm so that a clear cover of 25 mm to the additional reinforcement in the case of columns is ensured. The average thickness of shotcrete should be 50 mm, if no additional reinforcement is provided. Careful guniting ensures long service life of structures. However it is essential to observe several precautions at every stage in order to obtain the desired results and durable structures.
It is desirable to test the gunited components for soundness, homogenity and strength. The quality of guniting can be ascertained from the cores drilled from the final surfaces. But this is an expensive process and may not always be feasible, it is better to rely on non-destructive tests such as sounding, rebound hammer test and ultrasonic pulse velocity tests. The code of practice recommends casting test panels of size 300x300x75 mm periodically during guniting for quality control.
As a further precaution to enhance durability of gunited surfaces, it is desirable to apply epoxy coatings over gunited surfaces. Several types of epoxy coatings are available commercially.
Another development in the process is silica fume shotcrete. Silica fume, when added to dry mix shotcrete, improves substantially the adhesive and cohesive properties of fresh concrete by creating a dense sticky mix almost devoid of bleeding. Silica fume shotcrete facilitates overhead repairs, a greater thickness can be achieved in a single pass than that with ordinary portland cement shotcrete. Its superior adhesive characteristics are advantageous in wet conditions, the quantity rebound is significantly reduced.
There has been considerable increase in the use of shotcrete in Europe and U.S.A. during the last 15 years because of the good performance of many shotcrete applications. (Strictly speaking, guniting applications). Stiff, well-compacted concrete of the type used in shotcrete is a good structural material. A well designed and well executed job makes a satisfactory work.