Guniting or Shot Concrete (or Shotcrete)

An intimate mixture of cement, sand (or fine aggregate) and water is forced or ejected through a cement-gun and shot into place by means of compressed air. The usual equipment consists of a compressor, spray nozzle and flexible hose pipe. At the end of the hose, there is a nozzle to which water under pressure, by a separate connection, is added to form a slurry. Uniformly graded, thoroughly mixed dry materials, with low watermelon ratio are charged into the gun and shot under pressure by compressed air. Slightly moist sand works better. The usual proportions of cement and tine aggregate are 1:3 or 1:4. The fine aggregate should be well graded upto a minimum size of 10 mm usual size is 4.75 mm downwards.  Just only sufficient water necessary for the hydration of cement is used. 

There is usually 20 to 30 percent "rebound depending upon the wetness of the mixture". A very wet mixture will not stick.  While shooting, a nozzle under normal conditions is held at a distance of about 75 to 90 cm from the working face. The surface to be treated must be thoroughly cleaned of any dirt, grease or loose particles and should be fully wetted.  The correct no. of gun should be obtained for the maximum size of aggregate or sand to be used. Reinforcement, usually of 80 mm sq. mesh, may be incorporated to withstand structural or temperature stress. 

This method is very useful for rehabilitating or reconditioning old concrete, brick or masonry works which have deteriorated either due to climatic condition or inferior work.