Gunite is the general term for the process of applying sprayed concrete. Sprayed concrete was first developed in the early 1900's as a solution to repair the failing façade of a museum in Chicago. The process was later adopted as a construction/repair solution for tunnels, underground structures, slope stabilization, structural repairs and swimming pool construction. The process involves a mortar or small-aggregate concrete that is sprayed with air power onto surfaces at a high velocity. The force of the concrete spray consolidates and compacts the material and ensures that it adheres to the desired surface.
Gunite and shortcrete are essentially the same material, but they are applied with different processes. Gunite is the popular trade term for dry-gun concrete, while shotcrete is the common term for wet-gun concrete.
With the gunite method, the dry concrete ingredients are placed into a hopper, pre-dampened (to prevent static buildup) and then pushed pneumatically through a hose to a nozzle. The nozzle operator then controls the addition of water at the nozzle, turning the dry ingredients into concrete that is fully mixed by the time the material hits the desired surface. This gives the operator control over the water content being put into the mix, allowing for a better placement process without the need for additional accelerators. Gunite the recommended method when the process involves frequent stops during the application process.
Wet-mix shotcrete pneumatically pumps fully mixed concrete through a hose and out the nozzle without the addition of extra water. This is the most commonly used process, as it produces less rebound (material that falls to the ground and is wasted) and dust compared to gunite. The greatest advantage of the wet-mix process is that larger volumes can be placed in less time.
Repair to concrete structures where concrete has been damaged from external forces, environmental exposure, mechanical damage, or chemical exposure and attack. A spall in a concrete surface may be the result of localized distress or the symptom of a more widespread distress in the concrete element. In either case, an attempt should be made to determine the cause of the distress prior to taking corrective action. This can easily be accomplished by retaining the services of a reputable architectural firm, engineering firm, or contractor. Spall repair is not a band-aid to a structure in distress; it is a complex engineering task with challenges substantially different from those associated with new construction. This intricacy increases with the age of the structure and the severity of the deterioration. Standard practice for the repair of concrete spalls vary depending on the type of concrete element being repaired, cause and extent of damage, location of the spall and material selected. The repair material selection is typically based on such factors as properties of the repair material, climatic conditions, repair time frame, expected service life and cost. The repair process generally includes:
1) A physical inspection to document the details and extent of damages.
2) An assessment to determine the cause of distress.
3) The selection of repair materials.
4) The selection of an experienced contractor who specializes in concrete repair work. Repair methods include hand applied mortars, form and pump, shotcrete and gunite repairs.