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Sprayed concrete (also known as shotcrete and gunite) is a process by which concrete or mortar is conveyed through a hose and projected pneumatically at high velocity from a nozzle into its final location. The approach is used for both new construction (such as in situ linings for tunnels) and for repairs. No other compaction is required but the surface may be finished by hand if required.
Two different processes are used, either the wet process or the dry process. In the wet process, all the concrete constituents are batched and mixed together before being fed into a pump. The mixed concrete is conveyed under pressure to the nozzle where compressed air is injected, firing the concrete into position. In the dry process, only the dry ingredients are batched together and pumped under pressure to the nozzle. Pressurised water is introduced at the nozzle, which, along with the force from the pump, projects the concrete into the area being repaired. The addition of water is therefore under the control of the nozzle operator in the dry mix process and the end product tends to be more variable.
Acknowledgement: The Concrete Society
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