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Sprayed concrete is produced by the wet or dry process. In both cases, mortar or concrete (maximum aggregate size 10 mm) is conveyed through a hose and projected pneumatically at high velocity from a nozzle into the repair. No other compaction is required but the surface may be finished by hand if required. Many sprayed concrete surfaces are left in the as-sprayed condition. The skill of the nozzle operator determins the quality of the repair and to some extent the amount of rebound material.
In the wet process, all the concrete constituents are batched and mixed together before being fed into a pump. The concrete is conveyed under pressure to the nozzle where compressed air is injected, firing the concrete into position. Accelerators can also be injected at this point causing the impacted material to set very quickly. 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. In the dry mix process the addition of water is under the control of the nozzle operator and the end product tends to be more variable.
Preparation of the substrate is important if a good bond is to be achieved. Layer thicknesses are typically between 25 and 50 mm for overhead work and up to 100 or 150 mm for vertical repairs. If a second layer is to be sprayed, the first layer should be allowed to stiffen and all loose material removed before spraying further concrete.
Acknowledgement: The Concrete Society
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