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Sprayed concrete is pumped to the point of application and then pneumatically projected into place at high velocity, frequently vertical or overhead. This calls for a rapid-stiffening mix that will not fall away from the surface. Admixtures can be used in the fresh concrete to give stability and hydration control (a form of retardation) before spraying. An accelerating admixture injected at the spray nozzle then controls the rheology and setting of the concrete to ensure a satisfactory build-up with minimum rebound.
Admixtures used in wet-process sprayed concrete to control stability and hydration during pumping and before spraying include conventional superplasticisers and various retarders. Conventional accelerators for both wet and dry sprayed concrete include aluminates, silicates and carbonates. These are available in either liquid or powder form. They are usually alkali salts with a high pH that can give rise to health and safety issues. Alternatively, alkali-free accelerators may be used. These are much safer and environmentally friendly and give greater long-term strength but not usually such rapid early strength development.
Sprayed concrete admixtures normally form a package of products covering plasticisers, retarders and accelerators. They will often be purchased from a single manufacturer who will advise on those most appropriate. The main considerations include how long workability is required and whether early strength gain is important.
Admixtures are covered in; BS EN 934-5: 2007 Admixtures for concrete, mortar and grout - Admixtures for sprayed concrete
Acknowledgement: The Concrete Society
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