8.7 : Shotcrete

Shotcrete is a specialized technique where mortar or small aggregate concrete rich in cement content is sprayed onto a surface at high velocity. The force of impact compacts the material and enables it to cling to vertical or overhead areas without sagging. The technique involves layering the shotcrete in stages until it reaches approximately 4 inches in thickness. Operator skill in nozzle management is pivotal in deciding the quality of the shotcrete. Shotcrete is used in constructing tunnel linings, shell and flat roofs, pre-stressed concrete tanks, stabilizing slopes, constructing swimming pools, and restoring deteriorated concrete structures.

There are two primary methods for applying shotcrete, namely, dry mix and wet mix. The dry mix method starts by combining cement with moist aggregate. This mixture is then placed into a feeder and propelled through a hose by compressed air. As it arrives at the delivery nozzle, pressurized water is added to the dry mix, transforming it into wet concrete that is then sprayed onto the structure. Conversely, the wet mix process involves pre-mixing all ingredients, including water, before loading into the delivery system. This pre-mixed wet concrete is conveyed pneumatically through a hose to the nozzle, where compressed air is injected to enhance the force at which the shotcrete is applied to the surface.