In wet shotcreting, a concrete pump delivers ready mixed concrete with a predetermined water-cement ratio to the shooting nozzle at a rate of approximately 3 feet per second. Compressed air traveling at 150 to 200 feet per second is then introduced at the nozzle, a precise dose of fluid accelerator is usually injected into the propellant air. The mix of air and accelerating agent breaks up the concrete flow inside the nozzle, intensely mixing the concrete with the set accelerator. The propellant air drives the concrete out of the mixing nozzle at the ejection velocity required for compaction (typically 150 to 200 feet per second), and then guides it through the shooting hose to the surface. The plastic concrete begins to set a few seconds after impact with the base surface. The nozzle operator can vary the amount of air introduced, but has no direct control over the concrete's properties.
Since a wet-shotcrete pump is merely a conventional concrete pump that propels concrete down the hose to a point where compressed air can be introduced, all the principles of pumping concrete also apply to pumping wet shotcrete. Mix design, especially aggregate gradation, plays an important role in the pumpability of the concrete.
