The two basic methods for applying shotcrete are the dry-mix process and the wet-mix process. The term gunite is frequently used when referring to the dry-mix method; other terms such as air-placed concrete, gunned concrete and sprayed concrete have also been used in the past to describe the process.
DRY-MIX PROCESS SHOTCRETE
In this process, cement and moist aggregate are mixed and then placed into a device that meters the mixed material into a stream of compressed air. Material is carried by the compressed air through a delivery hose to the nozzle where water is added under pressure through a perforated ring. The water thoroughly wets the other ingredients as the mixture is jetted from the nozzle at high velocity onto the surface to be shotcreted.
In the wet-mix process, all ingredients are first mixed to produce mortar or concrete. The mortar or concrete is then placed into delivery equipment. The material is forced through a delivery hose to the nozzle where compressed air is injected to increase velocity.
THE SYSTEMS COMPARED
Bonding ability of dry-process shotcrete is excellent because of the low water-cement ratio and high impact velocity. One of the more attractive features of the wet-mix systems is that the water-cement ratio can be controlled more precisely.