Figure 1: Wet shotcrete application by truck mounted shotcrete robotShotcrete is something indispensable in tunnelling. It is a high-performance product, which is placed through spraying hydraulically/pneumatically and compacted by the impinging force of the material when shot from a nozzle under pressure. Shotcrete is placed either by dry or wet spraying methods. The recommended practice for shotcreting (both dry and wet) is covered in Indian Standard IS: 9012 – 1978.
Dry shotcrete
In this process, cement and moist aggregates are mixed and then placed into a device that conveys the mixed material pneumatically through a delivery hose to the nozzle where water is added under pressure. The water thoroughly wets the other ingredients as the mixture is jetted from the nozzle at high velocity onto the surface to be shot. The amount of water added is under the control of the nozzle man or placing operator and can be varied by means of a valve to produce concrete or mortar ranging from extremely dry to extremely wet mixes. Judgement is necessary in adjusting the water content when the dry-mix process is used and an inexperienced nozzle man could produce shotcrete of varying wetness and water-cement ratio.
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