M N Ramesh, Managing Director, Talrak Construction Chemicals
Figure 1: Wet shotcrete application by truck mounted shotcrete robot
Shotcrete 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.
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.
In the wet-mix process, all ingredients are first mixed to produce mortar or concrete. The wet shotcrete mix is usually delivered in a truck mounted mixer. The mortar or concrete is then placed into the delivery equipment which can be a Rotor-Stator worm pump, pneumatic-feed or positive displacement pump. In any case, the material is forced through a delivery hose to the nozzle where compressed air is injected to increase velocity. One more attractive features of the wet-mix systems is that the water-cement ratio can be controlled more precisely. The nozzle man can vary the amount of air introduced but has no direct control over the other properties of the concrete or mortar being placed.
NBM&CW August 2016