Alkali-free Accelerators for Wet Shotcreting in Tunnelling

Talrak Wet ShotcretingFigure 1: Wet shotcrete application by truck mounted shotcrete robot
M N Ramesh, Managing Director, Talrak Construction Chemicals

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.

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.

Wet Shotcrete

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.

Applications of Shotcrete in tunnelling

Wet mix system is ideal for applications in tunnelling, where access allows the application equipment and transit mixers to operate on a more or less continuous basis. The wet shotcrete is preferred in tunnelling due to its fast rate of application and better control on the quantity of water added to the shotcrete mix.

Further, generation of dust is less in wet shotcreting than in dry method. The cement content is usually between 360 and 420 kg/m³.

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NBM&CW August 2016

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