Shotcrete is something indispensable in tunneling. 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 the dry or wet spraying methods. The recommended practice for shotcreting (both dry and wet) is covered in Indian Standard IS: 9012 – 1978.
In this dry 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. Judgment is necessary for 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 of the more attractive features of the wet-mix systems is that the water-cement ratio can be controlled more precisely. The nozzleman 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 tunneling
Wet mix system is ideal for applications in tunneling, where access allows the application equipment and transit mixers to operate on a more or less continuous basis. The wet shotcrete is preferred in tunneling 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 by dry method. The cement content usually between 360 and 420 kg/m³.
Use of the wet-mix process generally results in less rebound, there is less waste to be handled and disposed of. There is also less of dust problem and this is an advantage whenever the material must be shot in a confined area like a tunnel. The rebound losses in case wet shotcrete is drastically low as compared to dry shotcrete.
Accelerators for wet shotcreting
Shotcrete accelerators work on the C3A (tricalcium aluminate) content of the cement, influencing the rate of hydration, resulting in heat evolution and early -age formation of calcium silicate hydrate (C-S-H) gel (Fig.2). Alkali-free accelerators are used in tunnel shotcreting works worldwide. These accelerators provide major advantages in the field of industrial health and safety.
There are two main types of accelerators used in wet – mix
– Sodium silicate-based accelerator.
– Non- Sodium silicate based accelerator.
Sodium Silicate based accelerators.
A sodium-silicate-type accelerator actually gels the water in the mixture to increase cohesion within the mixture as well as decrease the set time of the cement. Sodium silicate -based accelerators have a high alkali content and can be quite caustic, as the pH is above 11. Although these accelerators will display fast stiffening of the mixture, they can significantly decrease the final or 28-day strength of the mix — up to a 50% reduction. Even a dosage of 5% by weight of cement has been known to decrease ultimate strength by as much as 25%. Durability can also be considerably reduced. Sodium silicate can also decrease waterproofing characteristics because of the leaching of lime when the concrete is subjected to continuous moisture. One other, potentially negative, effect is that these accelerators increase the risk of alkali-silica reaction (ASR) and leaching of water-soluble portions. Sodium silicates are relatively low in cost, so they are popular in instances where shotcrete is needed to attain a quick stiffening and then gain some early strength rather quickly. They are not recommended for permanently exposed shotcrete.
Non-Sodium silicate based accelerators
These types of alkali-free accelerators are based on a combination of sulphates and oxides of aluminium compounds with some special additives to boost their effectiveness. These accelerators are a little more sensitive than sodium silicates to several environmental and material conditions and do not have quite as severe a detrimental effect on 28 -day strength when used at normal dosages. These accelerators promote the massive crystallisation of ettringite prisms which are responsible for rapid setting. Most modern-day specifications where accelerators are required insist on alkali -free accelerators do not have any effect on ASR. Although many alkali-free accelerators range from pH 2 to 3, which is fairly acidic. However, when spraying these, the acidity is tempered by the alkalinity of the concrete mixes so it is not as great a hazard to skin and eyes. Storage tanks for alkali-free accelerators must be either plastic or stainless steel and are never stored mild steel tanks.
Addition of Alkali Free Accelerators for Wet Shotcrete
The addition of Alkali-free accelerators set the concrete very quickly and allow the material to build to higher thickness. By adding shotcrete accelerator at the nozzle the shotcrete sets immediately following impact. Following up on this initial setting and rapid early strength development for the concrete is required in tunneling.
When using accelerator in wet-mix shotcrete, proper dosing pump should be used.
The diaphragm pump is not suitable, as it sends slugs of the accelerator to the nozzle and if it is out of sync with the concrete pump resulting in varying degrees of hard and soft shotcrete mixture on the sprayed surface. Positive displacement pump such as a peristaltic or rotor – stator pump can be used. It is important to ensure that the pump pressure is 0.07 to 0.10MPa higher than the air pressure to the wet -mix nozzle or the air can keep the accelerator from getting into the air stream. Control the accelerator dosage with the pump. The pump should have a variable speed drive and use that to control the flow. These pumps should all have stainless steel fittings due to the corrosively of alkali -free accelerators.
The wet shotcreting is ideally suited for stabilising the back strata and lining of the tunnels in transportation engineering projects such as roads, long distance railways and metros. An alkali-free wet shotcreting accelerator, right type of equipment and experienced nozzle men are the prerequisites for a successful shotcreting works in tunnels.
– IS:9102-1978 Indian Standard Reco-mmended Practice for Shotcreting
– Rocscience support document.
– Sprayed Concrete , Properties, Desi-gn and Application – Edited by Simon Austin &Peter Robins.
– Millette and Marc Jollin, ‘Shotcrete Accelerators for Wet Mix’
– Jerome A Moore ‘Dry and Wet Mix process Shotcrete’
For further details:
Talrak Construction Chemicals Pvt. Ltd.
No. 148 Sri Gururaghvendra Complex
Basaveshwara Circle, BEML III Stage
RR Nagar Bangalore – 98
Phone: +91 80 28608000
M N Ramesh
Director, Talrak Construction Chemicals Pvt. Ltd.