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2017SpringShotcreteEMag

Originally published for the “Tunneling Association of Canada-2016 Annual Conference” (www.tunnelcanada.ca), reprinted with permission from the authors Innovative Shotcrete Technologies for Advancement in Tunneling By Nicolas Ginouse, William Clements, and Scott Rand Sprayed concrete (shotcrete) is a well-established and proven component of ground support systems worldwide. Underground excavation projects currently involve more and more logistical and technical challenges to advance headings and keep the development cycles as short as possible. In this context, the installation of ground support has become one of the longest components of the development cycle. In addition, larger headings and deeper excavations lead to larger volumes of material requiring transportation from the surface over longer distances. The use of shotcrete as a ground support technique has undergone several key technological advances that are explored in this paper, which include the reduction of rebound and dust, rapid strength gain shotcrete and ultra-high performance shotcrete materials for high stress conditions. WET-MIX PROCESS In non-accelerated wet-mix shotcrete, all the ingredients, including water, are mixed together before the delivery phase. The fresh mixture is then pumped through hoses or pipes to be sprayed onto a receiving surface using compressed air, which is introduced at the nozzle. In this case, the amount of mixing water added to the mixture is predetermined before the pumping and the shotcreting phases, which therefore makes the implementation of in-situ quality control quite simple. However, because of the material delivery phase, the wet-mix process requires management and control of all parameters influencing workability/pumpability of the mixture to ensure the material is delivered properly. Even if a mixture is found to be adequate for pumping, that does not necessarily mean that the same mixture is shootable and will adhere to the receiving surface after impact. In the case of accelerated wet-mix shotcrete, the set-accelerating admixture is introduced at the nozzle to provide rapid hardening while overcoming any potential issues with workability. Normal dosages of accelerator used within the industry are typically between 2 and 6% by weight of the cementitious content of the shotcrete mixture. In addition to the obvious impact on the final material cost, the dosage of set-accelerating admixtures must be carefully selected and monitored on site as higher dosages reduce the later age compressive strength and durability of portland cement-based concrete/shotcrete (Jolin, Melo, Bissonnette, Power, and Demmard, 2015). In underground excavation projects, wet-mix shotcrete has become more and more popular. The popularity is mainly due to high application rates and the use of hydration-controlling admixtures. Hydration-controlling admixtures stabilize the shotcrete mix for long periods before spraying and provide additional flexibility/robustness in material delivery. However, the increased use of more sophisticated wet-mix shotcrete mixtures containing admixtures requires careful control. Increasing the dosage of hydration-controlling admixtures has an impact on cost, and requires a higher demand of set-accelerating admixtures to reactivate the hydration process and ensure rapid hardening during material placement. Once again, as reported in the literature, the overdosing of set accelerator on site must be prevented/limited because of its detrimental effect on material porosity that could significantly affect the durability of the shotcrete lining (Jolin, Melo, Bissonnette, Power, and Demmard, 2015). DRY-MIX PROCESS Fundamentally different to the wet-mix shotcrete process, the dry-mix shotcrete process consists of pneumatically conveying the dry mixture through hoses and adding the mixing water generally 10 ft (3 m) before the nozzle outlet (Fig. 1). In this case, all the mixture ingredients, including the admixtures such as set accelerator, are conveyed in dry form using compressed air. Mixing water is then added to the dry mixture in a fraction of a second before impacting the receiving surface. Therefore, the dry-mix process allows for very efficient delivery over long distances, and robust placement of mixtures onto vertical and overhead surfaces with limited use of admixtures. The dry-mix process has become very popular for shotcrete operations involving very challenging logistics and 18 Shotcrete | Spring 2017 www.shotcrete.org


2017SpringShotcreteEMag
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