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Unfortunately, the rapid formation of ettringite is also accompanied by a quick decrease in the workability of the mixture, leading to placement issues (Lemay, Jolin, and Gagné, 2014). Even though it is possible to increase the workability period by using retarding admixtures, their use also generally increases set time and therefore delays strength gain. To overcome this workability issue while ensuring a rapid hardening behavior, the use of dry-mix process has been shown to be the ideal choice since the mixing water is added to the mixture in a fraction of a second before impacting the receiving surface. By using ettringite based dry-mix shotcrete, it is typically possible to obtain approximately 2900 psi (20 MPa) after only 2 hours (Reny and Ginouse, 2014) in contrast to 1000 psi (7 MPa) after 4 hours with accelerated portland cement-based shotcretes. This makes the combination of CSA cement technology with the dry-mix shotcrete process an ideal solution for reducing the mining and tunneling cycle times. This technology is commonly referred to in the industry as RS Shotcrete technology. RS shotcrete technology has been successfully introduced and used in Canadian mines to accelerate the development cycle of daily underground operations and to increase the speed of construction for critical permanent infrastructure. Reaching 2900 psi (20 MPa) after 2 hours allows for a much quicker re-entry time under the sprayed openings/zones and leads to reduced lead-time to the next development step. Combined with structural fibers, the RS Shotcrete technology allows for developing ultra-rapid flexural toughness in challenging ground conditions requiring structural support and energy absorption as early as possible (Ginouse and Reny, 2015). This technology has also been tested (Ginouse and Clements, 2015) and used successfully for rapid repairs of critical civil infrastructure requiring fast re-opening to traffic and services (Jolin, 2016). Engineered High Performance Shotcrete Using design principles similar to ultra-high performance concrete (De Larrard, 1999), recent advancements have been made to adapt this technology for ground support lining in underground projects (Ginouse, Reny, and Jolin, 2015). This technology provides much higher tensile/flexural performance (see Fig. 13) than regular FRS resulting in higher spalling/impact resistance and energy absorption. On one hand, the overall ground support performance against seismicity and high-stress conditions are greatly improved. On the other hand, the technology is paving the way for thinner support innings, resulting in significant savings in material consumption, labor, time of application and overall logistics. This novel shotcrete technology has been recently been introduced and successfully tested in Canadian mines facing very challenging ground conditions requiring high performance ground support with minimal amounts of material required to be transported underground from the surface. Similar technology has been successfully used in tunnel repairs and surface protection in both Japan and the United States to extend durability of tunnel linings (Li, 2003). CONCLUSIONS Dry- and wet-mix shotcrete are practical, proven and welladopted methods for ground support in underground excavation projects. Among the different innovative technologies presented in this article and introduced during the past decades, here are the main technical, logistical and operational solutions/ advancements available for underground projects: • Enhanced shotcrete lining durability obtained using low water-to-cement ratio, supplementary cementitious materials, optimized aggregate gradation, high initial air content concept and minimal dosage of set accelerating admixture accurately monitored on-site in wet-mix shotcrete. • Flexible and robust shotcrete operations using the drymix process or hydration control additives in wet-mix shotcrete to extend pumping life. • High production of shotcrete material on-demand using either a bulk dry-mix loading/hauling/spraying system or a mobile self-loading mixer producing high quality wet-mix shotcrete on-demand using dry pre-blended materials. • Low-rebound shotcrete by following good industry practices, properly trained crews, well-maintained equipment and an optimized mixture design. Fig. 12: Typical early age strength gain provided by RS shotcrete technology and highly accelerated portland cement shotcrete Fig. 13: Improved post-peak tensile strength of engineered high-performance shotcrete (from Ginouse, Reny, and Jolin, 2015) 24 Shotcrete | Spring 2017 www.shotcrete.org


2017SpringShotcreteEMag
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