Page 25

2017SpringShotcreteEMag

Fig. 9: Shotcrete pre-dampening gunning rig with dry-mix shotcrete machine Fig. 10: Hydro-mix nozzle for dry-mix shotcrete (from Hutter, 2014) Fig. 11: Water atomizer used to reduce airborne dust Fig. 8: Bulk pre-packaged tote bag with hopper hood material is mixed with a small amount of mixing water prior to being introduced into the shotcrete machine (Fig. 9). The use of a hydro-mix shotcrete nozzle improves particle wetting prior to exiting the nozzle, by moving the water ring roughly 10 ft (3 m) back from the nozzle exit and allowing for increased mixing potential between the mixing water and dry-mix shotcrete (Fig. 10). Adequate ventilation and dust control devices such as a water atomizer that generates ambient fog is an efficient standard practice used in the mining industry to help control dust during shotcrete operations (Fig. 11). When using wet-mix shotcrete in tunnel applications, it is typical to use many different liquid chemical admixtures including retarder to increase the pot-life for long underground travel times. Set accelerator would then typically be added at the nozzle to improve early age strength development. Chemical additives can become airborne in the underground environment once atomized in the air stream during the spraying process. Therefore, it is important to minimize the amount of additives as much as possible and as previously mentioned; these chemical additives can be significantly reduced and potentially even removed in the dry-mix process. In the wet-mix process, additives are critical to producing good quality material and use of admixtures can be minimized when using an optimized aggregate gradation, supplementary cementitious materials to facilitate pumping and reduce rebound, and by increasing the initial air content of the shotcrete to improve pumpability. Ultra-Rapid Performance Gain Shotcrete Among the existing techniques to speed up the development cycle of underground excavation operations, the use of ultra-rapid strength gain shotcrete is beginning to receive more traction in the industry. By manipulating the cement chemistry and mixture design, this innovative technology allows much earlier re-entry to headings after the completion of shotcrete operations. Using current portland cement technology, high early strength cement (Type III or Type HE) and a high accelerator dosage, it is possible to provide a shotcrete mixture design capable of reaching early age compressive strengths of up to 1000 psi (7 MPa) at 4 hours. This is possible in both the wet- and dry-mix processes. However, to exceed this early strength level, the cement technology needs to be reviewed. From a cementitious matrix standpoint, the use of an ettringite based cement such as calcium sulfo-aluminate (CSA) cement can provide very rapid strength gain compared to high early strength portland cement (Type III or Type HE) with a high accelerator dosage (Reny and Ginouse, 2014). At early ages, this rapid strength gain is mainly due to a rapid formation of ettringite, which occurs when the CSA based cement comes into contact with the mixing water. www.shotcrete.org Spring 2017 | Shotcrete 23


2017SpringShotcreteEMag
To see the actual publication please follow the link above