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2017SpringShotcreteEMag

high application (output) rates and controlled dust emissions during spraying operations (Fig. 6). This type of system can be loaded via bulk bins erected on-site (loading area) or by using bulk tote bags (McDonald and Cruz, 2015). These systems allow for on-demand production with optimized accelerator dosages in the dry preblended shotcrete mixture accurately dosed at the manufacturing facility. Thus, the accelerator dosage is not affected by any other admixtures or on-site conditions, and since the mixing water for dry-mix shotcrete is added at the nozzle, there is no need for the use of a retarder or hydration stabilizer. Additionally, the long-term durability of the inplace dry-mix shotcrete is not affected by the high porosity that can be the result of excessive accelerator dosage use in the wet-mix process (Jolin, Melo, Bissonnette, Power, and Demmard, 2015). Practices for Reducing Rebound and Dust/Chemicals Emission Proper shotcrete nozzling technique, including the encasement of embeds, openings and reinforcing steel is an important aspect of reducing shotcrete rebound. Adequate lighting and clear access to the shooting face are also important factors for allowing the reduction of shotcrete rebound (Fig. 7). Shotcrete used for ground support in mining and tunneling applications should be applied in accordance with ACI 506R-16, “Guide to Shotcrete.” The ACI Shotcrete Nozzleman Certification Program developed and administered by ACI Committee C660 is an excellent program for qualifying and certifying shotcrete nozzlemen for most projects but does not directly cover all the requirements for a nozzleman working in mining and tunneling environments. For some large tunneling projects such as the Metropolitan Transportation Authority (MTA) CC/Long Island Railroad/East Side Access project in New York City, NY, it has become common practice for authorities to develop training and certification programs specific to the project (Drakeley and Rand, 2014). Specifying minimum requirements for shotcrete crew experience including nozzlemen, foremen and gun/pump operators also helps to ensure that the shotcrete will be applied properly. The shotcrete mixture design used for tunneling applications can also be optimized for rebound reduction for both dry- and wet-mix shotcrete. The aggregate gradation should meet the requirements of Gradation No. 1 or Gradation No. 2 in accordance with ACI 506R-16, “Guide to Shotcrete.” A properly designed shotcrete aggregate gradation will reduce rebound due to the optimal particle packing of the in-place shotcrete (Reny and Jolin, 2011). Supplementary cementitious materials used to replace Portland cement in the binder portion of the shotcrete mixture design can also help to reduce rebound. Silica fume has been shown to greatly reduce rebound in dry-mix shotcrete when used to replace a portion of the binder content (Wolsiefer and Morgan, 1993). For dry-mix shotcrete, certain equipment choices and practices can also help dramatically reduce dust emissions in the underground environment. Using bulk pre-packaged tote bags in conjunction with a hopper hood manufactured to fit the receiving hopper of the shotcrete machine helps to seal the hopper and prevent the release of excess dust (Fig. 8). Proper maintenance and adjustment of the shotcrete equipment including the water ring in the nozzle, and the exhaust port, wear pads and wear plates on the dry-mix gun will also help to reduce dust emissions. Pre-dampening equipment can also be used for dry-mix shotcrete to reduce the amount of airborne dust, as the Fig. 6: Bulk dry shotcrete hauler/sprayer Fig. 7: Adequate lighting and access to the shooting face in a tunneling application (from Croutch, 2014) 22 Shotcrete | Spring 2017 www.shotcrete.org


2017SpringShotcreteEMag
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