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Safety Shooter to do the job for which it was intended and will remain in place. When measuring the risk of a fallout, consid- eration should also be given to other factors that can influence the early-age strength of recently placed shotcrete—one of the most important being temperature. The temperature conditions under which initial compressive strength values were obtained may be much different than those where the shotcrete is actually being placed. Cold ambient temperatures will always increase the final set time and reduce the rate of strength gain, as will the temperature of the material and the mix water. Other factors that can influence the potential for a fallout are: surface preparation, nozzleman skill, application thickness, vibration, seismic activity, the presence of welded wire reinforce- ment, the process (wet- versus dry-mix), and of course the shotcrete mixture design. If changes are made to the shotcrete mixture design or there are changes in shotcrete supplier, early-age testing should be repeated to verify that the set times and early compressive strength results are meeting expectations. Shotcrete mixtures using new cement tech- nology that achieve compressive strengths as high as 2900 psi (20 MPa) after only 2 hours have recently been introduced to the mining and tun- neling industries. Vale’s Coleman Mine, located Fig. 2: Shotcrete mixtures using new cement technology in Sudbury, ON, Canada, has used this technology that achieve compressive strengths as high as 2900 psi in its underhand cut-and-fill mining process to (20 MPa) after only 2 hours have recently been accelerate the mining cycle and increase produc- introduced to the mining and tunneling industries tivity (Fig. 2). As a safety precaution, Vale’s shotcrete supplier offered to pigment the shotcrete mixture red so that shotcrete crews and other to minimum re-entry times. A safe re-entry time underground personnel could easily differentiate will depend on a number of previously mentioned between this high-performance mixture and the factors. Ignoring those minimum re-entry times conventional shotcrete used in other areas of the is a risk that can cause serious injury and, in some mine. Fallouts would be an obvious hazard if documented cases, cause workplace fatalities. shotcrete crews were unable to easily distinguish if the material placed reached 2900 psi (20 MPa) at 2 hours or 24 hours. We are all ultimately responsible for our own Joe Hutter is the Vice Presi- safety and understanding when a shotcreted area dent, Sales, for King Packaged is safe to enter is an important part of that respon- Materials Company, Burl- sibility. The information needed to understand ington, ON, Canada. He has safe re-entry times, however, must come from a more than 25 years of experi- number of sources. The shotcrete material suppliers ence in the cement/shotcrete should be able to provide set times and early-age industry. He is a former Presi- strength results for any mixture that is supplied dent and an active member of to the shotcrete crew. In the case of mine and ASA and has been Chair of the ASA Marketing/ tunnel excavations, ground control engineers Membership Committee since its inception. should establish guidelines for minimum compres- Hutter is also a member of the American Con- sive strength and correlate those strength values crete Institute. Shotcrete • Summer 2014 55


2014SumShotcreteEMag
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