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2013SprShotcreteEMag

Goin’ Underground macrosynthetic fibers in shotcrete; the result is a This heat helps the shotcrete sustain a high enough shotcrete that is really tough and durable that provides temperature to cure without the need for hot water. great abrasion resistance in areas where a lot of Although the hydration process does not take place scraping and gnashing of shotcrete surfaces occur, if it is too cold—say, below 40°F (5°C) as per such as in the grizzly screener, vertical shafts, and ACI 306R-102—work can be done in cool weather horizontal transport drifts in mines. without worrying about protecting the top layer of shotcrete from freezing during the hydration process. Is It Easy on the Environment? Spray in the day and it’s cured out before night, when green Technology because of the high water content.the colder temperatures freeze OPC-based shotcrete Rapid-setting cement is considered a sustainable (green) product, as it is manufactured at a fraction of the temperature that is used in kilns to make OPC. Conclusions The result? Fewer hot gases and CO2 into the atmo- CSA shotcrete is a very good choice for many sphere. We can do our part to help save the planet types and applications for shotcrete, including areas for future generations—pretty simple. that need special attention where abrasion is an issue and shaft liners, grizzlies, hoppers, chutes, open Will It Work in Dry-Mix Shotcrete? drifts, and stopes where there is a lot of equipment traffic. Only operators and nozzlemen who have It works really well in dry-mix shotcrete and been trained and are experienced with rapid-setting is being used to line hundreds of sewer pipes and shotcrete should be permitted to place it, especially drainage culverts throughout the United States. in underground conditions. Once the hydration It’s been used in underground mines for years and reaction starts, there is not much time to get it continues to gain favor in the mining industry. through the equipment. Equipment does not need to be modified to use the rapid-setting shotcrete, but it how About Wet-Mix Shotcrete? needs to be cleaned and maintained properly before Whoa—hang on a minute. This stuff sets up and after the use of rapid-setting shotcrete—and really fast, my friend. Don’t think that you are going straight away. Rapid-setting shotcrete will gain a to mix it and pump it and 10 minutes later pump it good deal of its final strength in the first 8 hours. through a typical wet-mix machine. That is not Postponing the cleaning of equipment and tools can advisable. You could get rapid-setting concrete into result in a great deal of extra effort if the shotcrete your shotcrete hoses that will set up in that length is allowed to harden; however, it’s fine if it is cleaned of time and cause major problems. The only prac- within a few hours. As with any product that is new tical method for placing wet-mix shotcrete is if a to us, it’s best to find out how it works on the surface mobile mixer or a “dry-to-wet” system is used. before using it underground in mines and tunnels. These work great and are standard in some mines in Australia. If the wet-mix shotcrete goes straight references from the mixer to the pump, things work out well; 1. Morgan, D. R., and Chan, C., “Technical Tip,” Under- however, use too much hose with a longer transport Shotcrete magazine, V. 3, No. 2, Spring 2001, pp. 26-30. ,standing & Controlling Shrinkage and Cracking in Shotcrete time to the nozzle and problems ensue. 2. ACI Committee 306, “Guide to Cold Weather Con- creting (ACI 306R-10),” American Concrete Institute, Farm- Mixture Design ington Hills, MI, 2010, 26 pp. Apart from not needing any shotcrete accel- erators, rapid-setting shotcrete is much the same as OPC-based shotcrete. The main difference is Mike Ballou works as a the pot life. Even with retarders, rapid-setting Tunneling and Mining Sales shotcrete starts to harden quickly. If extra time is Engineer for CTS Cement, needed to finish or screed off rapid-setting shot- Cypress, CA. He is a Graduate crete, it must be done soon after it is shot. Civil Engineer and has worked in and around the tunneling and Winter Work North America for over 25 years.mining industry throughout Because rapid-setting shotcrete reacts so He has served on several American Concrete quickly, the chemical reaction of hydration gives Institute (ACI) and ASA committees and subcom- off a lot of heat in the beginning of the curing cycle. mittees for underground, shotcreting, and grouting. Shotcrete • Spring 2013 47


2013SprShotcreteEMag
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