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Technical Tip amount of rebound varies with the position of the requirement for the nozzleman is that he retain work, nozzle angle, air pressure, impact velocity, control of the nozzle at all times, never pointing cement content, water content, maximum size and it in an unsafe direction and never setting it down gradation of aggregate, amount of reinforcement, while material or pressure are present in the lines. and thickness of layer. A blowpipe is sometimes Decreasing static electricity through the use of used to remove and control rebound.”1 Because predampening, the nozzleman will not be caught rebound primarily consists of aggregate, it should off guard by an electric shock, which could cause never be used in the structure. Instead, it must be him to lose control of the nozzle. Any undue stress removed either by shoveling, blow pipe, or other on the nozzleman could affect his performance means. Less rebound means less labor time to and the overall quality of the shotcrete placement. remove it. Predampening reduces rebound by allowing the cement binders in the mixture to give Return on Investment a more adequate coating over the aggregates. In addition to all of the benefits mentioned These components will then remain more closely previously, predampening is a sound business tied together with greater amounts of materials decision! Predampening equipment can “pay for adhering to the shot surface. Excessive rebound itself in a relatively short time.”3 Better mixing can lead to undesirable shrinking and drying of and uniformity from predampening results in the shotcrete, so it is important to keep it within concrete that is more structurally safe and sound. acceptable limits. For example, an expected range Less dust keeps your crews in better health, poten- of rebound losses on slopes and walls is 10 to tially saving your business on health care pre- 30%. By predampening, we can endeavor to miums, not to mention the peace of mind you get achieve the highest yield of material used for in- by doing the right thing for your people. Reduced place shotcrete while reducing labor and helping rebound allows you to get the most out of your to create safer concrete structures. material and improves your labor efficiency. Decreased wear on equipment may allow you to Less Wear on Equipment save on replacement parts for the dry-mix shot- Predampening dry-mix shotcrete materials crete machine and placement nozzle. Greatly makes them less abrasive and increases the life reducing static electricity reinforces your commit- span of various components in the dry-mix equip- ment to safe working conditions. Predampening ment setup. These components are recognized as materials in the dry-mix process is not only a “wear parts” and are primarily part of the dry-mix recommended procedure; it is also clearly a win- shotcrete machine, hose, and placement nozzle. ning solution for quality, efficiency, safety, and They include the feed system wear plate, wear profitability of your business. pads, discharge liner, internal material hose lining, placement nozzle washers, and liner. The expected References life span of these components is well-known and 1. ACI Committee 506, “Guide to Shotcrete (ACI 506R-05),” documented. However, it is not known to what American Concrete Institute, Farmington Hills, MI, 2005, 40 pp. extent the life span of these parts is improved when Pre-mixed Shotcrete Supply for Tunnels and Mines,” Shotcrete,2. Armelin, H.; Duke, N.; and Contreras, J.M.L., “Dry, using predampened materials. More research is V. 1, No. 3, Aug. 1999, pp. 8-11. needed to determine how predampening affects 3. Bridger, P., “The Value of Shotcrete Accessories,” the life span of these parts. Shotcrete, V. 7, No. 3, Summer 2005, pp. 10-12. 4. Sofis, T.W., “Gunite Refractory Installations,”Shotcrete, Greatly Decreased Static Electricity V. 10, No. 2, Spring 2008, pp. 26-27. If dry-mix materials are conveyed with low moisture content, a static charge can build up in the hose and can give a shock to the nozzleman Todd Ferguson is the Interna- if using ungrounded delivery hose. Predampening tional Sales Representative for can virtually eliminate this situation by adding Airplaco Equipment & Gunite the recommended 3 to 6% moisture content, Supply, a division of Mesa which reduces the likelihood of static charges in Industries, Inc. He has over a the line. Static electricity building up in the system decade of experience in speci- is not merely an annoyance to the nozzleman, it fications of shotcrete equip- is a safety concern. The static electricity “can ment, accessories, materials, shock the nozzle operator and cause a loss of and applications. For more information, call control of the nozzle.”1 The most important safety (513) 321-2950 or visit www.airplaco.com. 50 Shotcrete • Spring 2015


2015SpringShotcreteEMag
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