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2017SummerShotcreteEMag

| CONTRACTOR’S CORNER Fig. 6—This blow pipe is constructed from 0.75 in. (19 mm) steel pipe, approximately 3 ft (0.9 m) long. The tip has been shaped to a width of 0.25 in. (6 mm) with common tools to provide a wide fan pattern. It is efficient, durable, and requires very little air Fig. 7—The nozzleman and blow pipe operator working side by side 32 less air flow. It creates a wide fan pattern that is powerful enough to clear rebound, but cannot easily displace material. A blow pipe 3 ft (0.9 m) in length sweeping a foot or two (0.3 to 0.6 m) from the receiving surface yields effective cleaning and excellent range of motion while giving the nozzleman sufficient working room for placing concrete. An air compressor should be properly sized to adequately provide air flow to both the nozzle and blow pipe. If operation of the blow pipe causes a noticeable loss of air to the nozzle, the compressor is too small and must be upsized. It is important to remember that the control of rebound is the responsibility of the nozzleman and crew (refer to Fig. 7). Because rebound is continuously generated during shotcrete placement, the continuous use of proper nozzle techniques, in conjunction with the proper use of a blow pipe when needed, are necessary requirements to a quality job. NOZZLEMAN’S CHECKLIST There are two distinct steps to the control of rebound. The nozzleman must use placement methods to diminish the formation of rebound and prevent rebound from being trapped within the in-place material, such as: • Using visual cues to continuously maintain the correct slump for the project at hand; • Considering recommended mixture design choices such as the use of silica fume, admixtures, or fibers to diminish the formation of rebound; • Focusing the nozzle stream primarily into the developing puddle of material rather than directing the nozzle onto a hard receiving surface; • Positioning the nozzle stream perpendicular to the receiving surface—shooting from an angle causes excessive rebound; • Starting in corners or other areas that may trap rebound; • Directing the blow pipe operator to work immediately ahead of the nozzle stream by continuously sweeping the receiving surface as material is being applied; and • Using a blow pipe that uses a fan pattern that is powerful enough to clear rebound, but cannot easily displace material. ACI Certified Nozzleman Oscar Duckworth is an ASA and American Concrete Institute (ACI) member with over 25,000 hours of nozzle time. He has worked as a nozzleman on over 2500 projects. Duckworth is currently an ACI Examiner for the wet- and dry-mix processes. He serves on the ASA Board of Direction and as Chair of ASA’s Education Committee. He continues to work as a shotcrete consultant and certified nozzleman. Safety Tip The blow pipe is a dangerous tool. A highpowered compressed air stream can cause serious injuries. Never direct the stream towards someone. Exercise caution when handling a blow pipe. Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
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