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2017WinterShotcreteEMag

Fig 2: Although nice and clean outside, poorly maintained components may significantly reduce available air flow WHY AIR ENERGY MATTERS The dry-mix process requires much higher air volumes than wet-mix placement. Because dry-mix materials are conveyed through the delivery line by compressed air energy, more air volume is necessary to deliver material to the nozzle. With the dry-mix process, air compressors rated from 500 to 900 ft3/min (14 to 25 m3/min) at 100 to 125 psi (0.70 to 0.86 MPa) are necessary. Larger air flow also equates to the need for larger air delivery lines; 1.5 to 2 in. (38 to 50 mm) air delivery lines are common. With wet-mix, hydraulic pumping equipment, rather than compressed air, conveys material through the delivery line. Compressed air introduced at the nozzle accelerates the concrete mixture delivered through the hose to create the high-velocity stream that impacts the receiving surface—the critical component to achieving quality compaction and consolidation. Wet-mix nozzle air requirements vary by nozzle size and design. Robotic-style 2.5 to 3 in. (64 to 75 mm) tunneling nozzles are designed to accelerate far more material per minute than a typical hand nozzle. These nozzles require air volumes of 600 ft3/min (17 m3/min) or more to function properly. Supplying sufficient air to these powerful nozzles requires large air supply lines often 2 in. (50 mm) and larger. The most common hand nozzle is the 2 in. (50 mm) diameter nozzle. Hand nozzle designs differ significantly by manufacturer. Each configuration requires a predetermined minimum air flow rate and pressure at the nozzle to produce its designed impact velocity. Because these nozzles must be operated by hand, their weight and size limit efficiency at converting air flow energy into nozzle stream velocity (think short pistol versus long rifle). Therefore, unlike the larger (robotic) nozzles, small changes in supplied air volume and pressure to the nozzle directly correlate to very large changes in the impact energy generated from the nozzle material stream velocity. SIZE MATTERS: THE RELATIONSHIP OF AIR LINE DIAMETER TO DELIVERY DISTANCE Many shotcrete placement companies use 0.75 in. (19 mm) air delivery lines for wet-mix hand nozzle applications. Fig. 3: Larger-diameter air supply lines may be necessary to maintain sufficient nozzle velocity Fig. 4: Although identical in exterior appearance, non-full-flow couplers (left) can be easily identified by their restrictive interior diameter In very short delivery line length applications, a 0.75 in. (19 mm) air delivery line may supply adequate air volume for proper nozzle function to some, but not for all hand nozzle designs. Occasionally, jobsite configurations require greater distances between the air compressor and the placement location. As air delivery line length increases, internal resistance restricts the delivery lines available air-carrying capacity. For air delivery lines longer than approximately 100 to 150 ft (30 to 45 m), a small 0.75 in. (19 mm) air delivery line would never be capable of supplying adequate air volume to most modern wet-mix hand nozzles. Conveying air for greater distances requires an air delivery system with a larger inside diameter. Air supply lines of 1 in. (25 mm) or (much) larger are necessary to provide adequate air energy for hand nozzles in longer-distance placement conditions. Using hoses too small to convey air long distances defeats the design of the nozzle by restricting incoming air energy. Workers must move compressors closer or increase air delivery hose inside diameter to maintain sufficient air energy to the nozzle (refer to Fig. 3). www.shotcrete.org Winter 2017 | Shotcrete 69


2017WinterShotcreteEMag
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