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2017SpringShotcreteEMag

| SAFETY SHOOTER Downward force imparts pressures that act powerfully against the in-place materials, the receiving surface, and the reinforcement. As pressure increases, these pressures tend to distort the supporting previously placed concrete and embedded reinforcement. This can often push curtains of reinforcing apart and force them away from the receiving Fig. 4: (a) Saw cut reveals delaminations beneath reinforcements crack visible from formed surface caused by excessive weight 62 surface. The effects of the deformation and subsequent movement, whether within the material, forms, or reinforcement, tend to produce cracks and delaminations along the reinforcement within the work that will not reconsolidate (Fig. 4). Freshly placed materials that exhibit sags, cracks, or other delaminations lack structural properties and must be identified, completely cut out, and reshot before final set. Choosing the proper stack rate should not be guesswork. The best stack rate gauges are the nozzleman’s hands, eyes, and experience. Damage and risk due to aggressive stack rates are caused by the actions of the nozzleman. Rather than shoot a very tall lift, then hope for the best, nozzlemen should choose the appropriate stack rate by using all the tools available to them. Placement condition variables invalidate simple 3 to 6 ft/h (1 to 2 m/h) stack rate guidelines. Cold weather, concrete mixture, substrate surface, reinforcement, low nozzle velocity, or excessively thick elements are just a few of the varying conditions that require nozzlemen to limit the stack rate. Nozzlemen must determine the project’s safe stack rate by using keen sensory skills, their hands, eyes, and experience rather than simply reaching “as high as possible.” Prior to placement, nozzlemen should use their hands to tug the reinforcements and validate they are properly retained. During placement, they should feel in-place material, constantly validating sufficient set has occurred prior to placing additional shotcrete above previously placed material. Nozzlemen should keep a watchful eye for signs of movement within the reinforcement and the area below where fresh material is being stacked, and immediately stop if movement is noted. Nozzlemen should diminish risk by planning to allow more time for in-place materials to reach proper set before building additional height. Give in-place materials time by slowing the placement volume rate, increasing the horizontal length of a bench, or stop and move to another location. It is important to remember that choosing the proper stack rate is the nozzleman’s responsibility. Any efficiencies gained by shooting very tall lifts will be quickly erased by sagging, fallouts, or by hidden damage to in-place work. caused by downward movement; and (b) substantial ACI Certified Nozzleman Oscar Duckworth is an ASA and American Concrete Institute (ACI) member with over 15,000 hours of nozzle time. He has worked as a nozzleman on over 2000 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. Shotcrete | Spring 2017 www.shotcrete.org a) b)


2017SpringShotcreteEMag
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