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2017SpringShotcreteEMag

| SAFETY SHOOTER What’s the Hurry? Effective tools to determine the correct stack rate By Oscar Duckworth Apparently, seeing the concrete truck arrive does not bring out the best in people. I know seasoned concrete professionals who suffer painful anxiety on pour day in the agonizing moments before they must place tall form-and-pour walls. Experienced concrete workers know first-hand what may happen as enormous fluid pressures are contained within tall concrete formwork. Most days, the pour is uneventful and workers quickly return to their boisterous selves. With the shotcrete process, placement of tall walls tends to be a little less stressful. That is because only momentary impact pressure over a small area must be carried by the form, whereas, in the form-and-pour approach, the full fluid head pressure must be carried along the entire length of the cast section. Although placement of tall walls tends to be less risky with shotcrete placement, nozzlemen must be aware that shotcreted materials can be stacked in vertical lifts too quickly. When this occurs, internal stacking pressures can quickly escalate to levels potentially damaging concrete that has not yet reached sufficient strength to support the weight of the concrete above. Similar to the traditional form-and-pour operations, placing materials in very tall lifts can increase the risk of a failure, potentially collapsing large sections of freshly placed material. All nozzlemen know that stacking a wall in excessively tall lifts is a gamble, but collapse is only one of the potentially destructive effects that can occur. UNDERSTANDING THE STACK RATE Ask a nozzleman to identify the correct stack rate. Most will reply: as high as possible. But experienced nozzlemen can describe, usually from their personal experience, a collapse caused by stacking a wall too quickly. Shotcrete should never be stacked excessively before the in-place material below reaches a sufficient set to support its own weight without deforming. Nozzlemen know this as the Nozzleman Checklist • Aggressive stack rates do not save time. The risk of injury, fallouts, waste, and damage to in-place work is completely avoidable. • Movement can cause hidden cracks and delaminations of the in-place material. Learn to avoid potential trouble by recognizing the subtle cues of an excessive stack rate. • Completely cut out and reshoot materials that exhibits signs of cracking, sags, or delaminations. • Prior to placement, check reinforcement rigidity by giving it a firm tug. Visually validate that there is sufficient anchorage between front and rear curtains and to the substrate surface. • Use the best tools available to you—your hands, eyes, and experience—to determine a safe stack rate. • Plan to give in-place materials sufficient time to reach initial set before proceeding. Fig: 1: Moments after a potentially dangerous collapse of plastic material caused by stacking a wall too quickly 60 Shotcrete | Spring 2017 www.shotcrete.org


2017SpringShotcreteEMag
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