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material’s stack rate. Numerous job-specific placement conditions will influence how tall a given wall may be safely stacked per hour. To enhance productivity and minimize hose movements, nozzlemen tend to attempt the tallest stack rate possible. Unfortunately, nozzlemen and other in-field workers may not be aware of the potential damage that can occur to the in-place work from overly aggressive stack rates. SUBTLE CLUES OF TROUBLE The obvious risk shotcrete crews encounter when building up too quickly is the risk of creating fallouts or a dangerous collapse (Fig. 1)—at minimum, fallouts, sags, and cracks caused by aggressive stacking waste material and time. Many times, vertical wall fallouts can create a risk of injury to workers. Even small fallouts may weigh several hundred pounds (kg). Because a fallout can occur unexpectedly, workers may not realize that they are at risk. Head, neck, and shoulder injuries—even potentially deadly scaffold failures—have occurred due to fallouts. Far less obvious is the risk to in-place material quality from aggressive stack rates. Shotcrete nozzlemen who stack a wall too quickly negatively affect in-place quality well before the work is at risk of a fallout. Reinforcements specified for shotcrete placement must be firmly secured to remain in place and resist movement while the material is in its plastic state (Fig. 2). Aggressive stack rates subject reinforcement and formwork to strong downward pressures from immense weight. Freshly placed materials are vulnerable to cracking, sags, or delamination damage from internal movement due to the weight of higher levels being carried through the fresh concrete that has not yet reached enough set (and strength) to carry the loads (Fig. 3). a) b) Fig. 2: Reinforcements must be sufficiently rigid to resist movements as loads are transferred to them from the stacking of plastic materials. Note: (a) inverted U-shaped transfer bars between curtains; and (b) rigid anchorage to the substrate a) b) Fig. 3: (a) Visual evidence that excessive downward pressure has caused cracks within the plastic material; and (b) sagging materials have lost their bond with reinforcements and the substrate surface. These cracks, sags, or delaminations must be cut out and reshot www.shotcrete.org Spring 2017 | Shotcrete 61


2017SpringShotcreteEMag
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