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2017SummerShotcreteEMag

| CONTRACTOR’S CORNER What’s Happening in There? Part 1: Critical elements that dramatically affect shotcrete’s hardened properties occur hidden from sight within the nozzle stream By Oscar Duckworth Can the skill of the nozzleman be a factor in diminishing or even eliminating rebound? The answer may surprise you Is this much rebound normal? Bet you’ve heard that comment before. How much rebound is normal? Rebound and its effects can be a hot topic for not only nozzlemen and crews but also engineers and inspectors, all whose goal is to produce a quality job. Can the skill of the nozzleman diminish rebound? Rebound is aggregate that, due to impact at high velocity, ricochets off the receiving surface or other hard objects and falls onto lower surfaces. Its control during shotcrete placement will strongly influence both the amount of costly wasted material, and the quality and durability of the in-place work. Unfortunately, rebound is an unavoidable by-product of high-velocity shotcrete placement. Therefore, rebound can never be eliminated. Because rebound will always occur, obtaining a quality product is reliant on the nozzleman and shotcrete crew controlling it during placement. There are two distinct steps to the control of rebound: 1. Use placement methods to diminish the formation of rebound; and 2. Prevent rebound from being trapped within the in-place material. Concrete Mixtures + High-Velocity Impact = Separation Specifications for typical form-and-pour concrete placement have prohibited plastic concrete materials from being dropped excessively for decades, and for good reason. Does concrete bruise easily? Probably not. However, because concrete is a mixture of many individual components, if materials exiting the delivery chute or placement equipment can free fall more than a few feet (meters), the mixture will gain significant velocity. Separation of the aggregates and other particles is likely to occur as the fastmoving material collides with the reinforcing steel or other surfaces at the bottom of the form. This classic problem is easily mitigated by using well-established placement methods. A pipe, belt, or hose depositing materials at or near the bottom of the form slows the materials’ downward velocity; therefore, the potential for separation is eliminated. Clearly, with shotcrete placement, rebound is a form of concrete separation caused by the concrete’s velocity upon impact. Because shotcrete is placed with a high-velocity nozzle stream, the fast-moving, larger particles tend to bounce off all hard surfaces. This is the cause of rebound. Diminishing shotcrete’s velocity to stop rebound is not practical because all of shotcrete’s compaction and consolidation properties are derived from high-velocity impact. 28 Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
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