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2017SummerShotcreteEMag

WHY REBOUND MATTERS Rebound is mainly larger aggregates with little or no encapsulating paste. It is porous and does not possess structural properties. Essentially, it is just a pile of rocks and sand. Rebound that does not fall clear of the concrete section can become unintentionally trapped by the deposition of additional shotcrete material (refer to Fig. 1). Trapped rebound results in loose, unconsolidated aggregate lenses within the in-place concrete section. Loose aggregates, rather than well-consolidated concrete encapsulating the embedded reinforcement, reduce the structural strength. If embedded areas of rebound are widespread, the concrete sections may become structurally deficient and unable to carry the design loads. Loose, entrapped rebound will also provide internal moisture paths that can reduce water tightness and accelerate corrosion of the embedded steel. Over time, with moisture flow through the trapped rebound, unsightly efflorescence deposits can build up on the surface. With poor nozzling techniques, trapped rebound can occur anywhere within shotcrete work, but corners, joints, and areas of congested steel reinforcement are especially susceptible. Because rebound is continuously generated during shotcrete placement, rebound must be continuously controlled. Because the methods used by the placement crew to control rebound will strongly influence the structural integrity of the completed concrete section, it is the responsibility of the nozzleman and crew to use established techniques that both diminish the formation of rebound and continuously keep the receiving surfaces free of rebound during placement. THE NOZZLEMAN’S ROLE IN DIMINISHING THE FORMATION OF REBOUND The role of the nozzleman in preventing the detrimental effects of rebound cannot be overemphasized. It is the nozzleman’s responsibility to use appropriate techniques and continuously maintain the correct slump for the project at hand. Nozzleman skill and the proper choice of slump during placement is one of the primary factors affecting the formation of rebound. Slump is a numerical value indirectly corresponding to the flow characteristics or workability of a mixture at a given moment. The nozzleman must constantly use visual indicators rather than a specific slump range to determine whether the flow characteristics of material are correct, or need to be adjusted. Although recognizing the appropriate workability for a given project through its slump value is important, it is not exact. With experience, the nozzleman should be able to more accurately maintain proper workability by visually monitoring the surface and the freshly applied materials behavior as it is applied. The nozzle stream should flow easily but not excessively around reinforcements, and produce a surface that displays a glossy and smooth outer layer, rather than a sandy or rocky surface. A relatively smooth surface, glistening with paste, indicates that the mixture is sufficiently workable (refer to Fig. 2(a)). This surface is evidence that fast-moving aggregates within the nozzle stream have become deeply embedded within Fig. 1: Test specimen displays loose aggregates that did not fall clear of the work and became trapped behind reinforcements Fig. 2(a)—This freshly applied surface glistening with paste is evidence that the slump is ideal. Reinforcement bars are clean, without buildup. Fast-moving aggregates within the nozzle stream have become deeply embedded within the shotcrete surface, leaving a smooth, shiny paste layer Fig. 2(b)—Buildup on reinforcement and a dull, sandy, or rocky surface is evidence that the material is likely too stiff. The mixture lacks fluidity and is sticking to the reinforcement. Aggregates are not embedding and have either bounced off or remain at the surface, leaving a dull, sandy, or rocky layer the shotcrete surface, leaving a glistening, glossy paste layer. Buildup on reinforcement or a dull, sandy, or rocky surface is a red-flag indicator that the material is too stiff (refer to Fig. 2(b)). The mixture lacks fluidity and some of the www.shotcrete.org Summer 2017 | Shotcrete 29


2017SummerShotcreteEMag
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