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2017SummerShotcreteEMag

| CONTRACTOR’S CORNER aggregates are not embedding. Aggregates that have either bounced off or remain slightly embedded at the surface leave a dull, sandy, or rocky layer. Rebound increases dramatically if aggregates cannot embed at impact. Do not increase the slump excessively; too high of slump will certainly create other placement problems. visual indicators at the surface of the freshly applied material to continuously maintain a sufficiently workable mixture. MIXTURE RHEOLOGY Mixture rheology is a term used to define the physical properties of a concrete mixture in its fluid or plastic state. Mixture choices that affect rheology have a powerful effect on the rebound rate. Mixture choices such as the use of silica fume, fibers, or certain admixtures can be used to significantly reduce the rebound rate through the paste layer’s ability to retain, rather than ricochet materials as impact occurs at the receiving surface. Fig. 3—Correct and incorrect gunning positions from view from overhead (ACI CP-60(15), p. 58) Fig. 4—Blow pipe operator continuously maintains a clean receiving surface by working ahead of nozzle stream 30 Nozzlemen must control rebound by using important NOZZLE DISTANCE AND ANGLE The nozzleman must maintain a position that orients the nozzle stream perpendicular to the work’s receiving surface. Shooting at an angle other than perpendicular causes excessive rebound and should be avoided (refer to Fig. 3). Use nozzle technique, distance, and angle to focus the nozzle stream primarily into a developing puddle of material at the receiving surface rather than directing the nozzle onto a hard-receiving surface. Shooting into a puddle of material dramatically reduces rebound. Keep the nozzle close to the receiving surface, especially when working within congested reinforcement patterns. Nozzle velocity diminishes quickly as distance increases. Insufficient velocity cannot properly embed aggregates and will create excessive rebound. Use bench shooting techniques when working within congested reinforcement patterns and, when possible, direct the nozzle stream mainly through openings and around obstacles within the reinforcement pattern. PREVENT REBOUND FROM BEING TRAPPED WITHIN THE IN-PLACE MATERIAL Techniques used to control rebound created can substantially reduce the amount of rebound; however, they cannot eliminate it entirely—some rebound will always occur. Because rebound is continuously generated during shotcrete placement, the continuous use of proper nozzle techniques, in conjunction with the proper use of a blow pipe when needed, are necessary requirements to prevent trapped rebound. The continuous use of a blow pipe or other suitable device is a required placement step in the current ACI 506.2-13, “Specification for Shotcrete”: “3.4.3.4 Shotcrete crew shall continuously remove accumulations of rebound and overspray using a compressed air blowpipe, or other suitable device, in advance of deposition of new shotcrete.” During placement, the blow pipe operator should work immediately ahead of the nozzle stream, continuously sweeping the receiving surface as material is being applied (refer to Fig. 4). The blow pipe valve should be adjusted to supply enough air to easily displace rebound, but not displace freshly applied concrete materials. Rebound tends to accumulate in corners, ledges, joints, and behind reinforcement, where the nozzle stream cannot effectively embed the aggregates or blow them free of the work. The blow pipe operator must be especially vigilant in these areas to maintain a clean receiving surface. Placement should always begin in bottom corners or other areas that may trap rebound. Filling these areas first prevents accumulation of rebound. If possible, avoid aiming the stream directly toward, but rather on either side of reinforcement or other obstacles. Materials striking hard objects will rebound excessively. Use bench shooting techniques when working within congested reinforcement Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
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