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2017WinterShotcreteEMag

| TECHNICAL TIP Trouble in the Air: Common Air System Errors Influence Shotcrete Quality By Oscar Duckworth Shotcrete, as defined in ACI CT-16 Concrete Terminology: “Concrete placed by a high velocity pneumatic projection from a nozzle.” necessary to achieve optimum placement consolidation and compaction quality? To most, providing sufficient air energy is as simple as connecting a properly sized compressor to the opposite end of the hose supplying the nozzle (Fig. 1). A deeper look may reveal that the use of an appropriately sized compressor does not assure correct air energy. Many easy-to-overlook factors directly influence the ability to attain optimum air energy for wet-mix shotcrete placement. Do you know what they are? IS YOUR COMPRESSOR REALLY FUNCTIONING PROPERLY? Concrete material projected at high velocity differentiates shotcrete placement from traditional concrete placement methods. An adequate supply of compressed air is the energy source that provides sufficient impact velocity, the key element for full compaction and consolidation. Without sufficient air energy, shotcrete placement simply conveys and loosely packs materials onto a receiving surface, resulting in less than required compaction and consolidation. Air compressor pressure and volume output is rated by a numerical value derived from the developed air pressure in pounds per square inch (psi), and the amount of compressed air that can be produced in cubic feet per minute (ft3/min), at the rated pressure. Example: A 375 ft3/min (11 m3/min) compressor is designed to provide air at a volume of 375 ft3/min at an operating pressure of 125 psi (0.86 MPa). Although compressors will produce at or above their rated output in “as-new” condition, air bypass within internal components due to wear, dirty filters/separators, or other factors will impact compressor output. Because wear and deferred maintenance reduce output levels gradually, workers may not realize that a problem exists (Fig. 2). A compressor that is running smoothly does not assure that a compressor is delivering its rated output. Many compressor problems have been identified only after mechanical repairs or replacement with a similar-sized compressor that produced far more air. Ever try to place wet-mix shotcrete without enough compressed air? To many, a crash course on the importance of sufficient air energy immediately follows an unexpected compressor malfunction. Clearly, experienced shotcrete workers realize the importance of compressed air in shotcrete placement. However, what means do workers have to readily identify that they are actually receiving the CORRECT amount of air energy Fig 1: Longer hose lengths can lead to reduced air flow 68 Shotcrete | Winter 2017 www.shotcrete.org


2017WinterShotcreteEMag
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