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2014FallShotcreteEMag

Nozzleman Knowledge occasionally, necessary maintenance of the nozzle Shotcrete nozzles are precision tools. With use, is not completely understood, partially completed, they require continuous maintenance. Inspection, or ignored. Many nozzlemen may not realize that disassembly, and cleaning of all air metering poor or incorrect nozzle function can have a ports, and replacing worn components, must be profound effect on a shotcrete mixture’s plastic performed daily by the nozzleman. and hardened properties. Proper wet-mix nozzle function is primarily responsible for compaction What Must Happen Within and consolidation—the fundamental elements of shotcrete placement. Improper nozzle function the Nozzle? limits a nozzleman’s ability to provide acceptable Nozzles possess air rings that use multiple in-place shotcrete. small metered ports within the body. The air ring Although wet-mix shotcrete nozzle designs ports are designed to form high-energy air jets vary, all nozzles must effectively diffuse the which must diffuse (break apart) the low-slump incoming mixture into fine particles, then highly mixture delivered from the delivery system. Suf- accelerate the mixture’s components to produce ficient air volume and pressure is required to a high-velocity spray pattern. Shotcrete is defined diffuse the mixture into individual fine particles as a method of placing concrete with high velocity as it passes through the nozzle plenum. After the to achieve compaction. Concrete requires com- mixture is diffused into small particles, it can be paction to be an acceptable building component. accelerated and focused into a high-velocity Typically, cast-in-place concrete is placed, and nozzle stream. Proper diffusion and acceleration then consolidated by the use of a concrete vibrator. of the mixture by the nozzle is critically important This standardized procedure generally produces to successful shotcrete placement. The compac- well-consolidated concrete displaying good com- tion/consolidation properties of the in-place paction and consolidation quality. Shotcrete, material are highly dependent on the effective- however, must be physically driven onto a ness of the nozzle’s air ring port area to success- receiving surface at high velocity to achieve fully diffuse and accelerate the concrete mixture. similar or superior compaction/consolidation Air ring ports are prone to plugging through use characteristics. To accomplish this, a nozzle must or mishandling (Fig. 4). be properly chosen AND properly maintained or If even one of the ports becomes restricted, a placement quality cannot be achieved. significant percentage of the material flowing through the nozzle will pass through the air ring area as a cohesive mass that is too large for the nozzle to properly accelerate. These larger particles will exit the nozzle lacking sufficient velocity to achieve satisfactory compaction and consolida- tion at the receiving surface. Reduced nozzle velocity and an uneven spray pattern will diminish in-place compaction/consolidation quality. Distinct changes in velocity or spray pattern are sure signs of port plugging. A knowledgeable nozzleman must be aware of these changes and stop to correct the problem. Port plugging is a constant risk to placement quality. When laying down a wet-mix nozzle, always leave the air valve slightly open to keep air ring ports clear. Nozzle tips are a small cost when compared to other wear items. They must be regularly replaced to maintain proper nozzle function. When new, the nozzle tip’s interior shape is designed to produce a focused, high-velocity spray pattern. However, nozzle tips wear quickly. They must be replaced when wear increases the Fig. 4: Note plugged air ring port on lower tip’s internal contour or diameter. Worn tips nozzle. A single plugged port will dramatically dramatically reduce nozzle velocity and focus. reduce in-place compaction/consolidation quality When shooting production work, nozzle tips 50 Shotcrete • Fall 2014


2014FallShotcreteEMag
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