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|INTRODUCTION Shotcrete can be defined as spray-applied concrete with a maximum size aggregate in excess of 9.5 mm (0.375 in.). The concept evolved from guniting, which uses smaller aggregate and generally produces concrete with poorer physical properties. The larger aggregate used in shotcrete requires less water in the mix, while offering larger bonding surfaces, thus imparting greater tensile and compressive strengths. TYPES OF MACHINES Two basic designs of shotcrete machines in general use are the wet or prehydrated zero slump and low slump type, and the dry or nozzle hydrated machines. Fig. 1 shows a typical zero-slump machine in a con¬figuration commonly used for small-volume require¬ments. The prehydrated zero-slump machine mixes exactly proportioned batches in a sealed pressurized chamber. "Slugs" of hydrated shotcrete mix and com¬pressed air are sent through a distribution hose to the nozzle where the wet mix emerges as a spray. The spray is directed onto the rock surface by the nozzle operator. The low-slump machine is similar, except that a thick slurry mix of aggregate is delivered throughout the distribution hose to the nozzle. Fig. 2 shows a typical low-slump shotcrete machine used in moderate-volume applications. The nozzle-hydrated shotcrete machine accepts a premixed "dry" (4 to 6% moisture) batch of aggregate and cement. A constant feed of this mix is directed by compressed air through the distribution hose to the nozzle. At the nozzle, water for hydration is introduced into the emerging spray. The hydrated mixture is di¬rected onto the rock surface by the nozzle operator. Fig. 3 shows a typical dry-mix machine. The major difference between the two basic machine designs is the consistency of the shotcrete quality. Pre¬hydrated machines have high degrees of consistency and produce high-strength concrete, resulting from their ability to produce exact mixtures of zero or low-slump concrete within the machine. Nozzle-hydrated machines tend to produce poorer quality concrete because of the difficulty in controlling the water content at the nozzle. Water input essentially is an art. As such it is not precise, and the shotcrete produced is of variable quality. MATERIALS HANDLING Materials handling presents a problem to either type of shotcrete machine. At times considerable quantities|