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mining and tunneling industry. It is used to structurally support mine shafts and rail tunnels. In addition, countless highway bridge piers and decks have been repaired successfully with shotcrete, without structural failures (Fig. 1(a) and (b)). It is important that a shotcrete provision discuss materials and performance requirements to ensure the proper mixture designs are used to achieve the desired results. Dust is a concern on shotcrete jobs. The use of a predampener greatly reduces dust in dry-process shotcrete. When using dry-process shotcrete, a predampener adds 3 to 5% of the total water to the material prior to introduction into the shotcrete machine. This allows hydration to start, improves cohesion, and increases adhesion. The addition of microsilica to shotcrete mixtures has also helped to reduce dust by increasing adhesion. Excessive dust can also be a symptom of insufficient airflow. It is important that proper equipment, specifically air compressor size, type of shotcrete machine, and the use of a predampener in dry process be addressed in a state provision. While dry-process shotcrete is not completely dust-free, generally the demolition portion of a shotcrete project creates more dust than the shotcrete process itself. One type of project where dust is a specific concern is highway tunnels. The main concern is dust getting into the fan house and damaging equipment. The Liberty Tunnels in Pittsburgh, PA, were repaired with shotcrete. Over 1000 yd3 (914 m3) of dry-process shotcrete were placed in the tunnels on several different phases of construction. The dry-process material was microsilica-enhanced and a predampener was used during installation. With careful attention to materials and equipment there was absolutely no damage to the fan house or any other parts of the tunnel from dust related to the shotcrete process. In the event that work needs to be performed in an extremely dust-sensitive area, wet-process shotcrete is also an option. Dust concerns can be addressed in a provision by specifying proper equipment and mixture designs (Fig. 2). Another concern is that shotcrete can’t be finished to match the existing adjacent concrete surfaces. Shotcrete has a very low water-cement ratio (w/c) and low slump. This means shotcrete is stiffer and slightly more difficult to finish than traditional concrete one may see in a floor pour. However, multiple finishes can be achieved on shotcrete. Depending on the project aesthetics, the fresh shotcrete can be left with the natural gun finish, screeded or cut to the proper thickness, floated with a wood or rubber float, given a broom finish, or even given a smooth steel trowel finish. When shooting preconstruction panels, an inspector can have the contractor show a variety of finishes on the panels and make the appropriate selection from those examples. There are concerns that shotcrete can’t be used to replicate complicated shapes or large, round bridge piers. This is blatantly untrue; in fact, the creation of complex, irregular shapes is a distinct advantage of using shotcrete to creatively and efficiently create these types of sections. There are several ways to restore the original shape of a structure using shotcrete. Finish surfaces can be set using pins, pencil wire, or trim. Jigs can be constructed to match an original shape. Shotcrete is commonly used to create rockscapes in zoos, water parks, or pools. It’s also used to build complex, double-curved shapes to very tight tolerances for structures such as skate parks and even Olympic Fig. 2: Shotcrete being installed in the Liberty Tunnels by ACI Certified Nozzlemen using a predampener and a sufficiently sized air compressor. No damage to the tunnel was done by dust Fig. 3: Team Pain’s Construction Superintendent, James Hedrick, testing the freshly placed “capsule” at the Kortrijk, Belgium, skatepark Shotcrete • Spring 2016 19


2016SpringShotcreteEMag
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