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Sustainability an easier-to-access location. If higher volumes of air (and longer of the material can often lead to inconsistent material flow and hose lengths) are available, the distance over which the shot- hose blockages. Depending on the process (wet or dry), there are crete can be conveyed can be much greater. a number of steps that can be taken to minimize the effect of gravity on the material conveyed down to the point of placement. First, Wet-Mix Shotcrete Process the hose can be "looped" to decrease the velocity of the material When placing wet-mix shotcrete, the concrete mixture (with prior to it exiting the nozzle and to help balance the speed of the water already added) is pumped through a hose and air is added material. This is especially true in the dry-mix process when at the nozzle to increase the speed of the mixture to achieve a combined with a decrease in conveyance air, as required. high velocity. Benefits in terms of access are the same as those Although the effect of gravity must be monitored closely in experienced when using dry-mix shotcrete. The distance that the dry-mix process, the wet-mix process is more susceptible wet-mix shotcrete can be pumped is dependent on the capa- to hose blockages and nozzle pulsation due to the force of bilities of the concrete pump and, of course, the available length gravity on the material within the pipeline. As the conveying of the hose. One important difference between the two place- hose or pipe is filled, segregation can occur as the material ment methods is the weight of the hose. In the dry-mix shotcrete enters free-fall, and a plug can arise before shooting begins. process, much of the material composition (as it travels through The use of a sponge ball or "Go Devil" can eliminate plugs the hose) consists of air. In the wet-mix shotcrete process, the caused by segregation, as this ball restricts material flow and material composition also contains admixtures and the water forces the concrete pump to naturally convey material through required for hydration. The result is a much heavier material the line. As with the dry-mix process, a "loop" in the hose or hose that is more difficult to maneuver. the addition of an elbow (configured similarly to a "P Trap" under a sink) can help to regulate material flow and allow the Access in UndergroundEnvironments pump to convey material, instead of gravity conveying the Difficult access is a common challenge when placing con- material. For this reason, it is often easier to convey concrete crete in an underground environment. In mining environments, up, rather than down, especially when the conveying distance concrete placement is often required in locations that are is in excess of 100 ft (30 m). extremely difficult to reach (refer to Fig. 1). One example would be ore bins that often require concrete lining; or shafts and Access in Elevated Environments raises that require stabilization. A nozzleman can usually access The rehabilitation of elevated bridge structures is a common a raise (often from a working platform) hundreds of feet example of an application where the shotcrete process can mini- (100 ft = 30 m) above the access point where concrete can be mize the problems related to access (refer to Fig. 2). Once dete- placed using either the dry or wet shotcrete process. Both riorated concrete is removed, it must be replaced to ensure processes have challenges that are common to many under- structural integrity. On elevated structures, especially over water, ground environments—the most common being communica- railways, or heavy traffic areas, it can be extremely challenging tion between the nozzleman and the gun/pump operator. to access the areas where concrete placement is required (refer to It is imperative from a safety standpoint that communication Fig. 3). In a form-and-pump application, forms and form hardware between the nozzleman and the gun/pump operator always be must be lifted into place and removed several days later, after the maintained. A constant line of communication is critical to concrete reaches sufficient strength. This can be an arduous task, ensure that the material flow is cut off in the event of a plug or an injury. It should be noted, however, that in the case of underground environments, visible contact is not always pos- sible and must be substituted with verbal communication, usually through the use of voice-activated headsets. For an application in which dry-mix shotcrete material is conveyed vertically over significant distances, consideration should also be given to the available water pressure. Although the volume of air may be sufficient to convey the material to a waiting nozzleman, the water pressure must also be sufficient to ensure the mixing water also reaches the nozzle. This can be of particular concern in a confined space where lack of water at the nozzle would result in dry, cementitious material filling the space. This provides another example of the importance of communication between the nozzleman and the gun operator. When shotcrete equipment is located above the placement Fig. 1: In mining environments, concrete placement is often site and the shotcrete material is conveyed down hundreds of required in locations that are extremely difficult to reach, feet (100 ft = 30 m) into a raise or ore bin, the increased velocity such as ore bins Shotcrete • Summer 2013 61


2013SumShotcreteEMag
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