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Pool & Recreational Shotcrete Corner composition of the concrete changes in that the to the mix for the sake of speeding up the liquids, fines and cream begin to migrate to the placing process. Doing so will result in interior surface of the pipe while the aggregates shrinkage cracks and other failures. Next, when stay in the center. When you have that separation the concrete mix truck arrives, start shooting of material, the aggregate may not be properly right away. You always want to use the material coated by cement paste, which means you’re not within 90 minutes from when the truck was delivering a mix design to the nozzle that can be loaded or 45 minutes from when your material properly placed. is placed in the dry-mix gun to maintain proper • When shooting, shoot everything. What I mean moisture content. Next, you need proper ratio by that is never start hand-packing large volumes of cement, aggregate and water. A typical ratio of material. This does not mean finishers can’t is four to one aggregate to cement. If you hand-compact and work into place material at decrease the binder/Portland to save money, guide wires with finishing tools. Again, the you’re cheating the end product because you’re velocity is critical in that it is necessary to not properly coating the aggregate with compact the material and fully coat the aggregate. cement paste. When you start placing shovels full of material • Orchestration: The process of applying either by hand, the process is no longer monolithic. shotcrete, dry or wet, should be carefully Do not, under any circumstance, use rebound choreographed. A typical crew consists of the in steps, walls, benches, floors or anywhere nozzleman (the most important member of the else. When you hand-pack, you are no longer team), along with the pump operator or gun using pneumatically-placed concrete. You’ll operator, hose tenders and finishers. Everyone get weep holes, seams, separations and all sorts needs to know their job and be aware of what’s of points of weakness in the material. going on as the work progresses. The best • The distance between the nozzle and the substrate crews anticipate what’s happening and make should be no farther than 6 ft (2 m), always at appropriate adjustments for each other. a 90-degree angle to the receiving surface. • Always start in the corners or the radius first. Post Shoot It reduces trapped rebound, which is generated • Once you’ve finished shooting, a thorough to a greater degree when first applying the cleaning of the equipment is critical. All hoses material to the substrate. and the nozzle need to be free of any residual Shooting in layers: I’m a proponent of full- material. This is essential for proper function thickness shooting, meaning you don’t build up at the next shoot. the walls and floor in layers. The concern is that • Setting up for curing is the next step and of the material won’t stand up beyond a certain extreme importance. The concept is simple thickness. When shooting steps or other structures enough. You have to make sure the surface that are more than 12 in. (300 mm) thick, we will stays wet as the concrete cures to prevent the use a layering technique. For walls and floors up to evaporation of mix water. The idea is to allow 12 in. (300 mm), we use a full-thickness shooting the water in the mix to hydrate all the cement technique. Layered shooting is absolutely acceptable particles. The more particles you hydrate, the as long as each previous layer has been properly more strength you gain. Those chemical reactions prepped to receive new materials. generate heat, which promotes evaporation of • Air temperature can be an issue. According the mix water. Therefore, you want to keep the to ACI, you should be shooting between exterior temperature cool, which in turn approximately 40 and 100°F (4 and 38°C). reduces evaporation and leaves more water You can bypass those parameters with mix available for the hydration process—all of chemistry in the form of either retarding which leads to a stronger end product. agents or accelerants. Personally, I’m not a Contrary to what some people think, you’re big proponent of admixtures for the pool not adding water to the surface with the idea that it’s industry in a local short travel shoot because penetrating into the concrete. If that’s happening, in most cases simplicity of mix design is better the concrete is no good to begin with. You’re understood with most pool crews. curing to minimize temperature gain, which in turn • Thieves of strength: In order to achieve desired minimizes evaporation and maximizes strength. strength, you must avoid the “thieves” that will The surface should remain wet for the first reduce strength. First, do not add extra water seven days minimum. You can use misters, soaker Shotcrete • Spring 2013 35


2013SprShotcreteEMag
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