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2016SummerShotcreteEMag

Pool & Recreational Shotcrete Corner Safety 19. Hard hats, safety glasses, boots, dust mask, or respirator. 20. Proper coverage of worker’s skin against exposure to cement. 21. Screen must remain in place over the hopper of the wet-mix pump or dry-mix rotary gun while in operation. 22. Scaffolding: properly sized, supported, and braced (no jiffy jacks allowed). (a) (b) Fig. 8(a) and (b): Construction joints being prepared Rebound With shotcrete, the high-impact velocity used in the shooting process produces a certain percentage of material that doesn’t stick and bounces off the substrate or the previously shot concrete surface. This is called “rebound” and is mostly composed of the larger rock and sand particles that did not stick to the surface. These aggregates lack the concrete paste essential for a complete concrete mixture and MUST NOT be used in the structure. Cuttings and Overspray When shotcreting, the wall section is generally shot out to and slightly beyond a grade wire, and then finishers cut the shotcrete back to the surface defined by the grade wire (refer to Fig. 4). The cuttings from the finisher cutting the shotcrete back to the grade wire lack consolidation and may be quite variable in composition. They may also have passed the setting time for the mixture design used. Similarly, overspray cleaned off formwork or reinforcement doesn’t have the same aggregate and paste mixture that is desired in the concrete in place. Thus, cuttings or overspray MUST NOT be used in the structure. Summary Gathered from over 45 years of experience in swimming pool construction, I have listed some key performance aspects of the shotcrete construction process to ask your contractor. These should be your primary concerns in the construction of shotcrete swimming pools. The first and foremost step is to use a qualified shotcrete contractor with an experienced crew. Next, look at these key aspects of the shotcrete process: a. Does the contractor have the proper equipment for thorough mixing of the shotcrete material? (Dry-mix may be mixed on site or supplied in large premixed bags. Some wet-mix may be mixed on site, adding water to premixed bags.) b. A quality source of sand and aggregates (must be clean, washed, and well-graded and meets the ASTM requirements for concrete sand). Dry-mix aggregates entering the gun should have a 3 to 4% moisture content to help reduce dust, enhance hydration, and reduce wear on the equipment. This may require predampening equipment if the contractor is using bone-dry bagged shotcrete materials. No rebound or cuttings are to be reused in the structure. c. Properly functioning, well-maintained wetmix pump or dry-mix gun and a properly sized and functional air compressor are required. Construction joints Construction joints are one of the most important aspects in construction and performance of all concrete structures. Shotcrete is no different, but we do get a second chance. With pools having varying shapes and many accessory placements, construction joints (as many as 50 per pool) are a way of life. All construction joints are required to be prepped within the first 30 minutes of placing the fresh shotcrete. Preparation includes shaping the joint to a 45-degree angle, cleaning overspray from adjacent reinforcement not yet embedded, and roughening the surface of the joint with a stiff broom or brush. The joint can then stand for as long as needed before the next placement. When it is time to complete the area, the joint must be pre-dampened to a saturated surface-dry damp condition. When properly shooting and curing the subsequently placed shotcrete, the concrete will act as a monolithic section, just as if there was never a joint there to begin with. The secret in making this a joint that acts monolithically with perfect bond is the combination of the proper surface preparation of the joint and high-impact velocity (60 to 80 mph 27 to 36 m/s) of the shotcrete stream. The shotcrete is a cement-rich concrete paste that is pneumatically driven by impact into the rough surface left by the joint preparation. No bonding agents are needed, and indeed no bonding agents should be used because they interfere with the bond of the fresh paste to the rough substrate. 28 Shotcrete • Summer 2016


2016SummerShotcreteEMag
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