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Technical Tip body, installed at a distance of 12 to 36 in. (300 to 900 mm) from the nozzle tip. The benefits of predampening are worth exploring. Although predampening has been advised as a best practice by ACI and ASA, it would seem that until now we have yet to ade- quately correlate predampening benefits with the recommendations stated throughout ACI 506R. We will explore each benefit of predampening in the following, relating each to best practices while illustrating how they apply to safety. Benefits of Predampening • Higher-quality in-place concrete; • Reduced dust around equipment and nozzle; • Reduced rebound; • Less wear on equipment; • Greatly decreased static electricity; and • Higher return on investment. Fig. 2: Predampener shown with dry-mix shotcrete machine Higher-Quality in-Place Concrete Predampening in the dry-mix shotcrete process exposure to materials containing cement, silica helps ensure uniformity3 of in-place shotcrete, fume, and other chemicals not compatible with elevating the quality of the concrete structure. the human body. In enclosed spaces such as Uniformity can be seen in various stages of shot- refractories, tunnels, and mines where ventilation crete production, both before and after materials is minimal, dust could be a significant health are shot onto the surface. Before even reaching concern. Dust is minimized at multiple points of the nozzle, materials that are predampened will the shotcrete operation when using a predampener, have better mixing and uniformity. Predampening “greatly reducing dust at the machine as well as reduces separation “of the dry aggregates and the point of placement.”3 The dry-mix shotcrete cement binders as the predampened material machine contains a material agitator and rotating moves through the hose, which will result in a feed system, which may create dust if using dry better finished product in place. Also, the damp- material right out of the bag or “super sack.” ened material accepts the water better when the Luckily, some dry-mix machines have suppres- entire amount does not have to be introduced at sion systems to help prevent excessive dust in the nozzle.”4 Predampening leads to better uni- these situations. The predampener is the most formity of the in-place shotcrete because the effective means of reducing dust around the material has started to mix before even reaching machinery. That same dust “can contaminate the nozzle. The material then undergoes additional adjacent structures, equipment, and grounds. This mixing within the nozzle and is conveyed onto problem is especially aggravated on windy days.”1 the construction surface. Predampened materials In addition to reducing dust around the machinery, lead to consistent distribution throughout, giving predampening minimizes dust at the nozzle. More a “more homogeneous moisture content to the research is needed to determine just how much in-place shotcrete.”2 A more homogenous material dust reduction can be expected at the nozzle. allows a less-variable, more evenly distributed strength in the resulting concrete. When the shot- Reduced Rebound crete project results in more structurally sound Rebound is minimized in the dry-mix process concrete, the risk of failure is minimized. This by predampening. The benefit of reducing guarantees a safer environment for everyone who rebound can be realized by higher yields;—that lives, works, and plays on that concrete structure. is, less wasted material. Less rebound during the project also reduces labor time. “Rebound is Reduced Dust around Equipment aggregate and cement paste that ricochets off the and Nozzle surface during the application of shotcrete because Health and safety of the crew is a priority. of collision with the hard surface, reinforcement, Predampening reduces dust, which limits workers’ or with the aggregate particles themselves. The Shotcrete • Spring 2015 49


2015SpringShotcreteEMag
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