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Technical Tip A Closer Look at the Benefits of Predampening By Todd Ferguson Mmaterials before application; however, it aggregates, as well as other additives, which arebagged materials­wide range of specifications, precontain a binder cement, sand, some level ofuch has been written on the benefits ofpredampening dry-mix shotcrete (gunite) is worth exploring these benefits in greater detail. normally proprietary to their manufacturers. Oth- The purpose of predampening is to add moisture erwise stated, “dry, pre-mixed shotcrete consists content to dry shotcrete materials to ensure ben- of pre-blended aggregates (rock and sand) that efits during and after the project. Predampening are pre-dried to minimal moisture and mixed with is used in a diverse set of industries, including accurate amounts of silica fume, set accelerator, ground stabilization, refractory relining, tun- steel fibers, and any other addition required.”2 neling, mining reinforcement, and bridge applica- Obviously, not all materials contain the same tions, to name just a few. Virtually any dry-mix additives and only some contain steel fibers. Pack- shotcrete project benefits from the use of pre- aged dry shotcrete materials are devoid of mois- dampening materials. It is the industry “best ture by design—they have undergone a drying practice” as stated in the ACI 506R-05, “Guide treatment in their manufacturing so that they could to Shotcrete,” that “the crew should predampen be packaged. Premixed dry shotcrete materials the batch before introducing it to the shotcrete offer inherent benefits of long shelf lives, consis- delivery equipment.”1 tent specifications, engineered production stan- dards, and high early compressive strength and Dry-Mix Shotcrete (Gunite) long-term compressive strength. Other materials used in dry-mix shotcrete may be delivered by Materials ready mix or batched on-site with bulk sand and Dry-mix shotcrete is either pre-bagged, deliv- cement from bags or other storage containers. ered by truck, or batched on-site. Available in a Predampeners The predampener is machinery consisting of a material hopper for loading pre-bagged mate- rials and a combination auger/spray bar system that transfers materials as they are lightly misted with a water spray. The predampener’s auger system allows this predampened material to fall into the receiving hopper of a dry-mix shotcrete (gunite) machine. Contractors use predampeners to add 3 to 6% overall moisture content to dry-mix material before it is loaded into a dry-mix shot- crete machine. Batching and mixing equipment is also available for those producing materials on-site. This equipment performs a dual function of proportioning materials and predampening. Otherwise, the contractor will use a dedicated predampener or special hydromix nozzle to add moisture. Hydromix nozzles of various configura- Fig. 1: Dry-mix shotcrete nozzleman using predampened materials during tions have been used in the industry. The tradi- a parking garage repair tional hydromix nozzle contains a single nozzle 48 Shotcrete • Spring 2015


2015SpringShotcreteEMag
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