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Technical Tip Table 3: System Comparisons Component Dry Mix Wet Mix Cost Low to moderate; for example, $10,000 High; for example, $50,000 to >$100,000 Production rate (via Nozzle Person) Moderate; 5 yd3/h (3.8 m3/h) High; up to 16 yd3/h (12.2 m3/h) Complexity Air compressor, water Pump, compressor, water, plus Material control Good Good Single bag mixture Yes Yes Required clean-out Simple, blast nozzle with compressed air Must clean mixer, pump, hose Fiber capable Yes Yes system to use with the material. Wet-mix systems deliver the material as a paste, and compressed air is used to accelerate the concrete. Strong advantages include the ability to precondition the materials with a better control of heat and high delivery rates. However, highly reactive slurries can be difficult to manage and, based on our own experience, flash set can cause catastrophic equipment failure. Shotcrete can be reasonably divided into two types or systems—“dry mix” and “wet mix”—and each system has advantages and disadvantages Fig. 4: Test nozzles (Table 3). Wet-mix systems are, as implied, produced include major subway systems, roadways, air- with cement and water. The mixture is prepared ports, or other critical infrastructure. The delivery in a mechanical mixer and then the wet concrete vehicle is comprised of five essential components: is pumped through a hose—the end of which is water supply, air supply, cementitious material, a equipped with a high-pressure pneumatic nozzle. dry-mix shotcrete system, and the inline water The water and cement mixture passes through the heater for use in cold conditions. It has been hose and into a mixing nozzle chamber, where the determined that Tekcrete Fast will cure much nozzle accelerates the mixture to give the high quicker when the water is warm. velocity needed for impact consolidation. In dry- The mobile delivery system that has been used mix systems, the dry material is exposed to a water previously was deployed on a trailer. The hitch stream at the nozzle where mixing occurs, while end of the trailer houses the static components— also providing the high velocity needed for impact that is, the air compressor, water tank, and gen- consolidation. Several different dry-mix nozzles erator (if needed). The working area of the trailer were investigated (refer to Fig. 4) and tested houses the dynamic components that will require throughout the project, and all commercially operator access—that is, the dry-mix gun, water available nozzles performed very well. booster pump, hose reels, and material supply A dry-mix delivery system was determined to (refer to Fig. 5 and 6). This deployable delivery be best for the delivery of Tekcrete Fast due to vehicle includes everything needed for first the simplicity, the use of single-bag product for- responders to stabilize shock-damaged structures, mulations, and the ability to use very rapid-setting all on the back of a flatbed trailer. In addition, this materials. A variety of dry-mix systems were delivery system will also work in nonemergency tested throughout the project, including the Reed situations, allowing for easy deployment of all SOVA and the Meyco Piccola, and they all worked equipment and material for repairs of any type. very well for this process. Additionally, the delivery system was designed Disaster City Demonstration— to be a rapidly deployable, low-cost, integrated structure that can be engineered into a facility, or College City, TX a vehicle deployed by first responders to stabilize In November 2014, a civil engineering dem- damaged structures. Ideally, the system would be onstration of Tekcrete Fast and its dry-mix shot- maintained in a state of readiness in areas that are crete delivery deployment system took place in considered to be high-risk targets, which could Disaster City, TX. Disaster City is a 52-acre Shotcrete • Fall 2015 41


2015FallShotcreteEMag
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