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2016FallShotcreteEMag

look at shotcrete. ACI CT-16 Concrete Terminology defines shotcrete as “concrete placed by a high-velocity pneumatic projection from a nozzle.” This terminology comes from ACI Committee 506, whose membership includes contractors, engineers, suppliers, and educators intimately involved in the shotcrete business. The key in this definition is high velocity. A study at Laval University by Ginouse and Jolin (Shotcrete magazine, Fall 2013) studied particle speeds in both dry- and wet-mix and found average particle speeds in the dry-mix material stream ranging from 45 mph (20 m/s) at the edge to 78 mph (35 m/s) in the centerline. Wet-mix showed a velocity ranging from 56 mph (25 m/s) at the edge to 74 mph (33 m/s) in the centerline. Why do we need high velocity? Shotcrete placement depends on impact force to compact the concrete, allow the material to flow around obstructions such as reinforcing bars, and minimize voids within the concrete section. Imagine the force of your car driving 60 mph (97 km/h)down the highway, and then impacting an immovable concrete object. There is a massive transfer of energy from the car (and in our case the concrete material) flying through the air, and then suddenly stopping upon impact. One of the prime aspects of proper shotcrete placement is shooting perpendicular to the substrate. This is important because then we’re creating a head-on collision and imparting the maximum energy to consolidation, compaction, and densification of the material in place. Another aspect of high-velocity shotcrete is high abrasion, and correspondingly excellent bond to existing concrete or previously shotcreted layers. In effect, we are abrasively blasting the substrate, immediately before exposing it to the fresh paste of the cement-rich shotcrete mixture. The impact also drives the fresh paste into the substrate, creating excellent bond of shotcrete when using proper materials, equipment (air compressor size appropriate for the delivery hose and nozzle), and nozzleman placement techniques. Also, using high velocity helps to force fresh material around the back of reinforcement to be embedded in the concrete section. High velocity can be achieved by using proper air volume. Dry-mix requires more air flow because the full transport of the materials through the delivery hose is accomplished by the air flow. In wet-mix, material delivery is by mechanically pumping through the delivery hose, and air is added to accelerate the concrete mixture only at the nozzle. ACI 506R states that wet-mix shotcrete requires 200 to 400 ft3/min (5.7 to 11.3 m3/min) air volume at 100 psi (7 bar) and dry-mix 350 to 1000 ft3/min at 100 psi (10 to 28 m3/min at 7 bar). When compared to other methods, such as low-pressure sprayed mortar, shotcrete has the Fig. 3: Modern dry-mix shotcreting Fig. 4: Wet-mix shotcreting of structural wall potential for much higher volume and productivity. Wet-mix can use plant-batched concrete delivered in large ready mix trucks with many cubic yards (cubic meters) in every truck, batched with a concrete batching truck or site-mixed with pre-bagged mixtures in large “super sacks” or smaller individual bags. Dry-mix can similarly be batched with a concrete batching truck or sitemixed with pre-bagged mixtures in large “super sacks” or smaller individual bags. Shotcrete has some of the most sophisticated concrete mixtures used in the concrete construction market. We shoot overhead and vertical areas, thin or thick, fast set or normal set, and straight or curved sections, delivered from tens of feet to thousands of feet from the supply location. This allows shotcrete to be used in a wide Shotcrete • Fall 2016 23


2016FallShotcreteEMag
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