Shotcrete is mortar or small-aggregate concrete that is pneumatically projected onto a surface at high velocity. Also known as “gunite” and “sprayed concrete,” shotcrete was developed in 1911 and its concept is essentially unchanged even in today’s use. The relatively dry mixture is consolidated by the impact force and can be placed on vertical or horizontal surfaces without sagging. Shotcrete is applied by a dry or wet process. In the dry process, a premixed blend of cement and damp aggregate is propelled through a hose by compressed air to a nozzle. Water is added to the cement and aggregate mixture at the nozzle and the intimately mixed ingredients are projected onto the surface. In the wet process, all the ingredients are premixed. The wet mixture is pumped through a hose to the nozzle, where compressed air is added to increase the velocity and propel the mixture onto the surface.
As the shotcrete mixture hits the surface, some coarser aggregates ricochet off the surface until sufficient paste builds up, providing a bed into which the aggregate can stick. To reduce over spray (mortar that attaches to nearby surfaces) and rebound (aggregate that ricochets off the receiving surface) the nozzle should be held at a 90 degree angle to the surface. The appropriate distance between nozzle and surface is usually between 0.5 m and 1.5 m, depending on the velocity.
Shotcrete is used for both new construction and repair work. It is especially suited for curved or thin concrete structures and shallow repairs, but can be used for thick members. The hardened properties of shotcrete are very operator dependent. Shotcrete has a density and compressive strength similar to normal- and high-strength concrete. Aggregate sizes up to 19 mm can be used, however most mixtures contain aggregates only up to 9.5 mm; 25% to 30% pea gravel are commonly used for wet mixes.
Supplementary cementitious materials, such as fly ash and silica fume, can also be used in shotcrete. They
improve workability, chemical resistance, and durability. The use of accelerating admixtures allows build-up of thicker layers of shotcrete in a single pass. They also reduce the time of initial set. However, using rapid-set accelerators often increases drying shrinkage and reduces later-age strength.
Steel fibers are used in shotcrete to improve flexural strength, ductility, and toughness; they can be used as a replacement for wire mesh reinforcement in applications like rock slope stabilization and tunnel linings.
Steel fibers can be added up to 2 percent by volume of the total mix.
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