Since the development of New Austrian Tunneling Method (NATM), shotcrete in tunnels has been widely applied. Sprayed concrete or shotcrete concrete is a concrete that sprayed using a pneumatic hose under high pressure. The hose will be projected and directed to the surface, concrete will be applied at high speed and compacted instantly. Sprayed concrete or shotcrete is effective as a lining for tunnels due to the following reasons:
Shotcrete is a structural material, and it can be used as a permanent lining for tunnels.
Shotcrete concrete can be used to line tunnels with different profiles and different shapes. Shotcrete concrete high flowability allows concrete to form in any shape.
Shotcrete concrete gains strength rapidly, which provides support of the ground. This will limit the movement of the ground. Also, it will allow a degree of stress re-distribution.
Shotcrete can be mechanized, which reduces the risk and the potential adverse effects that may arise during the shotcrete process.
Shotcrete concrete consists of sand, cement, water, aggregates, and additives. The shotcrete concrete will contain a higher content of sand and cement ratio compared to normal concrete. The size of aggregates for shotcrete concrete is less than normal concrete. More additives added to improve shotcrete properties such as flowability and gaining strength. Mineral fines such as micro silica can be used to enhance shotcrete concrete properties. Using micro silica will increase the durability of concrete and produce a denser concrete. Also, steel fiber can be used to reinforce shotcrete concrete and improve flexural capacity.

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