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Tunnel shotcrete has two types: wet shotcrete and dry shotcrete. Tunnel shotcrete is mainly divided into dry shotcrete and wet shotcrete according to the process flow. The main difference is that the feeding procedures of each process are different, especially the timing of adding water and accelerator.
Wet spraying: the aggregate, cement and water are mixed evenly according to the designed ratio, and sent to the spray nozzle with a wet spray machine, and then sprayed out after adding an accelerator on the spray nozzle. The quality of wet shotcrete is easy to control, and the amount of dust and rebound in the spraying process is small. It is a spraying process that should be developed and applied.
However, the requirements for wet shotcreting are high, and mechanical cleaning and troubleshooting are troublesome. For soft rock and seepage tunnels with thick spray layer, it is not easy to use wet spray.
Dry spraying: Dry spraying is to mix aggregate, cement and quick-setting agent evenly in a certain proportion, and then load them into the dry spraying machine. Use compressed air to make the dry aggregate suspended in the hose and send it to the spray gun. The nozzle is mixed with high-pressure water and sprayed onto the rock surface at a higher speed. The mechanical structure used for dry spraying is relatively simple, and the mechanical cleaning and troubleshooting are easy. But its disadvantage is that it is prone to produce larger dust, and the amount of rebound is large. The water addition is controlled by the valve at the nozzle. The control of the water-cement ratio is more difficult and is related to the proficiency of the operator.
Due to the different spraying process, the strength of sprayed concrete is different, and the dry spraying strength is lower, generally only C20 and wet spraying can reach C30~C35.