Shotcrete for initial support of tunnels does have some special technical requirements, which are designed to ensure the quality of shotcrete and the stability of tunnel structures.
Here are some key technical requirements:
1. Concrete mix ratio: The mix ratio of shotcrete should be specially designed and tested to ensure good workability, suitable setting time and sufficient strength. A high-flow concrete mix ratio is usually used, and an appropriate amount of accelerator and water reducer are added.
2. Spraying equipment: It is crucial to choose the right spraying equipment, including sprayers, conveying pipelines, nozzles, etc. The equipment should have good sealing and wear resistance to reduce material loss and maintenance costs.
3. Spraying pressure and speed: The spraying pressure and speed should be properly adjusted according to the properties of the concrete and the conditions of the tunnel wall. Too low pressure may cause the concrete to not be fully compacted, while too high pressure may cause excessive rebound of the concrete.
4. Spraying angle and distance: The sprayer should adjust the spraying angle and distance according to the actual situation to ensure that the concrete can be evenly covered on the tunnel wall and well bonded with the surrounding rock.
5. Spraying sequence: Spraying should follow a certain sequence, usually from bottom to top and from both sides to the middle, to avoid concrete flow and hollowing.
6. Spraying layer thickness: The thickness of each sprayed concrete layer should not be too thick, and is generally controlled within a certain range to facilitate the solidification and maintenance of the concrete. Too thick a spraying layer may cause internal defects and insufficient strength.
7. Use of accelerator: The use of accelerator should be appropriate, both to ensure that the initial setting time of the concrete meets the construction requirements and to avoid adverse effects on the long-term strength of the concrete.
8. Maintenance: After spraying concrete, maintenance should be carried out in time to prevent shrinkage cracks caused by excessive water loss. Maintenance methods include spraying water, covering with wet cloth or using curing agents.
9. Quality control: During the construction process, the quality of the sprayed concrete should be tested regularly, including the strength, thickness, uniformity, etc. of the concrete, to ensure that it meets the design requirements.
The construction of shotcrete for initial support of tunnels is a technically demanding job that requires adequate preparation and planning before construction, and strict implementation of relevant technical standards and operating procedures during the construction process to ensure construction quality and safety.