In tunnel engineering, shotcrete is an important process for initial support and secondary lining, and its quality has a significant impact on the durability of the tunnel. This article will provide a detailed introduction to the key technical measures that cannot be ignored in shotcrete construction, helping construction personnel ensure the quality of tunnel concrete and guarantee the safety and durability of the project.
1、 Mix proportion design
1. Cement to bone ratio control: Control the cement to bone ratio of sprayed concrete between 1:4 and 1:4.5. Reasonably control the amount of cement used to avoid affecting the initial strength growth rate and shrinkage.
2. Sand ratio adjustment: The sand ratio should be maintained between 45% and 55% to prevent aggregate blockage.
3. Water cement ratio adjustment: The water cement ratio should be between 0.4 and 0.5. Ensure that cement and water are fully hydrated to form a dense cementitious structure, ensuring the strength of sprayed concrete and reducing rebound losses.
4. Addition of accelerators: Reasonably add accelerators, and be careful not to cause a decrease in later strength in order to pursue early strength.
2、 Precautions for construction preparation
1. Equipment inspection: Check the pipelines and joints to ensure that there is no air leakage in the air ducts and no water leakage in the water pipes. Install a valve joint every 40 to 50 meters.
2. Thickness marker embedding: Install shotcrete thickness inspection markers and record their exposed length. Pull the line, level with wooden bars while spraying, control the flatness of the spraying surface, and compact the concrete.
3. Excavation section inspection: Check the excavation section and make up for any under excavation. Reinforce unfavorable geological areas and provide sufficient lighting on the spraying operation surface.
3、 Maintenance measures
1. Initial setting leveling: After the initial setting of concrete, leveling should be carried out to avoid damaging the bonding between the internal structure of fresh concrete and the base layer.
2. Watering and curing after final setting: The last layer of concrete should be watered and cured for no less than 14 days after the final setting of 2 hours.
3. Low temperature maintenance: Water retention maintenance must be carried out when the temperature is below 5 ℃.
4、 Concrete thickness detection
1. Spray thickness inspection: Within 8 hours after the spraying is completed, drill holes to check the thickness of the reinforcement layer. If it is insufficient, it must be sprayed in a timely manner.
2. Thickness error control: The thickness error of the reinforcement layer should be controlled between -5 and+8 millimeters.
5、 Concrete strength testing
1. Sampling of test specimens: Cut or drill concrete specimens without cracks from the site, meeting the requirements of flatness and verticality.
2. Compressive strength test: Control the dispersion of the compressive strength test value of the specimen to ensure the rationality of the data.
3. Appearance inspection: Conduct appearance inspection to eliminate hollowing and cracking areas.
6、 Dust control measures
1. Selection of jet air pressure: Select the jet air pressure through experiments and control the jet force.
2. Spray material mixing * *:
-Uniformly mix the sprayed material to reduce rebound and dust volume.
3. Wet spray secondary mixing: Wet spray secondary mixing is adopted to reduce rebound.
4. Control the dosage of accelerators: Strictly control the dosage of accelerators.
Through reasonable mix design, meticulous construction preparation, effective maintenance measures, rigorous thickness and strength testing, and scientific dust control, the quality of tunnel shotcrete can be significantly improved, ensuring the safety and durability of the project. These technical measures not only provide practical guidance for construction personnel, but also lay a solid foundation for the smooth progress of tunnel engineering.