Reduce Shotcrete Rebound in Tunnel Applications
High rebound percentages of shotcrete will not only cause a great waste of raw materials, increase the construction cost and working time, but also lead to a construction site with high dust content, which seriously affects the health and safety of construction personnel.
Control of raw materials
In order to shorten the working time, reduce the rebound, and improve the working environment, the selection of raw materials should meet the engineering requirements and economic benefits. The referencing requirements for selection are:
Cement type: Ordinary portland cement PO 42.5;
Coarse aggregate: Uniform particles, crushed stone 5-10mm, clay content less than 1%, clay lump content less than 0.2%, flaky and elongated shapes less than 10%.
Fine aggregate: It is better to choose clean river sand with fineness modulus 2.5-2.8, clay content less than 3%, clay lump content less than 0.5%.
Accelerator: The initial setting time less than 5 minutes, the final setting time less than 10 minutes, a dosage between 2% and 4%.
Microsilica (silica fume): The dosage of microsilica is 10-13% by mass of cement. By using microsilica, the performance and production of shotcrete can be improved as follows:
- In dry-mix shotcrete, the rebound material on overhead and vertical surfaces can be reduced by about 50%, thereby improving the economic benefits of the material.
- The single-layer thickness on overhead and vertical surfaces can be increased without using accelerators, thereby improving production efficiency.
- The flexural and compressive strengths of the shotcrete in the early and late stages can be improved.
- The resistance to attack by chloride, nitrate, sulfate, alkali-aggregate reactions is enhanced.
- It can effectively reduce the dust content in the air, and the working environment is greatly improved.
It is strictly forbidden to use the rebound material as shotcrete again.
Spraying operation
When spraying shotcrete, the nozzle should be vertical to the surface and carried out in a spiral motion, with a distance of 0.6-1.2m between the nozzle and the surface, and a sprayer working pressure of 0.1-0.15MPa. The spraying operation is divided into sections, in sequence from bottom to top. The length of each operation section does not exceed 6m. The thickness of layers should be determined according to the design and spray positions: that of the first layer is 40-60mm, the arch part of the second spray is not more than 100mm, and side wall is not more than 150mm. The first spraying focuses on filling holes and cracks.
Inspection
Take corresponding improvement measures for the factors that may affect the rebound of the shotcrete. Sample, and statistically analyze the collected rebound data.
Results and Summary
(1) Without adding microsilica, the rebound rate of shotcrete in the arch and the side wall of the tunnel are 42.7% and 45.5% respectively, which can be reduced to 21.4% and 22.8% after adding microsilica. The use of microsilica significantly saves materials and creates considerable economic benefits.
(2) Adding microsilica also greatly increases the thickness of layers. This improves work efficiency, speeds up the construction progress.
(3) Microsilica also makes the mixture more uniform, and effectively reduces the dust content in the air, thereby greatly improving the tunnel operating environment, and ensuring the occupational health and safety of the operators.
(4) The use of microsilica improves the compressive and flexural strength of shotcrete, which can effectively resist the deformation and settlement of weak surrounding rocks and ensure the safety of tunnel construction.
