Reinforcement of discharge tunnel with carbon fiber cloth

Carbon fiber sheet reinforcement technology is mostly used in civil construction, bridges and tunnels, concrete structure surface reinforcement, and structural reinforcement. In recent years, it has also begun to be applied and promoted in water conservancy projects. Most of the technology is applied to the wall lining of newly-built water discharge tunnels and power transmission tunnels to play a role in reinforcement of the wall, erosion and wear resistance.

1 Project Overview

The construction content of the discharge tunnel of the Andi Reservoir includes: demolishing and reconstructing the inlet channel protection section of the water transmission tunnel and the water transmission tunnel shaft, machine room, and approach bridge; updating the imported maintenance gate, hoist and power distribution equipment. The reinforced concrete lining of the cave body is reinforced with carbon fiber sheet. The drainage tunnel body is reinforced concrete lining, the reinforced part is 159m long, the bottom slope is 1/200, the cross-sectional shape is circular, and the cross-sectional diameter is 3.4m.

The drainage tunnel was built in the 1960s. After several 10 years of operation of the original tunnel body concrete, the surface of the original sprayed concrete on the inner wall was undulating, with serious carbonization, leakage, dissolution and cracks, which increased the difficulty of carbon fiber sheet construction. For this reason, measures must be taken to solve the existing problems to ensure the smooth progress of the carbon fiber sheet pasting and to control the construction quality.

2 Difficulties in carbon fiber sheet reinforcement

1) Short construction period. According to the preliminary construction plan, workload and various conditions of the project, the construction diversion of the drainage tunnel is scheduled to be carried out from January to May 2020. Affected by the new crown epidemic, under the premise of preventing and controlling the epidemic, the time to resume work has reached March. The construction period is tight, the tasks are heavy, and the construction difficulties have become a major difficulty for carbon fiber sheet reinforcement.

2) The construction environment of carbon fiber sheet is poor. The sprayed concrete surface of the original inner wall of the drainage tunnel has undulations and serious carbonization, which does not meet the design cross-section size of carbon fiber sheet. Moreover, the hole body is long and narrow, and manual polishing is time-consuming and laborious, and the polishing dust is difficult to discharge immediately, which does not meet environmental protection requirements. At the same time, due to the serious leakage, dissolution and cracks of the cave body, and the humid construction environment, it is difficult to achieve a perfect fit with the water flow when the carbon fiber sheet is pasted in the water seepage ring.

3 Carbon fiber sheet reinforcement construction

1) Concrete surface treatment. After comparison and selection of schemes, first try to use a grinding wheel to remove the carbonized layer on the surface of the cave body concrete. Due to the serious carbonization of the cave body concrete, the effect is minimal. After research and discussion, it was proposed to use a high-pressure water gun to clean the concrete inner wall of the cavity. Use a grinder to clean the weak points, and finally use a blower and a double-row fan to dry the construction surface, and use the drying for some parts, and the final effect is good.

For the inner wall of the drainage tunnel washed by the high-pressure water gun, the surface is uneven. If the carbon fiber sheet is pasted at this time, the carbon fiber sheet will sink and crack to varying degrees, which will affect the construction quality. In order to meet the strength of the original concrete structure, a solution was obtained after demonstration: using C40 cement mortar and gelling agent to level the inner wall, and finally through the rebound test of the rebound instrument to produce a "rebound strength statistical table", the strength meets the requirements.

2) Apply primer. After the surface of the concrete is polished and leveled, use a hair dryer and welding torch to dry to keep the environment dry. Then configure the bottom glue according to the proportion, and evenly spread it on the surface of the concrete in the hole, and let the bottom glue dry to the touch for about 3-12 hours. In order to ensure the quality of the paste, a small amount of the bottom layer glue is required for multiple times. The bottom layer glue that has been stored for a long time cannot be used. The next step can be carried out after the glue is cured. The individual parts are leveled with epoxy mortar.

3) Paving and pasting carbon fiber sheet. The force direction of the carbon fiber sheet paste fiber is arranged in a ring shape along the hole body, and the carbon fiber sheet is cut according to the design requirements. The width of the carbon fiber sheet is 1000mm, and the lap length is not less than 100mm. Staggered laps along the direction of the water flow, equipped with an oil brush for dipping glue and evenly spread on the carbon fiber sheet surface. Then use a roller to repeatedly roll the surface of the carbon fiber sheet to remove air bubbles, so that the carbon fiber sheet is completely attached to the concrete surface. If the surface of the carbon fiber sheet has wrinkles, twists, and other non-fitting phenomena, it needs to be reworked and reworked to ensure quality control.

4) Curing and curing. Prevent rain or moisture within 24 hours after the completion of construction, and pay attention to protection to prevent hard objects from scratching the construction surface. When the average temperature is 20～25℃, the curing time should not be less than 3d; when the average temperature is 10℃, the curing time should not be less than 7d. After the carbon fiber sheet is pasted, the curing time is 4d.

5) Construction quality inspection. After the construction is completed, use the method of hand pressing and hammering the surface of the carbon fiber sheet to check whether there is an empty drum between the carbon fiber sheet and the concrete. If a hollow drum is found, it means that the carbon fiber sheet and the concrete are not fully bonded, and the purpose of joint force is not achieved. If the diameter of the hollow drum is less than 10mm, use a needle tube to inject glue to repair it. If the diameter of the hollow drum is greater than 30mm, the carbon fiber sheet needs to be removed and reworked. After testing, the total effective bonding area of carbon fiber sheet paste is not less than 95%, and the number of hollow drums per square meter is not more than 10, which meets the requirements.

4 Concluding remarks

Carbon fiber sheet reinforcement technology uses the good tensile strength of carbon fiber materials for the tensile, shear and seismic reinforcement of structural members. In the reinforcement of the inner wall of the discharge tunnel of the embankment reservoir, after pasting the carbon fiber sheet, the purpose of enhancing the bearing capacity and strength of the concrete of the cave was achieved. Therefore, the carbon fiber sheet technology has been well applied in the reinforcement of the inner wall of the discharge tunnel of the Andi Reservoir, and it has provided valuable experience for the construction of similar projects in the future.