Project Overview

A 49-story box-frame structure in Guangzhou is a commercial office building. The 3rd floor of the basement and the 4th floor of the podium are shopping malls. The 3rd to 7th floors of the underground were completed in 1999. The building is 38m long and 36.8m wide. The beams and slabs are cast-in-place, and the concrete design strength grade is C30. As the owner modified the original building drawings, an escalator was added on the second floor to allow customers to come and go. In order to ensure that the original structure can still be used normally after the added floor load of the escalator is added, the original frame beam with a cross section of 600mm X 1000mm and a span of 7400mm that bears the escalator load must be reinforced.

Carbon fiber sheet reinforced frame beam scheme

According to the requirements of the project construction schedule, in order to achieve safety and construction efficiency, after comparing the methods of adding section reinforcement, outer steel reinforcement, and carbon fiber sheet reinforcement, it was decided to use carbon fiber cloth to reinforce the components. The method can improve the bending and shear resistance of the frame beam without increasing the beam section, and it has the characteristics of light weight, high strength, and good durability. At the same time, site wet work can be avoided and the construction environment can be protected. Due to the increased load of the frame beam, it is necessary to calculate the reinforcement of the beam's normal and oblique sections by pasting carbon fiber sheets. 

Reinforcement plan

Substituting the newly added escalator load, the section size, span, concrete strength and original section reinforcement of the original frame beam into the calculation formula for the bending and oblique section shear reinforcement, and finally the reinforcement scheme of the frame beam is obtained. The bottom of the beam is pasted with a double-layer carbon fiber cloth of 500mm width and 7400mm length. Paste the middle-layer U-shaped hoops with a width of 200mm along the full length of the beam, with a distance of 350mm. The sticking height of the U-shaped hoop is taken as the height of the component section. After the U-shaped hoop is pasted, stick longitudinal carbon fiber sheet bead on the upper end of the beam and on both sides of the waist. The bead width is 200mm (see figure). Carbon fiber sheet specifications are 300g/m2, 0.167mm thick.

Whether the bond between the carbon fiber sheet cloth and the base layer is good can be judged by the method of thumping. The eligibility criteria are:

1) Single area hollow drum is less than 0.13 square meters

2) The hollow drum occupies less than 5% of the total area;

3) The average hollow drum is less than 10 places per square meter.

Conclusion

The technology of sticking carbon fiber sheet to strengthen and repair the concrete structure is to use a supporting adhesive resin to paste the carbon fiber sheet on the surface of the concrete to make the carbon fiber sheet bear tensile stress. It is coordinated with the concrete deformation and bears the force together, thus playing the role of structural reinforcement.

This project adopts the reinforcement method of pasting carbon fiber sheet to reinforce the frame beam. The construction quality is high, the construction efficiency is high, and the later use does not require regular maintenance. After the reinforcement is put into use, the condition is good and the expected effect is achieved. It is worthy of popularization and application.