1 Seismic damage analysis

1) Structural seismic conceptual design

In order to ensure that the structure meets the requirements of seismic safety, appropriate and reasonable separation or firm connection schemes can be adopted between the structural units according to the actual situation. The structure should be arranged as many anti-seismic defense lines as possible, as a seismic safety reserve, should consider the impact of the redistribution of internal forces of the structure after a certain line of defense is breached in the event of earthquake damage. The structure should have high ductility and appropriate rigidity, and the main energy dissipating components should not be selected to bear the vertical load. In the plane layout, the lateral force-resistant members should be arranged reasonably to reduce the torsion effect of the structure under the earthquake. The vertical rigidity and bearing capacity of the structure should be distributed continuously and evenly along the height of the house to avoid the formation of local weak parts of the structure. In the same floor, when the main energy-consuming component reaches the yield, ensure that the other lateral force-resistant components are still in the elastic working stage to ensure that the structure has a long-term restraint state, so as to improve the ductility and collapse resistance of the structure ability. Reasonably control the inelastic parts of the structure (plastic hinge area), understand the yield process of the structure, and clarify the final yield mechanism. The seismic design of the frame also observes the following principles: strong columns, weak beams, and stronger nodes in the core area. During the design process, effective measures should be taken to avoid brittle failures such as steel slippage, early concrete shear failure and crushing.

2) Common failure modes of reinforced concrete columns

2.1) The column is destroyed by shearing pressure.

When the cross-section of the column is too small, the restraint strength of the stirrup is insufficient, and the compressive strength of the concrete is insufficient, the column will undergo shear compression failure under the combined action of horizontal and vertical earthquakes when the earthquake occurs.

2.2) Shear failure of the column.

In the structure, the wall between the windows has a restraining effect on the columns, which causes the effective length of the columns to be greatly reduced, forming a short column. When an earthquake occurs, the pillars will undergo shear failure.

2.3) Plastic hinge failure of the column.

When an earthquake occurs, the top and foot of the reinforced concrete column in the frame structure absorb a large amount of seismic energy, and plastic hinges appear, resulting in large deformation of the structure, but the overall structure will not collapse

It can be seen that engineering building structures often use reinforced concrete columns as their important load-bearing components, which has far-reaching practical engineering significance for the reinforcement of reinforced concrete columns. Reinforcement or reinforcement of concrete columns requires not only to increase the vertical bearing capacity of the original structure, but also to further improve its ductility and flexural performance to enhance the seismic bearing capacity of the structure. Therefore, the research on the reinforcement technology of carbon fiber cloth for concrete columns has great economic value, which can guide and standardize actual projects.

2 Technical characteristics of reinforced concrete columns reinforced with carbon fiber cloth

2. 1 Introduction to carbon fiber reinforcement technology

Carbon fiber reinforced materials are divided into two types: carbon fiber cloth and carbon fiber board. In the 1980s, the United States, Japan and other developed countries began to study the application of carbon fiber materials to the reinforcement and repair of concrete structures. With the rapid development of domestic economic construction and transportation infrastructure projects, China also urgently needs to develop this reinforcement technology. In the existing buildings, there are many low standards of design load requirements at that time that do not meet the current use requirements, and there is an urgent need for maintenance and reinforcement. At present, there are many kinds of reinforcement methods commonly used in engineering, such as: outer steel reinforcement method, enlarged section method, carbon fiber reinforcement method, bonded steel reinforcement method and so on. Among them, the carbon fiber reinforced and repaired structure technology is a new type of structural reinforcement technical measure, which has obvious advantages compared with the traditional enlarged section method.

2.2 Advantages of reinforcement technology

First of all, in terms of material properties, the advantages of carbon fiber cloth are obvious: small thickness, light weight, good flexibility, acid and alkali resistance, high temperature resistance, fatigue resistance and creep resistance are extremely high. The material can meet the requirements of compression, bending and earthquake resistance, and at the same time can protect the reinforced column from various acid, alkali, and salt corrosion. After reinforcement, the carbon fiber cloth forms a second protective layer on the surface of the concrete column, which enhances the durability of the concrete. It has very little effect on increasing the weight and cross-sectional size of the column, and easily meets the appearance and use requirements of the reinforcement.

Secondly, in terms of stress characteristics, carbon fiber cloth has extremely high tensile strength and is easy to form a good bond on the concrete surface. By surrounding the column laterally, the stress state of the concrete is changed from two to three directions, forming confined concrete, which has been greatly improved and improved in terms of bearing capacity and ductility.

Finally, in terms of engineering applicability, it can achieve the advantages of non-destructive reinforcement of the original structure, simple construction and guarantee of construction quality. Unlike other reinforcement methods, during the entire construction process of carbon fiber cloth reinforcement, there is no need to be equipped with large machinery and wet working environment, and there is no need to implement a large number of fixed safeguard measures. The construction site occupancy rate is small and the efficiency is high. In terms of reinforcement quality, even if the surface of the original reinforcement member is uneven, due to the excellent flexibility of the carbon fiber cloth material, the bonding efficiency can be well guaranteed, the later maintenance costs are relatively low, and the service life is relatively long.

3 Research on seismic performance of reinforced concrete columns strengthened with carbon fiber reinforced cloth

The carbon fiber cloth reinforcement method can indeed effectively improve the seismic performance of reinforced concrete columns. The CFRP method of strengthening reinforced concrete columns has good performance in structural bearing capacity and ductility, dynamic response performance and consumption capacity, and has achieved effective improvements.

Secondly, consider the influence of the three factors of axial compression ratio, carbon fiber reinforcement amount and shear span ratio on CFRP reinforced columns. Research indicates:

1) Both the strong-shear-weak bending coefficient and the total hooping value are proportional to the displacement ductility coefficient, and increase as the former increases.

2) In order to prevent cracks in concrete columns and limit the development of diagonal cracks, carbon fiber cloth can be used to strengthen the concrete columns. The shear capacity of concrete columns has been significantly improved, so that the seismic requirements of strong shear and weak bends are realized.

3) The amount of carbon fiber reinforcement can directly affect the cross-sectional failure form of the concrete column. When the amount of carbon fiber reinforced cloth is increased, the shear failure form of the oblique section can be transformed into the bending failure of the normal section, and the ductility of the column is greatly improved.