Shortcomings of Carbon Fiber Reinforced Plastics in Strengthening Bridge Structures and Solutions

Insufficient ductility

Insufficient ductility, which is almost one of the biggest defects of carbon fiber. Whether it is cloth, plate or rib, it is composed of many extremely fine monofilament fibers, and the fibers rely on epoxy resin to transmit shear force to work together. Because the fiber and epoxy resin are unevenly distributed on the cross-section, it is doomed that the stress is not uniformly distributed on the cross-section of the material. The lack of ductility causes the monofilament carbon fiber to break after reaching the ultimate strength. In this way, the carbon fiber yarns in the high-stress area will break before the carbon fiber yarns in the low-stress area fully exert their strength, and all the carbon fiber yarns cannot exert their strength at the same time. In addition, an important characteristic of the fiber is the change in the strength of the fiber filament and the distribution of defects along the length of the fiber. The previous strength of the fiber filament is not the only specific value, but rather discrete. This also prevents the filaments from reaching the maximum stress at the same time. Therefore, the overall strength of the material is much lower than that of monofilament fibers, and the dispersion is quite large. The Weber function can be used to describe the strength distribution of carbon fiber reinforced plastics.

Carbon fiber reinforced plastic is applied to the structure, and the lack of ductility in its material properties is manifested as the insufficient ductility of the component. Excessive deformation of the component will cause brittle fracture of CFRP, which leads to brittle failure of the structure. This is very disadvantageous for structures that require large deformations or require relatively high seismic resistance.

The ratio of elastic modulus to strength is too low

The strength of carbon fiber reinforced plastics is very high, generally reaching more than 3000MPa, but its elastic modulus is relatively low. The commonly used ones are generally only about 230GPa, and the high elastic modulus is only about 380-640GPa. To exert greater strength, carbon fiber reinforced plastics need considerable deformation. When working with steel bars, CFRP can only play less than 20% of its strength when the steel bars fully exert their strength. It is difficult to restrain the deformation of the structure and the development of cracks. If a large amount of CFRP is used to control the deformation and cracks of the structure according to the rigidity requirements, it is difficult to be competitive in terms of economy compared with the reinforcement technology such as bonded steel, and there is too much unnecessary surplus in strength. Previous research work has also shown that the performance of the reinforced component is improved mainly in terms of strength when the flexural member is reinforced with carbon fiber reinforced plastic. The increase in rigidity, especially the early rigidity, is quite small, which is quite inapplicable to structures that also require reinforcement.

The shear force transmitted by the epoxy layer is limited

The shear strength of epoxy resin is constant. After the shear strength is exceeded, the shear stress transmitted by the interface no longer increases, while the shear deformation continues to increase, showing a softening phenomenon. When the ultimate shear strain is exceeded, interface micro-cracks will occur at the interface. With the continuous expansion of the micro-cracks, the interface will eventually peel off. Therefore, sticking CFRP sheet reinforcement has its limits. Excessive sticking will cause the interface to fail to transfer enough shear stress, so that the strength of CFRP cannot be fully utilized, and it is prone to bond failure when the component is subjected to a large load.

The above-mentioned defects limit the application of carbon fiber reinforced plastics in the reinforcement field to a certain extent. Especially because the structure often requires not only strength reinforcement, but also rigidity reinforcement, and the material properties of carbon fiber determine that ordinary technology cannot carry out rigidity reinforcement with better economic benefits. Even if a large amount of carbon fiber reinforced plastic is pasted to improve the structural rigidity, due to the material properties of the epoxy resin and the stress concentration at the bending and shear cracks, it is easy to cause the interface between the carbon fiber reinforced plastic and the concrete to peel off and bond failure. Reduce the reliability of the reinforced structure. In addition, due to the high strength of carbon fiber, the existing carbon fiber reinforcement technology cannot give full play to its strength, which also restricts the further application and development of carbon fiber reinforced plastics in the reinforcement field.

Solutions

In view of the above shortcomings of carbon fiber reinforced plastics, researchers and engineers have proposed various solutions. Regarding the defect of insufficient ductility of carbon fiber reinforced plastics, the researchers proposed that the carbon fiber is mixed with glass fiber or aramid fiber to make a hybrid fiber, and the good ductility of glass fiber and polymer fiber is used to make up for the insufficient ductility of carbon fiber.

Regarding the shortcomings of the carbon fiber elastic modulus to strength ratio being too low and the epoxy resin limited shear force transmission, the researchers proposed to prestress the carbon fiber reinforced plastic. In this way, the carbon fiber reinforced plastic has already begun to exert its strength before the structure bears the load. And it can effectively improve the rigidity of the structure, avoid the stress hysteresis caused by the secondary force during reinforcement, and solve the defects of insufficient strength and insufficient rigidity reinforcement. The initial bonding stress is concentrated in the anchoring area after the prestress is applied, and in many cases the bending moment of the anchoring area is not the largest, so that the bonding stress is more evenly distributed in the entire carbon fiber reinforced plastic paste area. The problem of the limited ability of epoxy resin to transmit shear force is solved, and adhesion failure is prevented.

In general, carbon fiber reinforced plastic is a new material with broad application prospects in the field of bridge reinforcement. Although there are certain shortcomings, considering the excellent performance of carbon fiber reinforced plastics that other materials have incomparable, as long as it is used reasonably, it is still very suitable for strengthening bridge structures. The application of carbon fiber reinforced plastics in the field of reinforcement will undoubtedly greatly promote the maintenance and reinforcement of bridges in my country.