Analysis of the short reinforcement life of carbon fabric

Analysis of the problem of short reinforcement period of carbon fiber cloth

External carbon fiber cloth reinforcement technology is a reinforcement technology that uses adhesives (commonly used epoxy resin) to impregnate carbon fiber cloth on site and paste it to the surface of the reinforced structure to improve the structural bearing capacity. Because carbon fiber has the advantages of light weight, high strength, corrosion resistance, etc., and the construction process is simple, it has gradually become one of the mainstream reinforcement technologies.

However, with the continuous practice of this technology, it has been found in the project that some bridge structures with a relatively short reinforcement period (5-10 years) have quality problems such as fracture or peeling of the external carbon fiber cloth, which is far from reaching the design service life after reinforcement stipulated in the code. Requirements seriously affect the structural safety.

What is the reason for the premature failure of carbon fiber cloth reinforcement and the short reinforcement period?

Carbon fiber cloth reinforcement technology is to improve the stress state of the structure by pasting carbon fiber cloth on the surface of the reinforced member. The key to achieving effective structural reinforcement lies in the quality of carbon fiber cloth and the reliable bonding between carbon fiber cloth and the interface. But in actual operation, it is not easy to solve the above two problems.

First of all, it is difficult to choose high-quality carbon cloth. There are not many manufacturers that meet the raw silk requirements for high-strength grade I carbon fiber cloth. Small tow carbon fiber has excellent performance but high price. It is generally used in high-tech fields such as aerospace and military industry, as well as product categories with high added value in sporting goods. The performance of large tow products is relatively low, but the preparation cost is also low, and they are often used in the field of basic industries.

With the release of carbon fiber production capacity, there will be situations where large tow or low-grade carbon fiber precursors with lower prices are used to replace small tow carbon fibers, resulting in uneven quality of carbon fiber cloth and affecting the life of carbon fiber reinforcement. Therefore Therefore, choosing high-quality and stable carbon fiber cloth through formal channels is one of the effective measures to avoid the "short-lived" problem of carbon fiber cloth reinforcement.

Secondly, there are many and complex factors affecting the adhesion between carbon fiber cloth and the interface.

①The performance of supporting adhesive.

Even if the two materials of carbon fiber and adhesive meet the design requirements, the strength of the carbon fiber cloth reinforcement system may not reach the design strength. The performance identification must be carried out at the same time as the selected supporting structural adhesive. The matching of the supporting adhesive and the carbon fiber cloth directly affects the performance of the carbon fiber cloth reinforcement system.

In addition, the viscosity of the supporting adhesive (impregnation glue) after mixing is also a key indicator. If the viscosity of the impregnation glue is too small, it may be precipitated from the carbon fiber cloth. If the viscosity of the impregnation glue is too high, it is not conducive to fully soaking the carbon fiber cloth and it is easy to entrain air bubbles. The viscosity of the impregnating adhesive is sensitive to temperature, and low temperature is not conducive to the effective infiltration of carbon fibers and affects the reinforcement effect.

② Uniformity of fiber bundle stress.

Carbon fiber cloth is impregnated with resin and pasted on the surface of the concrete structure at the construction site, and the formed carbon fiber sheet is prone to fiber bending. Fiber bending leads to uneven force between fiber bundles of fiber-reinforced composite materials during the stress process, resulting in a decrease in tensile strength and large discreteness. Therefore, a certain pre-tension is required during the construction of carbon fiber external bonding. In addition, fiber bending can lead to large creep deformation under long-term loading.

③ size effect.

With the increase of length, width and thickness, the performance of carbon fiber cloth will decrease to a certain extent. Size effects in the length direction are caused by non-uniform air bubbles along the longitudinal direction. Studies have shown that the tensile strength of 10m long carbon fiber cloth is only about 40% of the material test piece. In addition, the increase in width and thickness will reduce the reinforcement effect.

Therefore, the application specification requires that when carbon fiber cloth is pasted for reinforcement, it should be pasted in a multi-dense way instead of using the entire fabric that has not been cut into strips, and the number of pasting layers of fiber cloth should be controlled.

  In addition, the bonding performance of the carbon fiber sheet and concrete interface is affected by various on-site construction procedures, such as the treatment of the base surface, the stiffness of the epoxy adhesive, and the flatness of the base surface. The carbon fiber bends and generates vertical stress, which leads to damage such as pulling out and breaking of the carbon fiber.

There are many limiting factors for the reinforcement effect of carbon fiber cloth. It is necessary to ensure that the carbon fiber is straight during the reinforcement process, fully impregnated with glue, and effectively bonded between the adhesive and the interface. However, it is still very difficult to achieve these requirements in practice. In addition, with the release of a large amount of carbon fiber production capacity, cheaper large tow and low-grade carbon fiber precursors flood into the market, and the proportion of carbon fiber cloth that does not meet the reinforcement requirements will increase, which will greatly intensify the impact of carbon fiber. Hardened the probability of "short-lived" problems.