Application of Carbon Fiber Composite Materials in Bridge Engineering

I. Overview of carbon fiber performance

People first built bridges with natural wood and stone, and then slowly developed to use concrete, steel and special man-made materials. The continuous advancement of materials has promoted the continuous development of bridge engineering. In recent years, in addition to how to improve the performance of concrete and steel, how to apply composite materials such as fiber materials, smart materials and nano materials to bridge engineering to improve the safety and usability of bridges. As the span of the bridge increases, the problem of the self-weight of the bridge structure becomes more and more obvious. The bridge engineering materials gradually develop in the direction of high strength and light weight, improving the performance of the bridge and improving the ornamental of the bridge.

Carbon fiber composite is a new type of high-performance fiber material. It is a new type of industrial material in the high-tech field. It has attractive application prospects due to its unique performance advantages. At present, it is widely used in aerospace, military, automotive, electronics, machinery, chemical, textile industry, and civil sports. The performance characteristics of carbon fiber are as follows:

1 Graphite crystallites are stacked along the fiber axis and carbonized and graphitized to obtain carbon fiber, so the carbon fiber has high strength along the fiber axial direction. In recent years, countries around the world have invested heavily in the research and production of carbon fiber. The tensile strength of polyacrylonitrile (PAN)-based carbon fibers made in the United States is as high as 5.5-5.8 GPa, and the modulus is 354-375 GPa, with the advancement of production technology. This performance indicator will also be broken. After comparison, it is not difficult to see that these performance indexes of carbon fiber are much higher than that of steel, and the density of carbon fiber material is only 1.8g/cm3, only about 1/4 of steel, so its specific strength is much higher than that of steel, and the specific modulus is higher. Also higher than steel. Materials with higher specific strength and higher specific modulus are favored in bridge engineering.

2 Carbon fiber generally has a carbon content of more than 90%, which retains the inherent properties of carbon, that is, it is chemically stable, and its corrosion resistance is outstanding. It has good stability to common acids and bases, and only appears when the temperature reaches 400 °C. Significant oxidation, the formation of carbon monoxide (CO) and carbon dioxide (CO2), to avoid the problem of rusting steel materials. Some scholars have soaked PAN-based carbon fiber in a strong sodium hydroxide solution, and it still retains fiber morphology for more than 30 years.

3 carbon fiber material has no creep, has excellent fatigue resistance, and the axial thermal expansion coefficient is small, and the temperature change has little effect on the performance of the material. This performance advantage is particularly obvious in the environment with relatively harsh temperature conditions.

4 carbon fiber material has remarkable anisotropy, soft and processability, and can be processed into various fabrics such as carbon fiber cloth of a certain thickness. Carbon fiber cloth can increase the strength of the component without increasing the weight and cross-section of the component. This is because the carbon fiber has high specific strength and thin thickness, and retains the softness of the fabric itself. It is known as “outer and soft inside”. It is a new generation of fiber reinforced materials, so it can be used for reinforcement and repair of bridges, buildings, tunnels and other structures as well as seismic strengthening.

Based on the above advantages of carbon fiber composite materials, foreign countries began to study the application of carbon fiber composite materials in bridge engineering since the late 1970s. China began to study its application in bridge engineering after the 1990s.

Second, the common problems of carbon fiber application in bridge engineering

At present, most of the bridge projects are concrete bridges, steel bridges and steel-concrete composite bridges. After using for a period of time, the following problems often occur: 1 longitudinal cracks occur in the slab beam, internal steel bars are seriously corroded; 2 parts of slab beam concrete appear Severe corrosion, mildew; 3 mesh cracks on the bridge surface; 4 weathering is serious, concrete peeling and steel rust [5]. As a result, more and more bridges have potential safety hazards and even become dangerous bridges, which have a great impact on transportation and transportation. More difficult construction conditions such as strong winds, deep water, bad weather, etc.; broader functional requirements such as public railway construction, all-weather opening, high-speed rail transportation, etc.; more huge projects such as Qiongzhou Strait, Taiwan Strait, etc., future bridge engineering materials Performance requirements will face new challenges.

Faced with many problems in bridge engineering, combined with the performance characteristics of carbon fiber composite materials, scientists from all over the world have set off a research boom in the use of carbon fiber composite materials for bridge engineering. At present, the world's leading companies are mainly Japan, the United States, Germany and South Korea. The world's carbon fiber production has reached 40,000 t / a, of which 60% of carbon fiber production is occupied by Japan. China's research on carbon fiber started late, and the progress is slow. The preparation process has not been broken, and it basically relies on imports . Until the 21st century, domestic carbon fiber gradually entered industrialization, and enterprises that joined the ranks of carbon fiber production gradually increased and achieved certain results. It is expected that in the near future, China's carbon fiber industry will achieve a relatively fast growth rate. Polyacrylonitrile-based carbon fiber is widely used at present, because its production process is simpler than other methods, accounting for more than 90% of the total global carbon fiber production.

Third, the research and application of carbon fiber composite materials in bridge engineering

With the rapid development of transportation industry, bridge engineering as a transportation hub has become increasingly important, and the quality and longevity of bridges have become the top priority. The key to endangering the safety and durability of bridges is the corrosion of steel. Whether it can take effective measures to improve the material properties against steel corrosion, or to choose effective materials instead of steel for bridge engineering has been plagued by people. Carbon fiber composites attract people's attention because of their unique properties such as high strength, corrosion resistance and light weight. With the increasing maturity of carbon fiber material preparation technology and the reduction of carbon fiber production cost, and the performance is getting better and better, the commercialization and civilization of carbon fiber materials are just around the corner, and its application in bridge structure engineering is widely used. It will definitely bring about the rapid development of bridge engineering.

1. Carbon fiber composite material is used to build bridges

The carbon fiber composite material is made into a stranded wire and a bar is mixed with the steel bar, and even replaces the steel reinforced concrete structure. The carbon fiber composite material is corrosion-resistant and alkali-resistant, and does not easily rust as a metal in the concrete to affect the service life. For the first time in Japan, carbon fiber composites were used as concrete ribs in concrete bridges to build bridges. In 1991, Germany also applied carbon fiber composites into strands and applied them to a post-tensioned prestressed concrete high-speed overpass bridge, which was anchored to the beam by a wedge system. The application of carbon fiber composites in bridges, especially the use of strands and bars of carbon fiber composites as prestressing tendons and external tendons, lays the foundation for the application of carbon fiber composites as cable materials for super-long span bridges.

2. Carbon fiber composite material used for cable-stayed bridge cable

The main cause of damage to bridge cables and slings is corrosion, which requires high resistance to fatigue and corrosion. Carbon fiber materials have excellent corrosion resistance, fatigue resistance, strength, modulus of elasticity, etc., so some scholars have tried to use carbon fiber reinforcement as a material for making stay cables and slings. The cable-stayed cable in the famous Winterthern Storchenbrucke bridge in Switzerland is partially made of carbon fiber composite [7]. The bridge uses two stay cables made of carbon fiber composite material, each of which is composed of 241 5mm carbon fiber composite tendons. With the continuous advancement of technology, the performance of carbon fiber composite materials continues to increase and the cost is reduced, and the proportion of its application in the cable and sling will continue to increase.

3. Carbon fiber composite material for bridge reinforcement

With the increasing traffic of transportation, some highway bridges have been overloaded for a long time. Coupled with the phenomenon of aging and damage, about 30% of the bridges in China are in a state of being overwhelmed. If dismantling and reconstruction occurs, traffic disruption will affect the development of transportation. Therefore, strengthening it on the basis of the old bridge has become an effective measure to ease traffic.

(1) Reinforcement with carbon fiber composites

The carbon fiber composite material is repaired and repaired by using a resin material to adhere the carbon fiber material to the surface of the structure or component to form a composite material body, so that the original reinforced concrete and the carbon fiber jointly form a force surface, thereby increasing the shear resistance and bending resistance of the bridge structure. Improve mechanical properties.

Developed countries such as the United States and Japan began to study the application of carbon fiber composite materials in the reinforcement and repair of concrete structures from the late 1970s to the early 1980s. China has only begun to study this technology since 1997. Although it started late and developed slowly in the past few years, with the continuous advancement of science in China and the rapid development of economic construction, more and more universities and research institutes are investing in this. The research work has also achieved remarkable results. China's civil construction market has the most potential, and the application of carbon fiber reinforcement technology in building structures will continue to grow.

Carbon fiber reinforcement is divided into carbon fiber cloth reinforcement and carbon fiber board reinforcement. The carbon fiber materials used for reinforcement in actual engineering are mainly carbon fiber cloth. The main advantages are reflected in the following aspects:

1 Reinforcement effect is obvious: the carbon fiber cloth is bonded with the reinforced concrete members by epoxy resin, so as to achieve the effect of joint cooperation and force, the strength is obviously improved, such as the concrete structure and the bearing capacity and ductility of the members are greatly improved. Reinforce the repair.

2 Corrosion resistance of acid, alkali and salt medium: Carbon fiber cloth only uses epoxy resin in the reinforcement process, no metal material is added, so it will not rust, and it does not need regular rust prevention maintenance like the stick steel method. The carbon fiber is resistant to acid, alkali and the like, and can be applied to the reinforcement of components with a harsh working environment, and is particularly suitable for areas with high air humidity and high salinity in coastal areas.

3 Simple construction and fast speed: The carbon fiber cloth has low density, light weight and good flexibility. Therefore, the carbon fiber cloth can be brought to the construction site, and can be cut according to the size of the site, and the operability is strong; during the construction process, a small power tool can be used. No need for large equipment, convenient construction; no need for on-site fixed facilities, less space occupied, less operational space requirements, no wet operation, easy operation; fast construction speed, reinforced work efficiency of carbon fiber bonding is 4~ 8 times.

4 Keep the original structure unchanged, the appearance is beautiful: the operation is only to paste multiple layers of carbon fiber cloth on the surface of the original structure, cut and paste the carbon fiber cloth according to the shape of the member, without changing the original structure, and also apply decorative materials and fireproof materials on the outer layer. Improve their enjoyment and safety. In addition, the carbon fiber material prepreg is small and thin, the thickness of the single layer after bonding is only about 1.0 mm, and the mass per unit area after bonding is less than 1.0 kg (including resin), so the quality and volume of the construction are not increased after the construction, and the epoxy resin is bonded. There is no need to anchor the bolts, and there is no new damage to the original structure. And several layers can be pasted according to the force situation.

5 Extend the service life of the concrete structure by sealing the cracks generated by the concrete structure.

6 Carbon fiber composite reinforcement bridge is easy to construct and construction time is short.

7 Wide application: The carbon fiber composite material is flexible and repairable. It is not restricted by the type, shape and construction part of the structure. It can be widely used in various reinforcement and repairing. This is unmatched by any kind of structural reinforcement method.

(2) Principle of carbon fiber cloth in bridge reinforcement

Under the action of external force, the binder can effectively concentrate the strength of the carbon fiber filament, so that the carbon fiber filament forms a common force surface on the carbon fiber bristle, which generates a collective force and restrains the core concrete, thereby preventing the main rib from yielding. Improve the bearing capacity of reinforced concrete and finally achieve the reinforcement of the bridge.

The use of carbon fiber composite materials for the reinforcement of concrete bridge structures has been widely recognized by countries all over the world and is highly valued. It will become an effective measure for the reinforcement and repair of concrete bridges in the future.

Fourth, the outlook

With the advantages of carbon fiber composites in bridge reinforcement, the reinforcement and repair of concrete bridges has ushered in new developments, and at the same time brought about the development of bridge construction materials, which has promoted the advancement of bridge technology and thus the rapid development of bridge engineering. development of. As the carbon fiber composite material preparation technology continues to mature, the production scale continues to grow and the production cost continues to decrease, its application range in the bridge field will gradually expand. If steel and cement are the main materials for bridge construction in the 20th century, carbon fiber composites will likely become the main building materials for the 21st century and ideal materials for high-span bridges.

The use of carbon fiber composite materials for the reinforcement and repair of old bridges not only reduces costs, but also has an ideal effect. It is the material of choice for bridge reinforcement. The carbon fiber composite tendon is used as the strength of the prestressed concrete bridge or the cable of the cable-stayed bridge, and its excellent characteristics are gradually reduced. The proportion of the steel in the bridge construction should be gradually reduced, which should be the focus of the application of the carbon fiber composite material in the bridge.

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