Fiber-reinforced composites attract much attention from bridge engineering community

The advancement of engineering materials is the driving force behind the development of engineering structures. The history of bridge construction for thousands of years is also a history of bridge construction materials. The invention and promotion of new materials often greatly promote the development of bridge engineering. Historically, concrete and steel are good examples. Since the 20th century, these two "new" materials have gradually replaced ancient bridge construction materials such as wood, stone, and brick in the world, enabling human bridge construction to achieve unprecedented great development.

Bridge engineering urgently needs material breakthrough

The development of human economy and society in the 21st century has put forward new and higher requirements for bridge engineering. Traditional building materials such as steel and concrete have problems such as low strength, excessive weight, and insufficient durability. It has become increasingly difficult to meet the needs of bridge reinforcement and new construction. The demand for bridge engineering needs to achieve a breakthrough at the material level.

Fiber Reinforced Polymer (FRP: hereinafter referred to as fiber composite material) is a high-performance material with high strength, light weight, corrosion resistance, fatigue resistance, and low creep. It has been widely used in aerospace, automotive, sports , energy and other industries, the application in the field of civil engineering is in the ascendant. In bridge engineering, replacing traditional building materials such as steel and concrete with fiber-reinforced composite materials can further improve the mechanical properties and durability of bridge structures, reduce total project investment, reduce carbon emissions, and promote sustainable economic and social development.

Four commonly used fiber composites：

Fiber composites, as the name implies, are composed of reinforcing fibers and matrix materials. Among them, the fiber plays a load-bearing role and is the main body of the fiber composite; the matrix bonds the dispersed fibers into a whole, transmits external forces to the fibers, and protects the fibers. Fiber composite materials for bridge engineering mainly include continuous fiber reinforced resin matrix, etc.

There are mainly four types of fibers commonly used in fiber composite materials for bridge engineering, namely glass fiber, carbon fiber, basalt fiber and aramid fiber.

Excellent fiber composite performance

The density of the above four fiber materials is much lower than that of commonly used metal materials such as steel and aluminum alloy, and the tensile strength is more than an order of magnitude higher than that of metal, so the strength is also much higher than that of metal, and the characteristics of light weight and high strength are very prominent. Except for carbon fiber, the modulus of elasticity of the other three fibers is smaller than that of steel, which is equivalent to that of aluminum alloy. The coefficient of thermal expansion of the fiber is close to the dry dew or even negative, showing a strong temperature deformation inertia.

The mechanical properties of fiber composites are mainly determined by fibers. Due to the high strength of fibers, the tensile strength of various fiber composites is higher than that of steel, especially structural steel. Among them, the tensile strength of high-strength carbon fiber composites can reach more than 3000MPa. In addition to excellent static properties, fiber composites are especially The fatigue resistance of carbon fiber composites is far superior to that of steel, and the fatigue problem of bridge structures is more prominent. Using fiber composites in bridges to reinforce or replace traditional materials will help improve their fatigue resistance. In harsh service environments, The durability of fiber composites is significantly better than ordinary steel. Replacing steel bars/steel cables with fiber composite bars/cables can effectively improve the durability of bridge structures, fundamentally solve the problem of steel corrosion, reduce maintenance and repair costs during the operation phase, and prolong the service life of bridges.

In addition, fiber composites also have strong designability and are suitable for material-structure integrated design. During preparation, the strength, elastic modulus, heat resistance and other properties of fiber composites can be adjusted by selecting different fibers, resin matrix, fiber volume ratio, and fiber laying direction. At the same time, different molding processes can be used to mix or compound with other materials to prepare products with various shapes and properties to meet the needs of bridge engineering.

Fiber composites also have deficiencies

Compared with traditional building materials, fiber-reinforced composite materials also have disadvantages. Except for high-modulus carbon fiber composites, the elastic modulus of other fiber composites is generally 50~160GPa, which is lower than that of steel. Due to the different fibers used, the elongation at break of various fiber composites varies greatly. Among them, the elongation at break of glass fiber composites is the largest, but it is also much smaller than that of steel, showing that the ductility of fiber composites is insufficient.

In addition, due to the relatively low glass transition temperature of the resin matrix, the fire resistance and high temperature resistance of fiber composites are generally inferior to those of concrete and steel, but additives can be used to improve their heat resistance.

Fiber composites deserve attention

At present, fiber composites have received extensive attention from the bridge engineering community and are also a research hotspot in the academic circles. However, they are still only a substitute for steel or concrete materials in specific environments, and they are far from large-scale applications, especially in new bridges. There is still a long way to go, and it is urgent to overcome some obstacles that restrict the development of the industry, such as lack of awareness of fiber composite materials among practitioners, low degree of standardization of fiber composite materials, high prices, and no mature design and construction methods.

It is expected that bridge engineers and researchers will deepen their understanding of fiber composites, and jointly promote the large-scale application of this new type of bridge construction material in my country, and promote the further development of Chinese bridges in the 21st century.