Application of carbon fiber composite in bridge reinforcement?

The application of carbon fiber composites in bridge reinforcement is very important. Knowing the material can better combine with the actual situation to achieve the expected effect, and the treatment of every detail is very important. Zhong Da Consulting will explain the application of carbon fiber composite materials in bridge reinforcement.

1, the importance and significance of bridge reinforcement:

Traffic facilities are an important infrastructure to ensure the healthy development of national economy and society. As an important part of traffic facilities, reinforced concrete bridges play a vital role. In the construction of small and medium-sized bridges in China, bridge analysis was the main method in 1960s, hyperbolic arch bridge was the main method in 1970s, and T-shaped and deck assembled bridges were the representative in 1980s. Among them, the bridge built in the early 1990s has the worst quality. Due to the survey, design, construction, time and other reasons, as well as the increasing traffic volume, the representative vehicles tend to be heavy, and the load of highway bridges also increases. In addition, some old bridges are aging, damaged or limited by the original design load standard, and the bearing capacity of bridges is generally insufficient. Many existing bridges can no longer meet the requirements of modern transportation. These problems not only affect the normal use of bridges, but also bring great losses to the country and people. In order to meet the needs of modern traffic and the requirements of bridge design specifications, bridges must be reinforced.

There are tens of thousands of large and medium-sized reinforced concrete bridges and high-rise buildings in China, of which thousands need reinforcement and maintenance. By the end of 1998, there were 2 10822 highway bridges in China, including 3,053 temporary bridges and 4 105 dangerous bridges. There are 405 temporary bridges and 253 dangerous bridges in national and provincial highways, accounting for 1253 of temporary bridges and dangerous bridges respectively. There is a big gap with the Ministry of Communications "Highway Scientific Maintenance and Planning Management Requirements" (199 1-2000). These problematic structures are in urgent need of diagnosis, maintenance and reinforcement. According to incomplete statistics, there are more than 20,000 large and medium-sized reinforced concrete bridges in Guangdong Province alone, and now there are more than 2,500 old and dangerous bridges that need to be strengthened.

The development of civil engineering in the new century largely depends on the application and development of new materials with excellent performance. For traditional reinforcement materials, we should look for a new material with high strength, light weight, good corrosion resistance and durability; For the reinforcement, maintenance and transformation of existing structures, materials with excellent physical and mechanical properties such as high specific strength, fast construction speed and obvious improvement of structural bearing capacity after construction should be used instead of traditional steel. Therefore, it is an important research topic of sustainable development in the field of civil engineering in 2 1 century to use fiber composite materials with excellent performance to transform and strengthen civil engineering structures. The application of FRP in civil engineering has become a hot spot in the field of international civil engineering. Because of its excellent materials and properties, its application scope and usage are increasing at an alarming rate. As a new building material with development potential, FRP is not to replace traditional building materials-steel bars and concrete, but an important supplement to traditional building materials, and so is CFRP.

2. Introduction of traditional reinforcement methods

The traditional reinforcement methods are: 1. Bonding steel plate reinforcement method; 2. Enlarging section reinforcement method; 3. External prestressing reinforcement method; 4. Change the reinforcement method of structural stress system.

2. 1 bonded steel reinforcement method

When the bearing capacity of the bridge is insufficient or the longitudinal main reinforcement is seriously corroded, the main girder will generally produce serious transverse cracks. The steel plate is anchored and pasted on the edge or weak part of the tensile area of concrete structure by using anchor bolts and adhesives, so that the steel plate and the bridge structure become a whole, forming a composite system which is jointly stressed by concrete, adhesives and steel plates, and achieving the purpose of improving the bearing capacity of the bridge.

The characteristics of this reinforcement method are: (1) It is not necessary to destroy the size of the original structure after reinforcement; (2) The construction technology is simple and the construction quality is easy to control; (3) Short construction period.

2.2 Increase the section reinforcement method

When the strength, stiffness, stability and crack resistance of beams and columns are insufficient, reinforcement methods such as increasing the section of members, increasing the reinforcement ratio and improving the reinforcement ratio are usually adopted. This method is to increase the reinforced concrete section (with main reinforcement) and the column section (with column section reinforcement) at the bottom or side of the beam, thus increasing the bending section of the beam, increasing the bearing area of the column and improving the bearing capacity of the beam column. This method is widely used in the reinforcement of arch ribs of beam bridges and arch bridges.

2.3 external prestressing reinforcement method

For reinforced concrete or prestressed concrete beams or slabs, applying external prestress in the tension zone can offset part of self-weight stress and play an unloading role, thus greatly improving the bearing capacity of the beam.

The advantages of external prestressing reinforcement method are:

(1) can greatly improve and adjust the stress state of the original structure, and improve the stiffness and crack resistance of the load-bearing structure with little increase in self-weight;

(2) Due to the slight increase of the self-weight of the load-bearing structure, it has little influence on the stress state of the pier and foundation. Can save the reinforcement of pier and foundation;

(3) It has little influence on bridge operation, and can be reinforced without restricting traffic;

(4) Pre-stressed reinforcement method can be used as a temporary reinforcement method for bridges passing through heavy vehicles, and can also be used as a measure to permanently improve the load level of bridges.

1.4 reinforcement method for changing the structural stress system

This method is to improve the bearing capacity by changing the stress system of the bridge. For example, the bearing capacity can be improved by adding brackets or piers under simply supported beams, or connecting simply supported beams to form a continuous beam system, and adding superimposed beams or stiffening beams under beams. This method has good operability and is a common reinforcement method to solve the temporary traffic problem of overweight vehicles.

2 carbon fiber reinforcement technology

The strengthening method of carbon fiber reinforced composite plastics is to soak carbon fiber sheets in resin-based adhesive and stick them on the surface of the tensile area of concrete members, so that they become a whole with the same deformation and stress as the concrete matrix, thus achieving the purpose of improving the bearing capacity of the structure.

2. 1 properties of carbon fiber

2. 1. 1 classification of carbon fiber

Carbon fiber can be classified in the following two ways:

1 Classification by mechanical properties:

The tensile modulus of (1) high modulus (type I) carbon fiber is very high, reaching 380-640GPa, but the ultimate elongation is low, about 0.3%-0.5%.

(2) The tensile strength of high-strength (type II) carbon fiber is about 3000MPa, and the tensile strength of products with good technology can reach more than 4000MPa.

(3) The tensile modulus of medium modulus (type III) carbon fiber is generally between 200 and 300 GPA, and the ultimate elongation is 1.5%-2.0%. Its comprehensive mechanical properties are suitable for strengthening concrete structures, and it is most widely used in strengthening projects.

2 According to the different classification of fiber materials:

Carbon fiber (CFRP);

(2) glass fiber (GFRP);

(3) aramid fiber (AFRP) O.

3 according to the arrangement direction of different classification:

(1) unidirectional fiberboard;

(2) Bidirectional fiberboard. Table 2. 1 Fiber Material Table

2. 1 Mechanical properties of carbon fiber for reinforcement

The main mechanical properties of carbon fiber sheet can be determined by referring to the current national standard "Test method for tensile properties of oriented fiber reinforced plastics" GB/T3354, and the main mechanical properties should meet the requirements in the following table: Table 2.2 Main mechanical properties of carbon fiber sheet.

Although the fiber types and molding processes are different, the stress-strain relationship of all fiber composites is the same: elasticity rises until brittle fracture occurs in tension, which is an important feature of CFRP materials.

As can be seen from Figure 2. 1, fiber composites are brittle materials, and we know that steel bars are ductile materials. When used to strengthen reinforced concrete structures, the brittleness of fiber composites will have the following two important effects on the structural performance:

Firstly, the ductility of fiber composites is not as good as that of steel bars, and its brittleness will limit the ductility of reinforced concrete structures after reinforcement; Secondly, the brittleness of FRP material limits the stress redistribution. These factors affect the strengthening design of externally bonded FRP materials and cannot be simply replaced by steel bars with equal strength. It is necessary to deeply study and consider the brittleness of FRP materials and improve the existing design methods of reinforced concrete structures.

2.2 Performance of bonding materials such as adhesives

Bonding materials used in carbon fiber reinforced materials should include bottom resin, leveling material and impregnating or pasting resin. The performance of bonding materials must meet the requirements of specifications, and the performance of bonding materials is closely related to bonding quality and reinforcement effect. If the performance of the bonding material fails to meet the requirements, the reinforcement effect will inevitably decrease or even fail.

The function of bottom resin is to strengthen the surface of concrete and improve the bonding strength between concrete and leveling material or bonding resin. The normal tensile bond strength should be greater than 2.5MPa and should not be less than the standard value of tensile strength of reinforced concrete. The function of leveling material is to fill the holes and cracks in the upper surface of concrete, so that the flatness of the reinforced surface can meet the technological requirements, and it has reliable bonding strength with the underlying resin and impregnating resin, forming an integral bonding system. Positive bond strength should be greater than 2.5MPa, and should not be less than the standard value of tensile strength of reinforced concrete. When the smoothness of concrete surface meets the requirements, the amount of leveling material can be reduced appropriately.

Impregnating resin is the main bonding material for sticking carbon fiber cloth, and its function is to fully bond carbon fiber tow with concrete and jointly bear the load of concrete structure. Resin is also used as the matrix of FRP composites and the adhesive between FRP and concrete. The role of adhesives is particularly noteworthy. If the adhesive is too weak, the interface will be destroyed. When the bonding effect of resin is very important, such as bending and shear strengthening of beams, epoxy resin is generally used as adhesive. The strength of concrete structures that need to be strengthened is often relatively low, while the strength of epoxy resin is high, so interface failure often occurs in concrete. Bonding resin is the main bonding material for bonding carbon fiber board.

3. Introduction of carbon fiber composite reinforcement construction technology.

The reinforcement construction technology of carbon fiber composite materials is very important. Because the construction quality directly affects the reinforcement effect, the quality of construction, whether the parameters of materials meet the standards, whether the bonding between fiber composite materials and concrete surface is firm, whether the deformation is coordinated, and the difference of boundary conditions have great influence on the reinforcement effect.

3. 1 The overall construction process is as follows:

1 construction preparation stage

Read the design and construction drawings carefully, and make the construction scheme and plan according to the actual situation of the construction site and reinforced concrete structure; Prepare to use carbon fiber sheet to support resin and machinery before construction.

2 surface treatment

Degraded concrete such as spalling, looseness, honeycomb and corrosion on the surface of steel bar should be removed to expose the concrete structure layer, and the surface should be repaired with repair materials, and the cracks should be closed or sealed according to the design requirements. The concrete surface to be pasted should be polished and leveled, and impurities such as laitance and oil stain on the surface should be removed until the new surface of the concrete structure is completely exposed. Corner paste should be chamfered and polished into an arc with an arc radius of not less than 20mm, and the concrete surface should be clean and kept dry.

3. Brush the bottom resin and level it.

Prepare the bottom resin in strict accordance with the process regulations provided by the product manufacturer, and evenly coat the bottom resin on the concrete surface with a roller brush. After the bottom resin surface is finger-dried, proceed to the next process as soon as possible. Prepare leveling materials in strict accordance with the process specification provided by the product manufacturer. The concave part of the concrete surface shall be filled with leveling material, and there shall be no edges and corners. The corner shall be repaired with leveling material into a smooth circular arc with a radius of not less than 20 mm.

4 carbon fiber cloth

(1) Cut the carbon fiber cloth according to the design requirements;

(2) Prepare the impregnating resin according to the process regulations of the product supplier, and evenly spread it on the bonding part;

(3) Lightly press the carbon fiber cloth to the position where it needs to be pasted by hand, and repeatedly roll along the fiber direction with a special roller to extrude bubbles, so that the impregnated resin fully soaks the carbon fiber cloth and the carbon fiber cloth is not damaged in the rolling process;

(4) The above steps should be repeated when multi-layer bonding, and the next layer should be bonded as soon as possible after the impregnated resin on the fiber surface is dried;

(5) After pasting, evenly coat impregnating resin on the surface of the last layer of carbon fiber cloth.

5 surface protection

Do a good job of surface protection in accordance with the provisions of relevant standards, and ensure that there is a reliable bond between protective materials and carbon fiber sheets.

3.2 construction matters needing attention

(1) The temperature is lower than 5℃ and the relative humidity is RH; 85%, the water content of concrete surface is above 8%, which may cause condensation, so effective measures shall not be taken for construction;

(2) Because carbon fiber sheet is a good conductor of electricity, it should be far away from power supply;

(3) All kinds of resins used for construction shall be kept away from open flames and avoid direct sunlight;

(4) Construction and inspection personnel should wear protective work clothes, masks, gloves, protective glasses and other protective articles;

(5) The construction site is well ventilated;

(6) When all kinds of glue are attached to the skin, rinse with soapy water. If it gets into eyes, wash it with water immediately or be treated by a doctor.

(7) Fire prevention and other safety measures should be taken at the site.

4. Summary

Fiber reinforced polymer (FRP) can give full play to its light weight and high strength by combining with epoxy resin aggregate materials and bonding materials. At the same time, the construction technology is more convenient than the traditional construction method, which shortens the construction period and has a good application prospect in the processing of concrete structures. In the future, the prices of fiber materials and epoxy resin will drop, and FRP will be more widely used in civil engineering.

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