Application of strengthening concrete technology with carbon fiber unidirectional cloth in old bridge reconstruction?

What is the application of carbon fiber reinforced concrete technology in the reconstruction of old bridges? Here, Zhong Da Consulting Bidding Teacher will answer your questions for your reference.

I. Overview

Carbon fiber unidirectional fabric has the advantages of high strength, high elastic modulus and good durability. According to the design requirements, special resin and carbon fiber cloth are applied to the concrete surface to achieve the reinforcement effect of the structure. Through a large number of comparative tests of beams, slabs and columns before and after strengthening, the mechanical properties have been significantly improved. According to the experimental research, sticking a layer of carbon fiber unidirectional cloth can improve the bending performance of beams and plates by 5 ~ 8%. Compared with other reinforcement technologies such as steel bonding and shotcrete, this technology has the characteristics of small dead weight, simple construction, short construction period and good durability.

The United States, Japan and other countries began experimental research in the 1980s, and now this technology has been widely used. According to relevant statistics, in 1997, the amount of carbon fiber cloth used for strengthening and repairing in Japan alone reached 700,000 m2. In recent two years, scientific research institutions and universities in China have done a lot of research and application in this field, produced a series of carbon fiber composite materials, reduced the project cost, and gained mature design methods and construction experience, which created favorable conditions for widely popularizing this technology.

Second, brief introduction of overpass reconstruction project design

Jianxiang overpass is located at the intersection of Beijing North Fourth Ring Road and Badaling Expressway, and was completed at 1998. The superstructure of the bridge is a 6-span prestressed concrete simply supported beam with L = 18m. There is a separation belt in the middle, two-way motor vehicle lanes on both sides and non-motor vehicle lanes on both sides, with a total width of 55m. With the growth of urban traffic volume, the existing overpass width can no longer meet the needs of traffic volume. In order to ensure that the Fourth Ring Road becomes an urban expressway, it is necessary to widen the north and south sides of the existing bridge to 72 meters, transform the existing non-motor vehicle bridges on the two outer sides into auxiliary bridges, and build new non-motor vehicle bridges. At present, the load of the superstructure main beam and middle pier capping beam of the bridge is designed according to the automobile-super 20 class and trailer-120, while the load of the superstructure main beam and middle pier capping beam corresponding to non-motor vehicle lanes is designed according to the automobile-super 20 class and trailer -80 class. After being rebuilt into a highway bridge, the required load is automobile-super 20 class and trailer -65430. At present, the main beam of the non-motor vehicle bridge has been removed and replaced, but the middle of the motor vehicle and the non-motor vehicle of each pier cap beam has not been disconnected, and it is a continuous structure of variable cross-section ordinary reinforced concrete with three spans and one connection.

After repeated accounting and comparison, in order to minimize the workload of steel bars in the positive and negative bending moments of capping beams, on the one hand, measures are taken to reduce the self-weight of the bridge, and the concrete bridge deck is changed into a lightweight (bulk density is 19KN/m3) concrete bridge deck; On the other hand, the width of the pier column under the middle pier capping beam is widened, and the positive and negative bending moment and shear force of the capping beam are also reduced accordingly, so that the existing reinforcement configuration of the capping beam corresponding to the motor vehicle part can meet the design requirements. For the auxiliary road (now non-motor vehicle bridge), in order to save engineering investment and according to the situation that the traffic of the Fourth Ring Road is not allowed to be interrupted, with the approval of the relevant departments, it is determined to reinforce 20 positive and negative bending moment areas of the five rows of pier capping beams of the whole bridge. The reinforcement area of the capping beam of the whole bridge is 395m2, and the specific reinforcement position is

Third, the choice of reinforcement scheme and materials

1? Selection of reinforcement scheme

The original design of the capping beam of each pier is invisible, which brings great difficulties to the reinforcement work. The reinforcement scheme of sprayed steel fiber reinforced concrete wrapped with steel plate was put forward at the earliest. Because the beam end of the main girder is only 5cm away from the outside of the capping beam at the span, the above two schemes are less feasible after calculation and comparison and the limitation of construction space, and finally carbon fiber composite material is selected for reinforcement. The ultimate tensile strength of carbon fiber cloth is more than 10 times higher than that of ordinary steel bars, and its elastic modulus is similar to that of steel bars, so it is more suitable for strengthening reinforced concrete structures. In addition, the construction is simple and suitable for reinforcement projects with short construction period.

2? Selection of materials

In recent two or three years, China has introduced carbon fiber composites produced in the United States and Japan, and carried out some engineering reinforcement, which has achieved good benefits, but the high cost has affected its popularization and application. The recent appearance of domestic series of carbon fiber composites laid the foundation for the selection of this engineering material. By comparison, the carbon fiber cloth and supporting resin of Beijing Tiehua Dazhong Technology Development Co., Ltd. are adopted, in which the carbon fiber cloth is made of imported high-quality silk, with reliable and stable quality and low cost. The products have passed the strict inspection by the national quality inspection department, and all performance indexes have reached the level of similar products abroad. The following are the specifications of carbon fiber unidirectional fabric and matching resin and the performance comparison results with similar products in Japan (Nippon Steel series indicators).

By using the above-mentioned carbon fiber composite materials, different types of mechanical properties of beams, plates and columns before and after reinforcement are tested and studied, and the construction environmental conditions and construction process protection layer are discussed and studied. After repeated tests and tests, it is proved that the products have stable performance and excellent quality, and have been applied to aerospace, aviation and military fields, and have been mass-produced.

Fourth, the construction technology

In order to ensure that the original stressed steel bars and reinforced carbon fiber can work synchronously, the stress-strain difference between the original steel bars and carbon fiber should be shortened as much as possible, and unloading measures should be considered when strengthening the capping beam. So as to delay the transmission time of the stress of steel bars in the original capping beam structure to carbon fiber, actively improve the capping beam structure, make the original steel bars and carbon fiber enter the limit state respectively when damaged, and significantly improve the overall strength of the capping beam. ? Unloading measures for capping beam reinforcement: remove the main beam with low original load standard → paste carbon fiber sheets for reinforcement → install the main beam with new standard load.

Reinforcement construction technology

1? Grind the reinforced concrete surface, remove the weathered layer, expose new stubble, remove dust and keep it clean.

2? Surface treatment resin shall be evenly coated on the interface to be constructed, and there shall be no omission.

3? When the surface resin does not stick to hands, use leveling resin to level the base surface, so that the base surface is flat and free from depression.

4? After the leveling resin is basically cured, the impregnating resin is evenly coated on it, with a thickness of about 3 ~ 5 mm, without being too large, too thin or omitted.

5? Stretch the carbon fiber unidirectional cloth, lay it flat on the base surface coated with impregnated resin, and then roll it repeatedly with special tools to eliminate the air between the unidirectional cloth and impregnated resin, so that the unidirectional cloth is completely impregnated with resin, and the carbon fiber unidirectional cloth should be straight without holes and bubbles in the middle.

Reinforcement position map

6? The laying of one-way cloth on the second floor and above is the same as the operation and requirements in items 4 and 5.

7? After the last layer of unidirectional fabric is rolled, it should be flattened for the last time, and then coated with impregnating resin with a thickness of about 1 ~ 2mm.

8? The curing time of the reinforcement layer is about 7 days. After curing, protective and decorative topcoats can be painted according to design requirements.

The selected protective layer has excellent anti-aging and medium corrosion resistance, and can be used for a long time at -50℃ ~ 120℃. This material has been used in many key projects, such as CCTV Tower, Xiling Yangtze River Bridge of Three Gorges Project and so on, with good results. ? The reinforcement project of Jianxiang overpass started in early June 2000 and was completed in early July 2000, which lasted about 30 days. The bonding between carbon fiber cloth and beam surface is good, and the construction quality has achieved satisfactory results.

Verb (abbreviation of verb) Advantages and discussion opinions of steel bar

1? Carbon fiber cloth is light in weight, does not need any heavy equipment during construction, and is very thin, and does not need much space, especially in the case of tight space for steel bars in this project.

2? Sticking carbon fiber with high strength and high elastic modulus to strengthen reinforced concrete structures can also achieve the same reinforcement effect as steel plates, but its structural weight has not increased, and its appearance is light and beautiful.

3? It has excellent durability, because only carbon fiber and matching resin are used for reinforcement, it will not rust, and it has the properties of acid resistance, alkali resistance, salt resistance and atmospheric environment corrosion, and carbon fiber cloth has excellent chemical resistance.

4? In the reinforcement project of Jianxiang overpass, carbon fiber composite material was used to strengthen the middle pier cap beam. After preliminary calculation, it saved about 25 million yuan for the project, shortened the construction period, and solved the problem of uninterrupted traffic during the construction period, which achieved great economic and social benefits and was well received by relevant units.

5? In order to promote the rapid development of this new technology and new material, it is suggested that the relevant units organize their efforts as soon as possible to formulate complete technical specifications such as design, construction, quality inspection and acceptance standards.

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