Key Points Of Carbon Fiber Reinforcement Strengthening

 

Carbon fiber reinforcement material characteristics

Carbon fiber reinforced plastics are made by combining carbon fiber materials with specific resin materials through a certain manufacturing process. Its mechanical characteristic is that the stress-strain variable is completely linear elastic, and there is no yield point or plastic zone. Carbon fiber materials have excellent physical and mechanical properties. The carbon fiber cloth used to strengthening reinforce concrete components is a soft sheet made of carbon fiber filaments by weaving. When carbon fiber cloth is woven, a large number of carbon fiber filaments are evenly laid out in one main direction. The main direction carbon fiber yarns are braided and connected together with very few non-main direction carbon fiber yarns to form a very thin carbon fiber cloth that is stressed in the main fiber direction. The standard value of the tensile strength of carbon fiber cloth should be greater than 3000Mpa, and the elastic modulus should be greater than 2.1105MPa. Comprehensive analysis of the physical and mechanical properties of the material, in order to maximize the advantages of the material itself, it is suitable to use CFRP material as the tension or pre-stressed bending member of the bridge structure, especially suitable for pure tension members. Engineering practice has also proved at this point. At present, the carbon fiber materials used for bridge reinforcement mainly bear tensile stress and restrain the development of cracks.

Key points of carbon fiber (sheet) strengthening reinforcement treatment

Before carrying out structural reinforcement treatment, first take effective measures to unload and support the beams and slabs, and then carry out reinforcement construction according to the following construction procedures.

1. Surface treatment of damaged concrete. Remove the degraded concrete such as peeling, looseness, honeycomb and corrosion on the surface of the reinforced component, and expose the concrete structure layer.

2. After treating the damaged surface steel bars according to the design requirements, use M45 epoxy mortar for grouting or sealing, and ensure that the thickness of the steel bar protection layer is not less than 15mm. Use a grinder to smooth the surface, and use a steel brush to remove dust, oil and other impurities on the surface to make the reinforced surface of the component smooth, dry and dust-free. In addition, if the carbon fiber needs to be pasted around the corner of the component in the direction of the base fiber, the corner pasting should be chamfered and polished into an arc shape, and the arc radius should not be less than 20mm.

3. Brush the bottom resin. Use a special roller brush to evenly smear the bottom layer resin on the concrete surface. When the resin surface is dry to the touch, proceed to the next process.

4. Leveling treatment. The concave part of the surface should be filled with leveling material and should not have edges or corners.

5. Paste the carbon fiber sheet. Spread the prepared impregnating resin evenly on the part to be pasted, and use a rubber roller to roll it along the fiber direction several times to squeeze out air bubbles so that the impregnating resin can fully penetrate the carbon fiber cloth without damaging the carbon fiber cloth during rolling. Repeat the above steps for multi-layer pasting. When the fiber surface is dry, the next layer can be pasted. If it exceeds 60 minutes, wait 12 hours before applying adhesive to paste the next layer.

6. Spread the impregnating resin evenly on the surface of the last layer of carbon fiber cloth.

7. If necessary, carry out surface protection and appearance treatment.

Precautions for carbon fiber strengthening reinforcement

1) Matters needing attention in the selection of carbon fiber sheet

The weaving uniformity of the carbon fiber sheet will directly affect the reinforcement quality. Only when the carbon fiber bundles are evenly distributed and arranged and processed into composite materials can they exert a uniform force on the whole. If the uniformity of the material is not good, the fiber sheet after the component is stressed will not be uneven, so that the effect of carbon fiber reinforcement cannot be fully exerted. Therefore, choosing a carbon fiber sheet with better uniformity is the key to the quality of strengthening reinforcement.

2) Matters needing attention for carbon fiber (sheet) strengthening reinforcement

①Before surface treatment and pasting of carbon fiber sheet, the line should be positioned according to the reinforced design position.

② Cut the carbon fiber sheet according to the size required by the design. The width of the cut fabric should not be less than 150mm and should not be less than 100mm.

③Wipe the surface of the carbon fiber sheet clean to no dust. When two layers need to be pasted, both sides of the bottom carbon fiber sheet should be wiped clean.

④The cleaned carbon fiber sheet should be coated with bonding resin immediately, and the glue layer should be convex, with an average thickness of not less than 2mm.

⑤Apply the carbon fiber sheet coated with bonding resin to the position to be pasted by hand. Use a rubber roller to compact evenly and smoothly along the direction of the fiber sheet, so that the resin overflows from both sides, ensuring compactness and no voids. When the carbon fiber is pasted in parallel, the gap between the two sheets should not be less than 5mm. .

⑥ When two layers of carbon fiber sheets need to be pasted, they can be pasted continuously. If it cannot be pasted continuously, clean the bottom carbon fiber sheet before starting again.

⑦The construction should be carried out at an ambient temperature of 5℃ or higher, and should comply with the construction use temperature of the supporting resin. When the ambient temperature is lower than 5°C, a supporting resin suitable for low temperature should be used or heating treatment measures should be adopted.

⑧ The adverse effect of environmental humidity on resin curing should be considered during construction, and the moisture content of the concrete at the sticking position should not be greater than 4%.

⑨When preparing the resin, it should be weighed and placed in a container according to the proportion specified in the product instructions, and stirred with a stirrer until the color is uniform. There must be no oil and impurities in the mixing container. It is advisable to determine the mixing amount of the resin each time according to the actual temperature on site, and strictly control the use time.

⑩In order to ensure the quality of the paste, different types of paste resins should be used in different seasons and different temperature conditions, so that the operating time and curing time of the resin construction can be effectively controlled.

CFRP stengthening project - Longevity Palace

Longevity Palace, or Jingyang Temple, is thousands of cities and rural areas across the country, as well as Taiwan, Singapore, Malaysia and other regions and countries. It is also the representative of Chinese ancient hall culture, so it is also called Jiangxi Hall, Jiangxi Temple, Jiangxi Township Hall, Yuzhang Hall, etc. 

The renovation project was carried out in three phases, with a total planned reconstruction area of approximately 20.75 acres and a total investment of 320 million yuan, involving a total carbon cloth of 150,000 square meters. It was the largest carbon fiber cloth reinforcement and reconstruction project in China at that time.

Application of prestressed CFRP plate to strengthen bridge

Project Overview

The project is located on Huancheng Road in Taiyuan City, with an overpass span of 726 m. The net width of the bridge deck is 16.2 m, and the superstructure is prestressed box girder. The bridge was opened to traffic in the 1990s.

After a recent bridge inspection, it was found that the bearing capacity reserve of the bridge has declined seriously, which has begun to threaten the normal service of the bridge. It is determined by the government as an emergency rescue project, and must be dealt with immediately, and at the same time, it requires that the bridge deck be open to traffic during the reinforcement period. 

Therefore, it is planned to use prestressed carbon fiber plate reinforcement technology to complete the structural reinforcement.

Process Principle

(1) By applying prestress to carbon fiber plate, it can maximize the high tensile strength of carbon fiber material. The principle is similar to that of prestressed beam structure. Due to the prestress applied in advance, the material has a certain initial strain. After the second stress, it can improve the coordinated deformation ability of carbon fiber materials and concrete materials to improve the quality and effect of reinforcement.

(2) By applying prestress, decompression bending moment can be generated, which can offset a large part of the early load, improve the bearing capacity of the structure in the later period, significantly improve the crack resistance of the structure, and limit the appearance of new cracks, improve the overall rigidity and strength of the member, reduce the deformation of the bearing deflection.

(3) The deformation of the carbon fiber plate comes from two aspects, one is the deformation caused by the prestress, and the other is the deformation under the external load. The shearing deformation of the adhesive that shares the two parts of the deformation is distributed in the members. The two ends and mid-span positions of the adhesive can evenly distribute the shear deformation of the adhesive and prevent the brittle damage of the adhesive.

(4) For the proposed reinforcement structure, according to the mechanical performance and installation space of the engineering structure, the reinforcement position must be reasonably arranged, and the carbon fiber board coated with special epoxy glue should be used to pre-square the tension, repair the deformation and closed cracks of the component, and then paste the carbon fiber plate to the part of the bottom plate of the component to maximize the load-bearing capacity of the structure.

Bridge reinforcement materials

HM prestressed CFRP plate

HM-120cp carbon fiber plate adhesive

HM-200 chemical anchor bolt

Construction process

Scheme design-site construction preparation-beam bottom repairing and grinding-measurement lofting-drilling and planting reinforcement-anchorage installation-installation of carbon fiber board-stretched carbon fiber board-carbon fiber board coating-carbon board brush coating protection layer, anchoring at both ends of the anchor zone .

Conclusion 

The use of prestressed carbon fiber plate reinforcement technology can be widely applied to many reasons, such as damaged concrete structure, corroded steel bar, load lifting, standard improvement, improper design and construction, changes in use function, etc. In order to extend and improve the service life and capacity of the bridge structure as much as possible, improve the bearing capacity of the structure, and enable the bridge structure to exert higher service requirements, the above bridges must be reinforced and reinforced as necessary. Considering that the use of prestressed carbon fiber plate reinforcement technology has the advantages of simple construction and low technical threshold, only the construction personnel need to have a certain prestress application technology. In addition, the entire construction process does not require the support of large mechanical equipment, and can be completed using only various types of small equipment, such as electric drills, rhinestones and jacks. During the bridge reinforcement construction, since there is no need to close the traffic, it will not affect the normal passage of the bridge deck and will not cause congestion pressure on the local traffic. This article takes case analysis as the main method and summarizes the relevant technologies on the reinforcement of prestressed carbon fiber plate. It has been proved by practice that the use of prestressed carbon fiber reinforcement method at the bottom of the beam can significantly improve the bearing capacity of the bridge, and the construction process has a high cost performance, which is worth further promotion .

Applying structural strengthening technology in buildings

Problems of structural strengthening technology in building application

Relevant staff have insufficient knowledge of structural strengthening technology

In the use of house construction, due to the influence of physical factors, human factors and natural factors, it will shorten the service life of the house and affect the normal use of the house. The house provides people with a place to live, with strong integrity, and the stability of all structural components plays a vital role in the overall quality of the house. In the course of daily use of the house, if the structure of a certain place is damaged, it is very likely that a joint effect will occur, causing damage to other parts of the house, affecting the normal use of the house, and threatening the safety of residents’ lives and property. In the structural design of house buildings, adopting scientific design concepts can avoid the occurrence of such phenomena. During the construction of house buildings, if the construction personnel strengthens the emphasis on structural strengthening technology to ensure the overall quality of the construction, it will improve the quality of the house building. Overall quality. At this stage, many construction workers in my country have not yet realized the importance of structural strengthening technology, which has led to quality problems in the subsequent use of housing construction, resulting in serious safety accidents.

Seismic structure design of houses is unscientific

my country has a vast territory and ranks third in the world in terms of land area. Its geological structure is relatively complex. Some provinces are located in areas where geological disasters occur frequently, and earthquakes and other geological disasters often occur. If an earthquake occurs, it will seriously threaten people's lives and property and cause serious losses to society. In the process of designing housing construction, scientific design methods are adopted to strengthen the emphasis on building structure strengthening technology, improve the building's ability to resist earthquakes and other geological disasters, and reduce the losses caused by earthquakes. When designing the seismic structure of a house, scientific design measures can effectively improve the stability and safety of the building. Even if the earthquake really comes, it can effectively resist the damage caused by the earthquake and create more time for people to escape. At this stage, in the development process of my country's construction industry, the seismic design of buildings designed by designers is not very reasonable and scientific, and the seismic performance of buildings cannot yet reach the ideal standard. When the earthquake really came, due to the unscientific strengthening technology of the building structure, a large area of the building was damaged, threatening the lives and property safety of the residents and restricting the development and progress of the construction industry.

Construction personnel pay less attention to structural durability

In the process of housing construction, human factors can easily cause damage to the building. As the use time of the building increases, the building is prone to aging. The damage caused by the outside world to the building body gradually reduced the quality of the house building. Once the skeleton inside the building is destroyed, it will lead to the destruction of the entire building. Therefore, in the process of construction, designers must pay attention to structural strengthening technology.

Significance of applying structural strengthening technology in buildings

Improve the seismic performance of house construction

Based on the actual situation in China, China has a vast territory and a very complicated geological structure. Many places are located in areas with frequent geological disasters. These areas often have small-scale earthquake disasters, although they will not have a serious impact on people’s lives and property safety. But it will still bring some economic losses.

In traditional structural design of houses, advanced structural strengthening technology is generally not used, resulting in very poor seismic performance of houses. Even if a small-scale geological disaster occurs, the house will be severely damaged, forcing people to invest huge financial resources to renovate the house, causing serious economic losses. In the event of a severe earthquake disaster, houses built using traditional design methods are damaged very quickly, leaving people with little reaction time, which will magnify the losses caused by the earthquake indefinitely. On the contrary, if a scientific design technique is adopted and the building structure strengthening technique is used, the seismic performance of the house is greatly improved. For the less harmful earthquakes, the reinforced houses can successfully resist the earthquake threat, and for the more harmful earthquakes, they can buy more time for people’s escape and provide security for their normal lives.

Extend the life of the house

Due to the vast territory of my country, the climate varies greatly from place to place. The geological structure and climate of different regions are very different. Because of different natural conditions, the service life of houses and buildings is also different. In areas where the natural environment is relatively harsh, due to the impact of natural disasters such as earthquakes and some wind and rain corrosion, the service life of house buildings is very short. Once there is a problem with the building structure of the house, there will be associated side effects, and other places will also be affected to varying degrees, affecting the normal use of the house and threatening people's lives and property safety. In general, under such circumstances, repairing a house will cost a lot of money, and the repair results obtained are not very satisfactory, greatly reducing the overall robustness of the building. Adopt scientific strengthening technology to improve the overall stability of the house structure and improve the safety and stability of the building. Greatly improve the service life of the house building, improve the house's ability to respond to risks, save house maintenance and management costs, and achieve rapid development and progress in the construction industry.

New scientific strengthening technology: carbon fiber strengthening

• Easy installation

• Corrosion resistance

• Short construction period

• No maintenance required

• Light weight, no influence to original structure

• Low cost, cost effective compared with other methods

Carbon fiber reinforcement technology

Carbon fiber reinforcement technology as a new and efficient reinforcement technology. By sticking carbon fiber material on the surface of the member to make it firmly combined with concrete, it can improve the mechanical performance, achieve the purpose of strengthening the structure, improving the bearing capacity and seismic resistance.

Carbon fiber reinforcement technology is suitable for the reinforcement and repair of various structural types and various structural parts. Structures such as beams, slabs, columns, roof trusses, piers, bridges, cylinders, shells, etc., require that the strength of the base concrete is not lower than C15. In addition, some mechanical properties of brick masonry can also be reinforced with carbon fiber.

The working principle of carbon fiber reinforcement technology is to make carbon fiber into carbon fiber cloth. Combined with the relevant factors of the damage to the building components, a certain method is used to attach it to the surface of the building component, so as to spread the force and achieve the reinforcement effect.

First of all, carbon fiber cloth and bonding materials need to be used in carbon fiber reinforcement technology. Due to their different roles and characteristics, they need to form a perfect whole during the manufacturing process, which can enhance its stability and effective To avoid the breaking problem of single fiber under strong action.

Secondly, the carbon fiber cloth has high initial strength, strong load pulling force, and little change due to external force. The adhesive material is easily affected by external forces in the force transmission. In order to avoid the peeling of carbon fiber and building components under external force, Adhesive materials require very high initial properties and strength.

It is a technological innovation, because the traditional reinforcement method is often passively stressed, and the prestressed carbon fiber board reinforcement technology is an active force, the prestress applied by the carbon fiber board through the tensile anchoring end and the carbon sheet rubber work together and It is evenly and effectively transmitted to the beam body, which improves the bearing capacity of the component, and at the same time reduces deflection and deformation, or closes the cracks of the component, thereby improving the rigidity of the component and improving the performance at the stage of use.

CFRP composite strengthening bridge

General situation of Engineering

The maintenance renovation of the irrigation canal bridge of the North Jiangsu Expressway has been the first maintenance since the bridge was opened to traffic in December 8, 2000. In order to ensure the normal, safe and smooth transportation of this important north-south highway, the bridge maintenance repair project adopts non closed construction method with a construction period of 1 months.

Reinforcement time

September 2014

Reinforcement method

Strengthening method by bonding carbon fiber cloth

Material selection

HM carbon fiber cloth, HM carbon fiber glue

Application of strengthening beam bridge with carbon fiber plates

Bridge Overview 

A bridge was built in 1998. The original design load is automobile-over 20 tons, trailer -120 tons, simple support structure. The standard span of the bridge superstructure is 20m and 25m, the beam height is 0.9m and 1.0m respectively, with a total of 78 holes, and the total length of the bridge is 1951.6m. The whole bridge is divided into two, the two bridges have the same layout, and the total bridge has a total of 982 hollow slabs. The width of the middle plate is 1.24m and the width of the side plate is 1.64m. The post-tensioned partial prestressed concrete members of Class A are originally designed with concrete strength grade 40, the standard strength of prestressed steel strands is 1860 MPa, and the ordinary steel bars are HRB335 steel bars.

Bridge disease

Inspection of 982 post-tensioned prestressed hollow slabs of the full bridge revealed that there were transverse cracks at the bottom of 138 hollow slabs (see Figure 1), accounting for 14.05% of the total number of hollow slabs. The cracks are distributed within 5 m from the mid-span cross section, and are mostly located at the location of the transverse stirrups of the bottom plate, with a spacing of 0.2-3 m and a crack width of 0.02-0.22 mm. Among them, the cracks with a width of 0.02-0.10 mm account for more than 90% of the total, the crack depth is 20 mm-34 mm (the thickness of the protective layer is 35 mm), the cracks extend along the beam height to the web 0.4 y 0.6 m, and some cracks penetrate the bottom plate to form a U shape crack.

According to the inspection results of cracks, the width of the cracks has not met the requirements of the regulations, the actual bearing capacity of the prestressed hollow slab structure has significantly decreased compared with the design bearing capacity, so the prestressed slab beam should be replaced or repaired and strengthened

Bridge strengthening

Consider factors such as construction period, construction cost, the impact of structural diseases, material degradation, and whether new and old materials can be effectively bonded and coordinated after reinforcement. After comparison and selection of the scheme, 15 slab beams with dense cracks at the bottom of the beam and severe deflection were replaced with the main beam. The main beams with transverse cracks at the bottom of the remaining beams are reinforced with non-prestressed carbon fiber plates to improve the bearing capacity of the structure.

Paste a layer of 100mm wide and 1.4mm thick carbon fiber plate to the bottom of the beam.

Strengthening beam bridge with carbon fiber plates

After the carbon fiber plate is used to reinforce the slab beam, the deformation and strain of the bridge meet the design load standard requirements, and the flexural bearing capacity of the bridge is improved after the reinforcement. Since the bridge was reinforced and opened to traffic two years ago, no new disease has appeared on the bridge, and cracks at the bottom of the beam have not expanded. Therefore, the reinforcement method of pasting carbon fiber sheet has a good inhibitory effect on the development of existing cracks and the generation of new cracks. On the basis of reasonable economy and simple construction, the use of carbon fiber sheet to strengthen the bridge can meet the operational requirements, and the bridge is in safe use.