Generally speaking, bridge reinforcement includes the repair of diseased bridges and the transformation of original bridges. Among them, the repair of diseased bridges is generally due to the aging of the bridges and improper operation and maintenance. Natural causes such as the burning of garbage under the bridges have caused some bridges to become diseased and need to be repaired; the renovation of the original bridges is generally due to the increase in traffic volume and design. The improvement of load standards and the widening of highway subgrade width have made some bridges no longer able to meet the current transportation requirements and require reconstruction.
In addition to meeting the design specifications and complying with the principles of technological feasibility, economic rationality, and structural safety, bridge reinforcement must also go through certain procedures and steps, which requires targeted inspection of the bridge before reinforcement. Test and then make a specific reinforcement plan design. In the specific reinforcement design, the reinforcement principles of “durability, economy, and safety” must first be clarified.
1) For large bridges and extra-large bridges, when the main load-bearing components need to be reinforced, the reinforcement design plans should be diversified, and the plan comparison and economic evaluation should be carried out to select the best reinforcement plan to achieve the best results. Effect.
2) The reinforcement design should be closely integrated with the construction method, and effective measures should be taken to ensure that the new and old structures are connected reliably and work together;
3) During the reinforcement construction, as much as possible Reduce the interference to vehicles and pedestrians on and under the bridge, and take necessary measures to reduce pollution to the surrounding environment;
4) Reinforcement design and construction should not damage the original structure as much as possible, and retain useful features valuable components to avoid unnecessary dismantling or replacement;
5) During the reinforcement construction, safety monitoring measures should be taken to ensure the safety of construction personnel and structures.
2 Selection of reinforcement scheme
The reinforcement scheme is related to many factors. It is very important to choose a reasonable reinforcement scheme. The following factors are often considered:
1) Bridge structure Type;
2) Topography, hydrology, and natural conditions of the bridge location;
3) Analysis and research conclusions on the current status of the bridge;
4) Construction technology level;
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5) Whether traffic can be closed;
6) Expected reinforcement effect;
7) Amount of capital investment.
Common methods of bridge reinforcement
Common methods of reinforcement of bridge superstructure
External prestressed reinforcement method:
External prestressed method The reinforcement principle is to set prestressed materials in the tension zone of the lower edge of the beam, and generate eccentric prestress on the beam body through tension. Under the action of this eccentric pressure, the beam body will arch up, offset part of the self-weight stress, and reduce the The structural deformation and crack width improve the structural stress, which can greatly increase the structural bearing capacity. Compared with the usual prestressed concrete structure, the force bars and the original structure are only connected to the beams at the anchor points, similar to the unbonded prestressed structure. This method can greatly improve and adjust the condition of the original structure and increase the structural stiffness and crack resistance with a small increase in self-weight. This method is suitable for temporary reinforcement when heavy vehicles are passing through, and can also be used as a permanent reinforcement measure to improve the bearing capacity of the bridge.
The main applicable situations of this method are: when the prestressed tendons or ordinary steel bars in concrete beams are severely corroded and other diseases cause the structural bearing capacity to decrease; it is necessary to increase the load level of the bridge; it is used to control cracks in the beam body and steel bars Fatigue stress amplitude; suitable for structures under high stress conditions, especially the reinforcement of large structures.
The currently commonly used external prestressing methods include: the following prestressed tie rod reinforcement method and the external prestressed steel wire bundle reinforcement method.
System conversion and reinforcement method
Changing the structural system to strengthen the old bridge usually means adding additional components or carrying out technical transformation, which changes the stress system and stress conditions of the bridge, thereby causing To reduce the stress of load-bearing components, improve bridge performance, and achieve the purpose of increasing load-bearing capacity. This technology has the advantages of improving the structural bearing capacity, increasing structural stiffness, and reducing deflection.
Add component reinforcement method
Add component reinforcement method mainly refers to adding longitudinal beams to improve load-bearing capacity or widening and reconstruction, and adding diaphragms to strengthen lateral connections.
When the pier foundation has good safety performance and has load-bearing capacity, and the superstructure is basically intact, but its load-bearing capacity cannot meet the requirements, and the bridge deck is required to be widened, new longitudinal beams with high load-bearing capacity and high stiffness are generally used to make the new and old The beams are connected to each other and bear the same force. For those that need to be widened and reconstructed, the piers and abutments also need to be widened.
Commonly used methods can be divided into: adding longitudinal beam reinforcement (without widening the bridge deck) according to the combined stress form of adding components and new and old main beams; adding side beam reinforcement; unilateral widening technical transformation; bilateral widening Technical transformation; add auxiliary beam reinforcement.
Steel plate reinforcement method
Steel plate reinforcement is to use an adhesive to paste the steel plate on the tensile edge or weak part of the reinforced concrete structure to make it integral with the structure. A reinforcement method that improves the load-bearing capacity of beams. If anchor bolts are used to anchor the steel plate to the beam, it is also called the anchor steel plate method. At this time, the steel plate can be appropriately thicker. The steel plate fixed on the surface of the tensioned concrete can increase the bending stiffness of the concrete structure, reduce the deflection of the structure, and limit the development of cracks. And the steel plates can be cut according to the design needs during construction, which can effectively exert the bending resistance, compression resistance and shear resistance of the bonded steel components. The stress will be uniform and no stress concentration will occur in the concrete. In addition, this method It also has the advantages of simple and fast construction, does not affect the structural appearance, has low reinforcement costs, does not reduce the bridge clearance, and does not increase the load much. The disadvantage is that the quality and durability of the adhesive are the main factors affecting the reinforcement effect.
Carbon fiber reinforcement method
Paste carbon fiber reinforcement technology refers to using high-performance adhesive to paste carbon fiber cloth on the surface of the building structure. When the structural load increases, the two will collapse. Work together to improve the bearing capacity of components, thereby achieving the purpose of reinforcement. The mechanical characteristics of fiber composite materials are that their stress and strain are completely linear elastic and there is no yield point or plastic zone. Because carbon fiber has excellent physical and mechanical properties such as high strength, light weight, corrosion resistance, and fatigue resistance, as well as fast construction speed, short construction period, and easy to ensure bonding quality, it is an ideal material for reinforcing old bridges. The performance of the bonding material in the carbon fiber reinforcement method is the key to ensuring that carbon fiber and concrete work together. It is also the weak link in the force transmission path of the two. Therefore, the bonding material should have sufficient stiffness and strength to ensure the shear force between carbon fiber and concrete. transmission, and should have sufficient toughness to prevent brittle bond failure due to concrete cracking. Compared with other reinforcement methods, the use of carbon fiber to strengthen the old bridge can minimally change the stress distribution of the original structure, ensuring that it bears the same stress as the original structure within the design load range.
Bridge deck reinforcement method
The bridge deck reinforcement method is to lay a layer of reinforced concrete layer on the top of the beam (bridge deck) to make it consistent with the original It is a common and effective method to form a whole with the main beam, thereby increasing the effective height and compression section of the main beam, increasing the overall stiffness of the bridge deck, and improving the load-bearing capacity of the bridge. In order to reduce the dead load increased by the reinforcing layer, the original bridge deck pavement layer is often cut away, and the new and old can be well combined and bear the same force.
Commonly used reinforcement methods for reinforcing bridge substructure
Pier abutment widening and reinforcement method
This method is suitable when the foundation bearing capacity is insufficient or the burial depth is too shallow, and the pier This is the case when the platform is a rigid solid foundation of masonry or concrete. The expansion of the foundation area should be determined by foundation strength verification.
Supplementary pile foundation reinforcement method
When there is a soft bed layer under the bridge pier base, the pier abutment will sink; for this purpose, the supplementary pile foundation reinforcement method is a common and effective method. method. This method is to add bored piles around the pile foundation to increase the bearing capacity of the foundation and enhance the stability of the foundation.
Reinforced concrete hoop or sheath reinforcement method
Reinforced concrete hoop or sheath reinforcement method is a method of reinforcing steel and concrete hoops or sheaths due to insufficient foundation burial depth of bridge piers or due to construction quality control. When the pier abutment cracks due to looseness or other reasons, penetration cracks may sometimes appear. Reinforced concrete girdle or steel hoops can be used for reinforcement.
Reinforcement method of new auxiliary retaining wall on bridge abutment
Because the horizontal soil pressure on the back of the abutment is too large, causing the abutment to tilt, a retaining wall should be built behind the abutment. to resist excessive earth pressure.
Expanded foundation reinforcement method
Using the old bridge foundation, the cantilever widened part is protruded from the pier cover beam to facilitate the installation of the superstructure. In this case, only the pier is widened, and the upper cap beam, pier body and foundation do not need to be reinforced.
A variety of common reinforcement methods can be used neutrally and optimized in combination to better reflect the reinforcement effect and economic benefits, but the following points should also be noted:
1) Different Reinforcement methods have corresponding design calculation methods;
2) The improvement in the load-carrying capacity of the reinforced bridge structure is restricted by the original structure, such as the original structure reinforcement ratio, cross-sectional size, etc., and it is impossible to increase it without limit Bearing capacity;
3) For the reinforcement calculation of complex long-span bridge structures, the overall structure analysis is generally required. The effective tool is the finite element method, and nonlinear effects should be considered when necessary.
4) When widening and strengthening are carried out at the same time, it is advisable to connect the widened part with the original bridge as a whole to give full play to the unloading effect of the newly added part.
Characteristics and advantages of carbon fiber
The use of carbon fiber in the reinforcement and repair of concrete structures is a new technology. It uses impregnated resin to place carbon fiber cloth along the direction of stress or perpendicular to the direction of stress. It is pasted on the damaged concrete components to work together with the original steel bars of the components, thereby reinforcing the concrete components. After reinforcement, the tensile and shear resistance of the structure can be increased, its strength, stiffness, crack resistance and ductility can be effectively improved, and the continued development of cracks and deflection can be controlled. Carbon fiber composite materials include carbon fiber cloth and bonding materials. A brief introduction is as follows: 4.1 Carbon fiber cloth has the following characteristics
1) High strength;
2) Small change in strength between individual fibers;
3) High elastic film to provide effective reinforcement;
4) The strength is stable and remains unchanged during manufacturing and processing.
5) It has good chemical properties and is not easy to react with the outside world, causing changes in its original properties.
The bonding material has the following characteristics
The performance of the bonding material is the key to ensuring the perfect cooperation between carbon fiber and concrete. It is also the weak link in the stress-bearing process between carbon fiber and concrete. It should have sufficient stiffness and strength to ensure the transmission of shear force between carbon fiber and concrete, and at the same time apply sufficient toughness to prevent brittle bond failure due to concrete cracking. In addition, it should be adapted to on-site construction conditions, that is, it can be cured under normal conditions, has appropriate fluidity and viscosity, and has a small curing shrinkage rate. Bonding materials mainly include three categories: primer, leveling material and impregnated resin. Their functions are as follows:
Primer: immersed in the surface of the concrete to strengthen the surface strength of the concrete and thereby improve the adhesion between the concrete and the carbon fiber cloth. The connectivity is improved;
Smoothing material: Carbon fiber cloth is likely to be damaged or hollowed out by sharp protrusions, misalignments and corners on the concrete surface, resulting in reduced strength. The leveling material can fill holes or slopes caused by concrete surface deterioration or surface treatment. On the other hand, it can be used to grind 90o corners of rectangles and fill them into arc shapes.
Impregnated resin: The continuously pasted carbon fiber cloth is combined together to harden it into a plate shape, so that the fibers can be combined with each other, uniformly resist external forces, and exert the overall strength of the fiber. At the same time, the carbon fiber cloth and concrete are bonded together to form a composite whole that can simultaneously resist external forces.
Characteristics and advantages of carbon fiber reinforcement technology
Excellent mechanical properties can be effectively used for various forms of structural reinforcement, including bending resistance, shear resistance, tensile resistance, Anti-fatigue, earthquake resistance, wind resistance, control the expansion of cracks and deflection.
Excellent chemical stability. Carbon fiber cloth has strong acid, alkali, salt, ultraviolet and waterproof capabilities. Have sufficient ability to adapt to temperature changes. Adding a fireproof layer can effectively prevent fire. Therefore, the structure's adaptability to harsh external environments can be greatly enhanced and the structural life can be extended.
The material itself is lightweight and strong. It does not increase the structural volume and does not change the structural appearance. The added structural weight is negligible, allowing it to be painted in the desired color without leaving traces of reinforcement.
The construction process is simple and can be operated with small power tools.
There are fewer types of work, less labor, short construction period, and fast progress. More data show that construction can be carried out under continuous traffic vibration without affecting the reinforcement effect, thus greatly shortening the construction interruption time and having greater economic and social benefits.
Common cracks in bridges and their treatment methods
Because bridge cracks are a very common bridge disease and are potentially dangerous, cracks must be dealt with in a timely manner.
Cracks can be divided into categories according to the causes of formation
The first category: cracks caused by external loads, called structural cracks (also called stress cracks). The distribution and width are related to the external load. The occurrence of such cracks indicates that the structural bearing capacity may be insufficient or there may be other serious problems.
The second category: cracks caused by deformation, called non-structural cracks. When the structural deformation caused by factors such as temperature changes and concrete shrinkage is restricted, self-stress will be generated inside the structure. When this stress reaches the ultimate tensile strength of concrete, it will cause concrete cracks. Once cracks appear, the deformation is released and the self-stress disappears.
There are obvious differences between the two types of cracks, and the harmful effects are also different. Sometimes the two types of cracks merge together. Survey data show that among the two types of cracks, cracks caused by deformation account for about 80%; cracks caused by load account for about 20%. Analysis of the causes of cracks is the basis for assessing the hazards of cracks, repairing and strengthening cracks. If cracks are dealt with blindly without analysis and research, not only will the expected results not be achieved, but the risk of sudden accidents may also be hidden.
Joint grouting
Joint grouting is to pour a certain proportion of cement (sand) slurry and epoxy resin (sand) slurry into the structure through a syringe at a certain pressure. In the gaps, it plays the role of filling cracks, preventing corrosion of steel bars and improving the overall strength of the structure. Cracks are common in bridge diseases, and the causes of cracks are many and complex. Once a crack occurs in a structure, stress redistribution occurs in its stressed section, which means that the effective stressed section becomes smaller, the structural stress increases, and the load-bearing capacity is reduced. Joint grouting is to fill the cracks in the structure with cementing materials, so that the action and transmission of force can be restored to the original state as much as possible.
Joint grouting is generally used to treat cracks in the upper and lower structures of bridges. Grouting is divided into cement slurry, cement mortar, epoxy resin slurry, epoxy resin, mortar, etc. Which one is used depends on It depends on the actual situation. Usually cement (sand) slurry is used for cracks in stone piers, platforms and arch rings. The size of the cracks determines whether to add sand to the grouting. The use of cement (sand) slurry has low cost and good effect. Epoxy resin mortar is generally used in reinforced concrete structures because the cracks produced by reinforced concrete components are smaller, easier to fill, and have good adhesion; epoxy resin mortar is mostly used for cracks in bridge decks.
First use 1:1 cement mortar to joint. When jointing, grouting holes with a diameter of about 6-8mm must be reserved. The hole spacing depends on the width of the crack. The hole spacing at the joint width is 0.3-0.4 m. , the hole spacing at the small joint is 0.4-0.5m. Grouting can be done after the jointing mortar reaches a certain strength. The cracks in reinforced concrete beams are small and should be jointed with epoxy resin. Any cracks larger than 0.1mm must leave holes for grouting. The hole spacing is generally 0.4 to 0.5m. The grouting method is roughly the same as grouting. In the reinforcement of old highway bridges, joint grouting is one of the comprehensive treatment methods. It is commonly used. Through test loading and use observation, the effect is good.
Conclusion
Therefore, the reinforcement and maintenance of bridges are essential and are one of the technical measures to ensure smooth roads. It saves a lot of manpower, material, financial, resources, etc. How to properly utilize bridges is a problem we urgently need to solve now.
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