Bridge reinforcement should not only meet the design specifications, conform to the principles of technical feasibility, economic rationality and structural safety, but also go through certain procedures and steps, which requires the concrete inspection of the bridge before reinforcement, and then the concrete reinforcement scheme design. In the design of concrete reinforcement, the principle of "durability, economy and safety" should be defined first.
1) For bridges and extra-large bridges, when the main load-bearing members need to be reinforced, the reinforcement design schemes should be diversified, and the schemes should be compared and evaluated economically to choose the best reinforcement scheme to achieve the best effect.
2) The reinforcement design should be closely combined 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, the interference to passing vehicles and pedestrians on and under the bridge should be reduced as much as possible, and necessary measures should be taken to reduce the pollution to the surrounding environment;
4) The design and construction of reinforcement shall not destroy the original structure as far as possible, and the valuable components shall be reserved to avoid unnecessary removal or replacement;
5) During steel bar 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, so it is very important to choose a reasonable reinforcement scheme, usually considering the following factors:
1) bridge structure type;
2) Topography, hydrology and natural conditions of the bridge site;
3) Analysis of the present situation of bridges and research conclusions;
4) Construction technical level;
5) Can traffic be closed?
6) Expected reinforcement effect;
7) Capital investment.
Common methods of bridge reinforcement
Common reinforcement methods of bridge superstructure
External prestressing reinforcement method:
The reinforcement principle of external prestress method is to set prestressed materials in the tension area of the lower edge of the beam, and eccentrically prestress the beam through tension. Under this eccentric pressure, the beam arches, which offsets part of the dead weight stress, reduces the structural deformation and crack width, improves the structural stress, and can greatly improve the bearing capacity of the structure. Compared with the usual prestressed concrete structure, the prestressed tendons and the original structure are only connected to the beam at the anchorage point, which is similar to the unbonded prestressed structure. This method can greatly improve and adjust the state of the original structure, and improve the stiffness and crack resistance of the structure under the condition of little increase in self-weight. This method is not only suitable for temporary reinforcement when passing through heavy vehicles, but also can be used as a permanent reinforcement measure to improve the bearing capacity of bridges.
The main applications of this method are as follows: when the prestressed steel bars or ordinary steel bars in concrete beams are seriously corroded, the bearing capacity of the structure is reduced; Need to improve the load grade of the bridge; Used to control the crack of beam and the fatigue stress amplitude of steel bar; It is suitable for structures under high stress, especially for the reinforcement of large structures.
At present, the commonly used external prestressing methods are: post-tensioned prestressed tie rod reinforcement method and external prestressed steel strand reinforcement method.
System transformation strengthening method
Strengthening the old bridge by changing the structural system usually means adding additional members or carrying out technical transformation to change the stress system and stress state of the bridge, thus reducing the stress of the load-bearing members, improving the performance of the bridge and improving the bearing capacity. This technology has the advantages of improving the bearing capacity of the structure, increasing the stiffness of the structure and reducing the deflection.
Strengthening method of adding components
The strengthening method of adding members mainly refers to increasing longitudinal beams to improve the bearing capacity or widening the transformation, and increasing diaphragm to strengthen the transverse connection. When the pier foundation has good safety performance and bearing capacity, and the superstructure is basically intact, but its bearing capacity can not meet the requirements, and the bridge deck needs to be widened, the new longitudinal beam with higher bearing capacity and stiffness is generally used to connect the old and new beams and bear the same force. If it needs to be widened, the pier needs to be widened.
Common methods can be divided into: adding longitudinal beams to strengthen (without widening the bridge deck); Adding side beams for reinforcement; Technical transformation of unilateral widening; Bilateral widening of technological transformation; Add auxiliary beam reinforcement.
Strengthening method by sticking steel plate
Bonding steel plate reinforcement is a kind of reinforcement method which uses adhesive to bond steel plate to the tensile edge or weak part of reinforced concrete structure to form a whole with the structure, thus improving the bearing capacity of the beam. If the steel plate is anchored to the beam with anchor bolts, it is also called anchor bolt steel plate method, and then the steel plate can be appropriately thickened. Fixing steel plates on the surface of tensile concrete can increase the flexural rigidity of concrete structure, reduce the deflection of the structure and limit the development of cracks. Moreover, the steel plate can be cut according to the design requirements during construction, so that the bending, compression and shear properties of the bonded steel members can be effectively exerted, and the stress is uniform, and the phenomenon of stress concentration will not occur in concrete. In addition, this method also has the advantages of simple and rapid construction, no influence on the structural shape, low reinforcement cost, no reduction in bridge clearance and little increase in load. The disadvantage is that the quality and durability of binder are the main factors affecting the reinforcement effect.
Carbon fiber reinforcement method
Sticking carbon fiber reinforcement technology refers to sticking carbon fiber cloth on the surface of building structure with high performance adhesive. When the structural load increases, the two work together to improve the bearing capacity of the components, thus achieving the purpose of reinforcement. The mechanical property of fiber composite is that its stress and strain are completely linear, 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 the advantages of fast construction speed, short construction period and easy guarantee of bonding quality, it is an ideal material for strengthening old bridges. The performance of bonding materials in carbon fiber reinforcement method is the key to ensure that carbon fiber and concrete work together, and it is also the weak link in the way of force transmission between them. Therefore, the bonding material should have enough rigidity and strength to ensure the transmission of shear force between carbon fiber and concrete, and at the same time, it should have enough toughness to prevent brittle bonding failure caused by concrete cracking. Compared with other strengthening methods, strengthening the old bridge with carbon fiber can change the stress distribution of the original structure to the minimum extent and ensure the same stress as the original structure within the design load range.
Bridge deck reinforcement method
Bridge deck reinforcement method is a common and effective method to increase the effective height and compression section of the main beam, increase the overall stiffness of the bridge deck and improve the bearing capacity of the bridge by laying a layer of reinforced concrete on the top of the beam (bridge deck) to make it form a whole with the original main beam. In order to reduce the dead load of the reinforcement layer, the original bridge deck pavement is often chiseled off, and the old and the new can be well combined and stressed together.
Common reinforcement methods for bridge substructure reinforcement
Widening and strengthening method of pier and abutment
This method is suitable for the case that the bearing capacity of the foundation is insufficient or the buried depth is too shallow, and the pier is a rigid solid foundation of masonry or concrete. Enlarging the foundation area should be determined by checking the foundation strength.
Supplementary pile foundation reinforcement method
When there is a soft cushion under the pier foundation, the pier settlement occurs; It is a common and effective method to adopt supplementary pile foundation reinforcement method. This method is to add bored piles around the pile foundation to improve the bearing capacity and stability of the foundation.
Strengthening method of reinforced concrete hoop or sheath
Reinforced concrete hoop or sleeve reinforcement method is a method that causes cracks in piers and abutments and sometimes through cracks due to insufficient foundation burial depth or lax construction quality control. Reinforced concrete hoop or steel hoop can be used for reinforcement.
Strengthening method of new auxiliary retaining wall of abutment
Because the horizontal earth pressure on the abutment back is too large, the abutment is inclined, so a retaining wall should be built behind the abutment to resist the excessive earth pressure.
Enlarged foundation reinforcement method
Using the old bridge foundation, the cantilever widening part is picked out by the pier cap beam for the installation of the superstructure. In this case, only the pier is widened, and there is no need to reinforce the upper capping beam, pier body and foundation.
Various commonly used reinforcement methods can be used neutrally to optimize the combination, which can better reflect the reinforcement effect and economic benefits, but the following points should also be noted:
1) Different reinforcement methods have corresponding design and calculation methods;
2) The improvement of the bearing capacity of the reinforced bridge structure is limited by the original structure, such as the reinforcement ratio and section size of the original structure, and it is impossible to improve the bearing capacity indefinitely;
3) For the reinforcement calculation of long-span complex bridge structures, it is generally necessary to analyze the structure as a whole. The effective tool is finite element method, and the nonlinear influence should be considered when necessary.
4) When widening and strengthening are carried out at the same time, the widened part should be integrated with the original bridge to give full play to the unloading function of the new part.
Characteristics and advantages of carbon fiber
Strengthening and repairing concrete structures with carbon fiber is a new technology. It uses impregnated resin to stick carbon fiber cloth on the damaged concrete component along the stress direction or perpendicular to the stress direction, so that it can work together with the original steel bar of the component, thus realizing the reinforcement of the concrete component. The reinforcement can improve the tensile and shear capacity of the structure, effectively improve its strength, stiffness, crack resistance and ductility, and control the continuous development of cracks and deflection. Carbon fiber composites include carbon fiber cloth and adhesive materials, which are briefly introduced as follows: 4. 1 carbon fiber cloth has the following characteristics.
1) high strength;
2) the strength variation between individual fibers is small;
3) High elastic membrane provides effective reinforcement;
4) The strength is stable and remains unchanged during manufacturing and handling.
5) It has good chemical properties and is not easy to react with the outside world, causing changes in the original properties.
The adhesive material has the following characteristics
The performance of bonding material is the key to ensure the cooperative work between carbon fiber and concrete, and it is also the weak link in the process of stress between carbon fiber and concrete. There should be enough rigidity and strength to ensure the transmission of shear force between carbon fiber and concrete, and at the same time, enough toughness should be applied to prevent brittle bond failure due to concrete cracking. In addition, it should adapt to the site construction conditions, that is, it can be cured under general conditions, with appropriate fluidity and viscosity and small curing shrinkage. Adhesive materials mainly include primer, leveling material and impregnated resin, and their functions are as follows:
Primer: Immerse in the surface of concrete to enhance the surface strength of concrete, thus improving the adhesion between concrete and carbon fiber cloth;
Leveling material: carbon fiber cloth is likely to be damaged or hollowed out due to sharp protrusions, dislocations and corners on the concrete surface, thus reducing its strength. On the one hand, leveling material can be used to fill holes or inclined planes caused by surface deterioration or surface treatment of concrete, and on the other hand, it can be used to polish the 90o angle of rectangle to make it arc-shaped.
Impregnating resin: combine the continuous bonded carbon fiber cloth to make it plate-like, so that the fibers can be combined with each other, evenly resist external forces and give full play to the overall strength of the fibers. At the same time, carbon fiber cloth and concrete are bonded together to form a composite whole and resist external forces.
Characteristics and advantages of carbon fiber reinforcement technology
Excellent mechanical properties can be effectively used in various forms of structural reinforcement, including bending, shearing, tensile, fatigue, earthquake and wind resistance, as well as controlling crack propagation and deflection.
Excellent chemical stability. Carbon fiber cloth has strong acid resistance, alkali resistance, salt resistance, ultraviolet resistance and waterproof performance. Have enough ability to adapt to temperature changes. It can effectively prevent fire after adding fire layer. Therefore, it can greatly enhance the adaptability of the structure to the harsh external environment and prolong the service life of the structure.
The material itself is light in weight and high in strength. It will not increase the structure volume and change the structure appearance. The increased structural weight can be ignored, which is convenient for painting the required color and will not leave any reinforcement marks.
The construction process is simple and can be operated by small electric tools. Less jobs, less labor, short construction period and fast progress. More data show that the construction can be carried out under the condition of continuous traffic vibration without affecting the reinforcement effect, which greatly shortens the construction interruption time and has great economic and social benefits.
Common cracks in bridges and their treatment methods
Because bridge crack is a very common bridge disease, its potential danger is relatively large, so it should be dealt with in time.
According to the formation reasons, cracks can be divided into
The first type: cracks caused by external load are called structural cracks (also called stress cracks), and their distribution and width are related to external load. This kind of crack shows that the bearing capacity of the structure may be insufficient or there are other serious problems.
The second kind: cracks caused by deformation are called non-structural cracks. When the structural deformation caused by temperature change, concrete shrinkage and other factors is limited, self-stress will occur inside the structure. When this stress reaches the ultimate tensile strength of concrete, it will cause concrete cracks. Once cracks appear, the deformation will be released and the self-stress will disappear.
There are obvious differences between the two kinds of cracks, and their harmful effects are also different. Sometimes, these two types of cracks will merge together. The survey data show that about 80% of the two types of cracks are caused by deformation; Cracks caused by load account for about 20%. Cause analysis of cracks is the basis of damage assessment, repair and reinforcement of cracks. If the cracks are treated blindly without analysis and research, not only the expected effect will not be achieved, but also the danger of unexpected accidents may be hidden.
Embolization grouting
Plug joint grouting is to inject a certain proportion of cement (sand) slurry and epoxy resin (sand) slurry into the cracks of the structure through a syringe under a certain pressure, which plays a role in filling the cracks, avoiding the corrosion of steel bars and improving the overall strength of the structure. Cracks are common phenomena in bridge diseases, and the causes of cracks are many and complicated. Once cracks appear in the structure, the stress on its stress section is redistributed, which means that the effective stress section becomes smaller, the structural stress increases and the bearing capacity decreases. Plug grouting is to fill the cracks in the structure with cementing materials, so that the function and transmission of force can be restored as much as possible.
Plug grouting is generally used to deal with cracks in the upper and lower structures of bridges. Grouting can be divided into cement slurry, cement mortar, epoxy resin slurry, epoxy resin and mortar. Which one should be decided according to the actual situation. Usually, cement (sand) slurry is used for cracks in stone piers, abutments and arch rings, and whether sand is mixed in grouting depends on the size of cracks. The use of cement (sand) slurry has low cost and good effect. Epoxy resin slurry is generally used in reinforced concrete structures, because reinforced concrete members have small cracks, easy filling and good cohesiveness; Epoxy mortar is mostly used for bridge deck cracks.
Use 1: 1 cement mortar for jointing first. When jointing, a grouting hole with a diameter of about 6-8 mm should be reserved. The spacing of holes depends on the width of cracks. The hole spacing at the crack width is 0.3-0.4 m, and the hole spacing at a small part of the crack is 0.4-0.5 m. After the jointing mortar reaches a certain strength, grouting can be carried out. Reinforced concrete beams have small cracks, so they are jointing with epoxy resin. Cracks larger than 0. 1mm should be grouted, and the hole spacing is generally 0.4-0.5m m, and the grouting method is roughly the same as grouting mud. Plug grouting is a widely used comprehensive treatment method in the reinforcement of old highway bridges. Through fitting and observation, the effect is good.
Concluding remarks
Therefore, the reinforcement and maintenance of the bridge is very important, which is one of the technical measures to ensure the smooth road. Save a lot of manpower, material resources, financial resources and so on. How to make good use of bridges is an urgent problem for us to solve now.
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