In recent years, with the rapid development of China's economy, transportation infrastructure construction has achieved leap-forward extraordinary development in scale and construction standards. As a part of traffic trunk line, with the development of national economy and the increase of highway traffic volume, the bridge load presents the following characteristics: (1) the traffic volume is increasing; Heavy vehicle and serious overload; The increase of over-limit transportation. Due to the limited bearing capacity of bridges, many bridges will be damaged by cracks, subsidence and so on because they are overwhelmed. If the new bridge is rebuilt after removing the working bridge or replacing all the main beams of the working bridge, and a new beam with high load level is erected, the construction will be time-consuming, laborious and costly. Therefore, the reinforcement and maintenance of bridges have obvious practical value.
1. Work contents and steps of bridge reinforcement
Bridge reinforcement is a measure to improve the bearing capacity or passing capacity of some or all bridges by expanding or repairing bridge members. The main contents of bridge reinforcement and reconstruction work include: strengthening the upper components of existing bridges; Reinforce the substructure of the working bridge; Widening the carriageway or sidewalk of the bridge; Raise the height of the bridge structure; Replace the pavement structure of bridge traffic or approach bridge. Bridge reinforcement and reconstruction should make full use of the original components, and those that can be reinforced shall not be rebuilt. The working procedure of bridge reinforcement is generally as follows: check the present situation and damage of the bridge; Investigate the historical and technical data of the bridge and the existing traffic conditions; Put forward maintenance, reinforcement or reconstruction schemes and make analysis and comparison; Determine the scheme and put it into practice, that is, carry out maintenance, reinforcement or construction.
2. Common methods of bridge reinforcement
The discussion of bridge reinforcement technology in China started late, but with the rapid development of infrastructure construction in China, the reinforcement theory is becoming more and more perfect. Based on the full application of mechanical effects and new material characteristics, an effective bridge reinforcement method has been formed.
2. 1 Increase the reinforcement method of concrete section
There are two ways to increase the concrete section: thickening the bridge deck and increasing the height and width of the main girder ribs. Thickening the bridge deck means laying a layer of reinforced concrete on the top of the beam. Generally, the old bridge deck is chiseled away first, so that the strengthening layer and the original main beam form a whole, the effective height of the main beam is increased, the compressive section strength is improved, and the lateral load distribution ability of the bridge is improved, thus achieving the purpose of improving the bearing capacity of the bridge. Increasing the height and width of the main girder ribs, that is, when the strength, rigidity, stability and crack resistance of the beam are insufficient, usually adopt reinforcement methods, such as increasing the cross section of members and improving the reinforcement ratio, that is, increasing the size of the bottom or side of the beam, increasing the main reinforcement, improving the effective height and bending strength of the beam, thus improving the bearing capacity of the beam.
2.2 Anchor Shotcrete Reinforcement Method
With the help of high-speed spraying machinery, new concrete mixture is continuously sprayed on the spraying surface of anchoring steel mesh, which solidifies and hardens to form reinforced concrete, thus increasing the stress section of the bridge, increasing the steel bars, enhancing the integrity of the structure and enabling it to bear greater external loads. The reinforcement technology of anchor-shotcrete has high mechanical strength and good durability. At present, sprayed synthetic fiber concrete is popular, that is, sprayed concrete is mixed with synthetic fibers with three-dimensional distribution to improve the performance of concrete.
2.3 Bonding steel plate reinforcement method
When the traffic volume increases, the bearing capacity of the main girder is insufficient, or the longitudinal main reinforcement is seriously corroded, serious transverse cracks will appear in the main girder of the beam slab bridge. Using adhesive and anchor bolts, steel plates are stuck and anchored at the tensile edge or weak part of concrete structure, forming a whole with the structure, thus improving the flexural and shear capacity of members, improving the stiffness of the original structure, limiting the development of cracks, improving the stress state of steel bars and concrete, and achieving the purpose of improving the bearing capacity of beams. The characteristics of this reinforcement method are: (1) it is not necessary to change the size of the original structure after reinforcement; The construction technology is simple and the construction quality is easy to control.
2.4 Change the reinforcement method of structural system
By changing the stress system of bridge structure, the bearing capacity of bridge can be improved. Brackets or piers are added under simply supported beams, or simply supported beams and simply supported beams are connected longitudinally to make them continuous beams, or stiffening beams such as steel trusses or composite beams are added under beams to reduce the internal stress of beams and improve the bearing capacity of bridges. There are many ways to change the structural system, but it is often necessary to operate or set up permanent facilities under the bridge to reduce the clearance under the bridge. When strengthening with temporary support, the stress system of the original simply supported beam is changed, and bending moment will occur at the fulcrum, so stress calculation must be carried out.
2.5 Bonding carbon fiber reinforcement method
Bonding carbon fiber reinforcement technology is a new type of bridge reinforcement and reconstruction technology. Carbon fiber is a high-tech material with excellent mechanical properties. It is a scientific and technological product manufactured at a high temperature of several thousand degrees through a special process. Carbon fiber is bonded to the tensile edge or weak part of concrete structure with epoxy resin glue to form a whole with the structure, and carbon fiber cloth is used instead of steel bar to improve the bearing capacity of beam. The construction technology of carbon fiber reinforcement method is simple, carbon material has excellent mechanical properties and chemical stability, and the material itself is light and high strength, which will not increase the structural volume and change the structural shape.
3. Detection and evaluation of bridge reinforcement effect
Bridge reinforcement technology is difficult, which requires higher construction technology. After reinforcement, it is necessary to inspect and observe the bridge to determine the reinforcement effect. Conventional detection methods are static load test and dynamic load test, and the test results are compared with the calculation results of reinforcement design to judge whether the bridge reinforcement is successful or not.
3. 1 bridge static load test.
Static load test is to judge the bearing capacity of the bridge by testing the strain, displacement or crack analysis of the control section of the bridge structure under the test load. The selection of load cases should reflect the most unfavorable stress state of bridge design. At the same time, in order to ensure the safety of loading and understand the relationship between structural strain and displacement with the increase of test load, bridge test loading should be carried out in stages. Loading equipment often adopts feasible vehicles, and the layout of measuring points should not be too much. However, in order to ensure the quality, in general, the layout of main measuring points should be able to control the maximum stress and deflection of the structure.
3.2 Bridge dynamic load test
The dynamic load test tests the response of the bridge under dynamic load, analyzes the dynamic characteristics of the bridge such as frequency, resistance and vibration mode, and evaluates the bearing capacity of the bridge according to the dynamic response and dynamic characteristics. The dynamic characteristics of bridge structure are only related to the inherent characteristics of the structure itself, but have nothing to do with other conditions such as load. Under the actual dynamic load, the stress state of the bridge under the dynamic load is analyzed according to the dynamic responses of each control part of the structure, such as amplitude, frequency, speed and acceleration, and the impact coefficient reflecting the overall dynamic action of the structure.
4. Concluding remarks
Any "new" bridge will eventually become an "old" bridge after years of erosion and use by nature. Bridge reinforcement and maintenance has become a topic of universal concern all over the world. China's new bridge technology has developed rapidly, but the development of bridge maintenance and reinforcement technology is relatively backward. The reinforcement and maintenance of the old bridge will be a permanent technical topic, so we need to create and summarize various practical technologies and methods in the inspection, evaluation, reinforcement and transformation of the old bridge.
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