1, Introduction
There are more or less uncontrollable, insurmountable or easily overlooked unfavorable factors in the construction process of the bridge itself. After its completion, it is constantly impacted by vehicle load, eroded by external forces such as wind, rain and water flow, affected by temperature and humidity changes for a long time, and even severely damaged by earthquakes, making the contradiction between its carrying capacity and traffic demand increasingly prominent. Therefore, on the basis of ensuring the safety performance of the bridge, we should fully consider the economy, science and rationality of the engineering construction, and reinforce and repair the related defects and hidden dangers of the existing bridge. This paper summarizes the structural defects of the bridge, briefly analyzes the bridge reinforcement and maintenance technology, and introduces and shows the bridge reinforcement and maintenance work through examples.
2, the purpose and basic principles of bridge reinforcement and maintenance
The fundamental purpose of bridge reinforcement and maintenance is to eliminate and reduce the safety defects and quality hidden dangers of the bridge and ensure the normal traffic function of the bridge. Therefore, its basic treatment principles should be based on reality, eliminating hidden dangers, comprehensive comparison, coordinating stress, preventing damage and orderly implementation.
3. Common technical schemes and treatment methods for bridge reinforcement and maintenance.
The commonly used technical schemes for reinforcement and reconstruction include: reducing the dead load of bridges, strengthening key components, replacing newer components, and improving the original structural stress system. It also plays an important role in increasing the capacity of the bridge to bear live load, maintaining the healthy operation of the bridge system, improving the service performance of the original bridge and prolonging the service life of the existing structure. Common treatment methods are as follows:
3. 1 Strengthening method by enlarging the section of members
The reinforcement method of increasing the section of members, also known as the reinforcement method of wrapping concrete, is to improve the strength, stiffness, stability and crack resistance of members by increasing the section and reinforcement of members, and can also be used to repair cracks, which is suitable for beam bridges and arch bridges.
3.2 Bonding (steel plate, carbon fiber, nylon fiber) reinforcement method
Bonding reinforcement method is reinforcement bonding method. Generally, epoxy resin or building structural adhesive is used to bond materials with high tensile strength, such as steel plate, steel bar or glass fiber reinforced plastic, to the tensile edge or weak part of reinforced concrete structure, so that it can bear the same force as the original structure, so as to improve its stiffness, improve the stress state of steel bar and concrete in the original structure, and limit the further development of cracks, thus achieving the purpose of strengthening and improving the bearing capacity of the bridge.
3.3 external prestressing reinforcement method
For reinforced concrete or prestressed concrete beams or slabs, applying external prestress in the tension zone can offset part of self-weight stress and play the role of unloading, thus greatly improving the shear capacity of beams. The advantages of external whisker stress strengthening method are:
3.3. 1 can greatly improve and adjust the stress condition of the original structure, and improve the stiffness and crack resistance of the load-bearing structure with little increase in self-weight;
3.3.2 Because the load-bearing structure is slightly increased, it has little influence on the stress state of the pier and foundation, and the reinforcement of the pier and foundation can be omitted;
3.3.3 It has little influence on bridge operation, and can be reinforced without restricting traffic;
3.3.4 Pre-stressed reinforcement method can be used as a temporary reinforcement method for passing through a heavy truck bridge, and can also be used as a measure to permanently improve the load level of the bridge.
4. Examples of bridge reinforcement and maintenance
4. 1 project overview
The bridge body of WLD overpass project in Hefei has four floors, which are divided into three floors above ground and one underground. The bridge covers an area of 40,000 ㎡ and 8.9 hectares. It is a four-story interchange with a double "Y" circular ramp, which is divided into five intersections and radiates around to solve the traffic in 17 direction.
4.2 Major diseases
After the completion of the bridge, it has been in operation for more than ten years. During this period, through continuous regular inspection and evaluation, the bridge has found different degrees of diseases:
(1) The thickness of asphalt pavement on bridge deck is generally thin, some longitudinal cracks intermittently run through the bridge deck, and the pier top cracks laterally;
(2) There are longitudinal cracks at the bottom of many hollow slabs, and transverse cracks at the bottom of some plate beams, and the bottom plate is thin;
(3) The hinge joints between hollow slab beams with partial spans are invalid, which tends to stress the single slab;
(4) Cracking and leakage of integral concrete correspond to multiple transverse cracks on pier;
(5) Some bearings are missing or empty;
(6) Some plates have different degrees of exposure, rust swelling and insufficient thickness of the bottom protective layer;
(7) There are many cracks in the cast-in-place continuous beam, and there are large cracks in the mid-span of the box girder. The crack development extends from the bottom plate to the web plate, and the crack exceeds the standard, and the maximum width of the crack reaches 0.5㎜.
4.3 The main contents and methods of this maintenance and reinforcement.
According to the specific disease status and mechanism, combined with the test results and traffic conditions, this maintenance mainly includes: bridge deck pavement replacement and reconstruction, continuous large box girder reinforcement, hollow slab beam reinforcement (partial replacement), crack repair, bearing replacement and maintenance, approach bridge reconstruction, approach road maintenance and traffic engineering. , by sticking (carbon fiber) for maintenance and reinforcement.
4.4 Characteristics and principles of paste reinforcement method
4.4.65438+ Reasonable and safe design should control that the concrete at the bonding site will not be damaged by shear before the medium yields and deforms;
4.4.2 When the diagonal bars in the main tensile stress area of the bridge structure are insufficient, in order to reinforce and increase the shear strength of the structure, steel plates can be pasted on the side of the structure and obliquely pasted along the direction perpendicular to the shear cracks to bear the main tensile stress;
4.4.3 In the reinforcement design, the steel plate can be regarded as the section of reinforcement. The steel plate is converted into steel bar, and the original component bears dead load and live load, and the added steel plate bears the live load that the original component cannot bear;
4.4.4 In structural design, carbon fiber or steel plate for reinforcement can adopt different shapes according to actual needs, but the thickness of steel plate must be greater than the calculated thickness. The steel plate used to strengthen the bending capacity should be as thin and wide as possible, and the thickness is generally 4 ~ 6 mm The thinner steel plate can have enough elasticity to adapt to the surface shape of the component.
4.5 Main construction technology and technical points
4.5. 1 surface defect repair measures
(1) For surface defects of concrete with an area less than (25×25)cm2 and a depth less than 5㎝, remove loose concrete, expose aggregate, remove corrosion of steel bars, and repair with polymer cement-based repair materials; (2) For the surface defects of concrete structures with defect area greater than (25×25)cm2 and depth greater than 5㎝, remove loose concrete, expose aggregate, remove steel corrosion, spray interfacial agent and repair with dry RCC;
(3) For shallow defects, polymer cement-based repair is adopted.
4.5.2 Bonding steel plate reinforcement construction technology
(1) Requirements for carbon fiber, adhesive and primer materials
1) pan-based carbon fiber materials must be small tow fibers below 12K or 12K, and large tow fibers are strictly prohibited. Fiber composites must be composed of continuous fibers and modified epoxy resin adhesive. See table 1 for technical indicators.
Table 1 Main mechanical properties of carbon fiber sheet
The standard values of tensile strength (MPa), tensile modulus of elasticity (MPa), elongation (%), bending strength (MPa), interlaminar shear strength (MPa), positive tensile bond strength (MPa) and mass per unit area (g/㎡) of fiber composite materials and concrete in the state of upward bonding.
Unidirectional fabric (cloth) ≥ 3100 ≥ 2.1×105 ≥1.5 ≥ 600 ≥ 35 ≥ max {2.5, ftk}, bond failure of concrete 300.
2) The adhesive for sticking carbon fiber must be a specially prepared modified epoxy resin adhesive, whose basic performance meets the requirements of Class A adhesive, and it is a single adhesive for impregnation/bonding and repair.
3) The primer for sticking carbon fiber is single glue, which is used for dipping/sticking and repairing.
(2) process program
Surface treatment of components → base treatment before pasting → repairing surface defects of components with epoxy crack repairing agent FCS → coating adhesive → pasting carbon fiber sheets → curing → coating.
(3) Process requirements
The key of carbon fiber bonding technology lies in the compactness and firmness of carbon fiber bonding, which ensures that it can be integrated with the original structural form and work together. At the same time, the bridge deck is not suitable for driving during the bonding and curing of carbon fiber.
In order to meet the above requirements, it is necessary to master the construction technology correctly, and each process should be conducive to ensuring the tight bonding of carbon fibers.
(4) Construction quality requirements
1) The carbon fiber sheet has been cut as required;
2) Air temperature, air humidity and surface water content of components meet the requirements;
3) Primer and epoxy putty are dried (resin surface hardening);
4) The type of epoxy resin and the construction temperature are suitable, accurately weigh the main agent and curing agent according to the specified proportion, put them into a container and mix them evenly with a stirrer, and the first dosage should be used up within the available time;
5) The specification, patch position, length, width, fiber direction and number of layers of carbon fiber sheet meet the design requirements.
6) Maintenance shall strictly comply with the time requirements of natural maintenance.
5. Conclusion
Bridge reinforcement is a systematic and complicated project. With the gradual investment of national infrastructure, large and extra-large bridge projects have become quite popular. After a certain period of load operation, a series of defects and common faults will inevitably appear, so the reinforcement and maintenance of old bridges will be the general trend of later engineering construction. Bridge maintenance and reinforcement is a technically feasible and economically reasonable measure, and it is also a project that saves energy and pays attention to environmental protection.
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