1, Introduction
1 and 1 Durability of concrete In the early 1990s, the durability of concrete bridges has attracted worldwide attention. Through the maintenance and management practice of a large number of concrete bridges with a bridge age of more than 20 years, it is found that the concrete cracking, peeling, decay and corrosion of steel bars (insufficient grouting of pipelines) have caused serious damage to the bridge, which has become an urgent problem to be solved, and the seriously damaged bridge has endangered traffic safety.
There are two technical ways to improve the durability of concrete bridges. One is to use high-performance concrete to improve the impermeability, uniformity and frost resistance of concrete, thus improving the ability of concrete to resist carbonation and freezing erosion; The other is an effective way to improve the durability of existing bridges, that is, to strengthen and transform defective bridges and extend their service life.
1 2 Bridge reinforcement
Bridge reinforcement and reconstruction is to reinforce and widen the bridge structure through effective and feasible technical means, and its fundamental purpose is to restore and improve its bearing capacity and durability. At present, there are many commonly used bridge reinforcement methods, which can be broadly divided into superstructure reinforcement and substructure reinforcement. There are two kinds of reinforcement methods for bridge superstructure: the method of changing the structural stress system and the method of not changing the structural stress system. From the point of view of whether the post-reinforcement material has prestress and basic stress principle, the reinforcement of the upper part of the bridge can be divided into two categories: active reinforcement and passive reinforcement.
1 and 2 1 bridge active reinforcement principle
Reinforcing materials are directly added to the tensile area (or shear weak area), such as repairing welded steel bars, pasting steel plates, pasting high-strength composite fiber materials (carbon fiber, aramid fiber), etc. This way of strengthening belongs to the category of passive strengthening from the principle of action. In fact, the characteristics of load reinforcement and phased loading must be considered in the design, and the self-weight and dead load of the component should be borne by the original beam; The live load is borne by the reinforced composite section, and the strength of the reinforced material is limited by the deformation of the original beam.
Passive reinforcement principle of 1 and 22 bridges
In order to solve the "strain lag" of post-reinforcement materials and improve the utilization rate of post-reinforcement materials, the principle of pretensioning method is used to pre-stress the post-reinforcement materials. From the principle of action, prestressed reinforcement belongs to the category of active reinforcement. The pre-tension of the material strengthened by active stress improves the stress state of the original beam, thus improving the bearing capacity and crack resistance of the original beam.
At present, there are three kinds of prestressed reinforcement systems used in bridge structures: external prestressed reinforcement system, high-strength composite fiber prestressed reinforcement system and bonded prestressed reinforcement system.
2, external prestressed reinforcement system
2. Development background of1
In 1970s, with the development of transportation, the reconstruction of old roads and the reinforcement of old bridges attracted worldwide attention. Many countries have studied the reinforcement of old bridges. Some European countries use external prestressing technology to strengthen bridges and improve the load grade of the original bridges, which has received obvious economic benefits. The application of external prestressing technology in strengthening old bridges has promoted and deepened people's understanding of external prestressing technology.
In fact, the external prestress technology appeared with the appearance of prestress technology, and the external prestress technology built concrete bridges before the internal prestress technology, so it is an ancient new technology. As early as 1934, De Schuengel of Germany obtained the patent of external unbonded prestressed tendons in Germany and France, and built a real bridge in 1936. However, due to the immaturity of external corrosion protection technology of steel bars, the cost of bridge maintenance is high, which has hindered the development of this technology for a long time.
Since 1990s, external prestressing technology has developed rapidly abroad. On the one hand, people are eager to create a structure with corrosion resistance due to the improvement of the understanding of the durability of concrete, which provides a broad market for the development of external prestressing technology. On the other hand, with the development of cable-stayed bridge technology, the problems of steel wire anticorrosion, design and construction of large-tonnage anchor head have been successfully solved, which has eliminated the biggest obstacle to the development of external prestressing technology and made the development of external prestressing technology get a new life.
With the continuous development of external prestress design theory and practical technology, in turn, it further promotes the application of external prestress in the reinforcement of old bridges.
2, 2 action principle
External prestressing reinforcement is to arrange anti-corrosion prestressed tendons outside the beam (or inside the box), to prestress the beam, and to offset some internal forces generated by external loads with the counter-bending moment generated by pre-tensioning, so as to improve the service function of the beam and improve the bearing capacity of the beam.
External prestress reinforcement is one of the most widely used reinforcement methods at present, especially for long-span prestressed concrete continuous box girder and continuous T-box girder bridge. The external prestressing tendons are anchored on the diaphragm beam at the end (or middle) of the beam, and the angle of the prestressing tendons is adjusted by turning blocks between spans to meet the stress requirements of the beam.
Strictly speaking, external prestress and internal unbonded prestress, including cable-stayed bridges, belong to unbonded prestressed structures. The core problem of theoretical analysis of unbonded prestressed structures is the calculation of stress increment of prestressed steel tendons in each stress stage.
2, 3 technical characteristics
Compared with ordinary prestressed concrete structures, externally prestressed concrete structures have the following main advantages:
(1) External prestressing tendons can be non-replaceable, and can also be designed as replaceable, pullout and replaceable external tendons, which is convenient for maintenance, maintenance and reinforcement;
② The structure of the old bridge strengthened by external prestressing is simple, which simplifies the construction; Because there is no pipeline in the beam rib, the section weakening is avoided; The external reinforcement can realize the year-round construction by canceling the pipeline grouting process, which is of great practical significance to the provinces in the northern cold region.
(3) The friction loss of tendons is reduced, the utilization efficiency of prestressed tendons is high, and the stress variation range of tendons is small, which is beneficial to fatigue resistance. The popularization and application of external prestress in strengthening concrete bridges in China is to solve the long-standing problem of concrete durability in engineering.
If possible, improving the durability of the bridge and prolonging its service life (the service life can be extended for up to 20 years by replacing or supplementing the cable once) can bring considerable direct economic benefits for the maintenance, management and reinforcement of the bridge, and its indirect and long-term economic benefits are also quite remarkable.
3. High-strength composite fiber prestressed reinforcement system
At present, the high-strength composite fibers used in engineering mainly include aramid fiber and carbon fiber (FRP). Because carbon fiber materials are widely used in bridge reinforcement and the technology is mature, this paper mainly introduces carbon fiber prestressed reinforcement.
3. Ask the question 1.
In engineering, the widely used reinforcement method is to directly paste fibers in the tensile area or shear weak area of the structure. As far as the bearing capacity reinforcement is concerned, the passive reinforcement method of directly sticking carbon fiber sheets in the tensile area and adding reinforcement materials can not give full play. According to the characteristics of stress in stages, the reinforcement material only bears the internal force of live load after direct pasting; Compared with the original beam reinforcement, its strain is seriously "lagging". In the limit state, the exertion of its strength is limited by the deformation of the original beam, and the design value of its tensile strength is generally not obtained.
The calculation shows that the reinforcement design is controlled by the ultimate compressive strain of concrete when the height of the original beam is small and the reinforcement ratio is large. In the limit state, the stress of the reinforced material is only 700 ~ 800 MPa, which is only equivalent to the standard value of the tensile strength of carbon fiber (2 1.2% ~ 24.2%). For the case that the height of the original beam is large and the reinforcement ratio is small, the reinforcement design is controlled according to the limit value of 0.0 1 of the original beam reinforcement strain. In the limit state, the stress of the reinforced material in the later stage is only about 2000MPa, which is equivalent to 60% of the standard tensile strength of carbon fiber. Due to the deformation limitation of the original beam, the high tensile properties of high-strength composite fiber can not fully play its role in the limit state, resulting in great waste. Moreover, blindly increasing the amount of post-reinforcement materials without analysis may lead to brittle failure of reinforcement members and unsafe design.
3,2 principle of action
In order to improve the utilization efficiency of carbon fiber materials and enhance the reinforcement effect of old bridges, it is an effective way to apply prestress to carbon fiber materials.
The function principle of carbon fiber prestressed reinforcement is to use carbon fiber cloth strips (or slats) anchored on the reinforced beam to prestress the beam and improve the stress state of the reinforced beam. The key technology is to solve the tension and anchorage problems of prestressed fibers (or plates) suitable for bridge site construction.
At present, this reinforcement system is still in the experimental research stage.
3, 3 technical characteristics
① High strength and high efficiency. Due to the excellent physical and mechanical properties of FRP, it can make full use of its high strength and high elastic modulus in the reinforcement and repair of concrete structures, improve the bearing capacity and ductility of concrete structural members, improve their mechanical properties, and achieve the purpose of efficient reinforcement and repair.
② The linear expansion coefficient is close to that of concrete, which ensures that FRP and concrete can work together when the temperature changes.
(3) convenient construction, high work efficiency, no wet operation, no need for large construction machines and tools, less construction area and high construction efficiency. According to statistics, the construction efficiency of sticking FRP is 4 ~ 8 times that of sticking steel plate. Glass steel is light and soft, easy to attach, and the construction quality is easier to ensure than sticking steel plates.
(4) The weight and volume of the parts are not increased. FRP is light in weight and thin in thickness. After reinforcement and repair, the weight and size of the original structure will not be increased basically, and the use space of the building will not be reduced.
⑤ It has good corrosion resistance and durability. Experiments show that carbon fiber, glass fiber and aramid fiber have good corrosion resistance and durability, and can resist the corrosion of acid, alkali and salt often encountered in buildings. After reinforcement with this material, it not only does not need regular maintenance, but also protects the internal concrete structure.
⑥ Widely used. It can be widely used for reinforcement and repair of various structural types (such as buildings, structures, bridges and tunnels, culverts, chimneys, etc.). ), various structural shapes (such as rectangle, circle, curved structure, etc.). ) and various structural members (such as beams, plates, joints, arches, shells, piers, etc.). ), does not change the shape of the structure, does not affect the appearance of the structure, which is unmatched by any structural reinforcement method at present.
4, bonded prestressed reinforcement system
4, 1 principle of action
The small-diameter prestressed tendons anchored on the reinforced beam are used to prestress the beam, and then composite mortar with high tensile strength is sprayed to bond the prestressed tendons and the reinforced beam into a whole, forming a bonded prestressed reinforcement system.
Bonded prestressed tendon system has attracted the attention of civil engineering circles at home and abroad because of its technical advantages of simple anchorage, convenient tensioning construction, good structural durability and high material utilization efficiency. The characteristics of bonded prestressed tendon system are suitable for strengthening small and medium-span reinforced concrete T-beams, hollow slab beams and box girder bridges, especially for medium-span reinforced concrete and prestressed concrete continuous box girder bridges widely used in expressways and urban interchange projects. Due to the height limitation of box girder, it is difficult to arrange external prestressing tendons in the box. It is one of the ideal reinforcement schemes to add prestressed tendons to the bottom plate of box girder and then spray high-performance tensile composite mortar.
4, 2 technical characteristics
Compared with external prestressed reinforcement system, bonded prestressed reinforcement system has the following outstanding advantages:
① It can give full play to the role of post-reinforcement materials and improve the utilization efficiency of materials;
(2) Spraying a protective layer of high-performance tensile composite mortar. Strong carbonation resistance and chloride ion corrosion resistance, protecting steel bars from corrosion, improving structural durability and prolonging structural service life;
(3) It is safer and more reliable to anchor prestressed steel bars with small anchorage and high-performance tensile composite mortar.
4, 3 development prospects
The SRAP process of M&S company in South Korea has the characteristics of bonded prestressed steel bars, but the AP mortar and prestressed galvanized mild steel wire used are imported from abroad, and the project cost is high, which affects the market competitiveness to some extent. In order to meet the demand of bridge reinforcement market, the high-performance tensile composite mortar (HTCM mortar for short) developed in China has been supplied to the market, and its tensile strength, bonding performance, carbonation resistance and chloride ion corrosion resistance are slightly higher than those of AP mortar. The experimental research on using domestic steel products such as two or three strands of steel stranded wire, spiral ribbed steel wire and small-diameter high-strength thick steel bar as the tensioning and anchoring system of prestressed tendons has achieved initial results, which has opened up a new way for popularizing bonded prestressed tendons in our province.
Using bonded prestressed reinforcement system can improve the durability of the structure, prolong the service life of the structure by 20-30 years, and bring huge economic benefits.
The above pre-stressed reinforcement technologies and characteristics of roads and bridges were collected and sorted by Zhong Da Consulting.
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