With the rapid development of China's transportation industry, the traffic density of large and heavy vehicles on highway bridges is increasing day by day. The problems of low design standards of bridges built in the early stage and aging trend have seriously affected the smooth and safety of highway traffic. 

Using reinforcement method to strengthen and maintain the original bridge not only saves a lot of construction funds, but also solves the contradiction between the bearing capacity of the bridge and traffic demand in a short time. At present, the main methods used to reinforce bridges include enlarging section and reinforcing steel bar, bonding steel plate, external prestressing and carbon fiber sheet.

1. Reinforcement Method of Increasing Section and Reinforcing Bar and Key Construction Points

1.1 Reinforcement with Increased Section and Reinforcement

Increasing the section size and reinforcement of members is a bridge reinforcement method suitable for slab-girder bridges and T-girder bridges with smaller span. This method usually pours a certain thickness of reinforced concrete reinforcing layer on the deck, side and bottom of the main girder of the original bridge. The flexural strength and crack resistance of the bridge can be improved by increasing the section area and reinforcement ratio. The advantages of this reinforcement method are: simple technology, mature construction experience, the stiffness and bearing capacity of the original bridge have been greatly improved, especially for the rib girder bridge, the effect is better. The disadvantage is that site construction is usually carried out under the beam, which affects navigation or traffic. Maintenance time is also longer, construction quality is not easy to control. In addition, in order to make the new and old concrete bond well, the workload of concrete chiseling is large. Because of the dense arrangement of reinforcing bars, it is not easy to compact when tamping concrete. Because the method enlarges the self-weight of the main girder, the bearing capacity of the lower structure and foundation of the bridge is more demanding.

1.2 Key Construction Points of Increasing Section and Reinforcing Technology

The key point of using this reinforcement technology is to ensure the good combination of old and new concrete. Therefore, the following construction points should be followed in construction:

1) When chipping the surface of the original bridge components, the edges and corners of the concrete should be removed. After chipping, the dust and scum should be cleaned up. At the same time, the surface of the additional stressed main reinforcement should be treated with rust removal.

2) When planting reinforcement holes on the surface of the original bridge members, the reinforcement inside the original members should not be destroyed.

3) Before pouring new concrete, cement slurry should be applied on the surface of old concrete to enhance the bonding force between new and old concrete.

4) In order to avoid the redistribution of structural internal forces due to the different shrinkage of old and new concrete, it is not appropriate to use excessively high-grade cement in newly-poured concrete. Concrete materials similar to the performance of original components should be used as far as possible, or reinforcement with steel mesh should be installed in newly-poured concrete.

2. Reinforcement Method of Sticking Steel Plate and Key Points of Construction

2.1 Reinforcement with Sticking Steel Plate

Bonded steel plate reinforcement method is to use building structural glue, epoxy resin and other binders and anchors to bond and anchor steel plate or FRP with high tensile strength to the tensile edge or other weak parts at the bottom of the main beam, so as to form an overall resistance to moment shear force with the original main beam, in order to limit the crack development and improve the bearing capacity and durability of the bridge structure. The advantages of this reinforcement method are: simple technology, less consumables, small increase in self-weight, flexible reinforcement parts, fast construction, reliable construction quality, no change in the shape and size of the original components, basically no impact on the traffic under the bridge. The disadvantage is that the quality and durability of the binder itself will directly affect the reinforcement effect of the bridge. In addition, in order to prevent steel plate corrosion, surface protection is needed after bonding steel plate reinforcement, so the maintenance cost of later period is increased.

2.2 Key Construction Points of Reinforcement Technology with Bonded Steel Plate

The following construction points should be followed in the construction of this reinforcement technology:

1) The concrete surface of the reinforced members should be polished first, and then the dregs should be washed with clean water.

2) The surface of the bonded steel plate should be pre-treated with rust removal. When welding the steel plate, the weld should be polished.

3) Reinforced members should be unloaded before bonding steel plates. When bonding steel plates, the steel plates should be positioned correctly and bonded smoothly.

4) In winter construction under low temperature conditions, the binder should be heated in time. In order to speed up the solidification of the binder, infrared light can be used to heat the steel plate bond.

3 External Prestressing Reinforcement Method and Construction Points

3.1 External Prestressing Reinforcement Method

External prestressing reinforcement method is a post-tensioning method which produces unbonded prestressing force by tensioning the high-strength steel outside the stress components of the bridge. External prestressing reinforcement system is mainly composed of external prestressing pipeline, cement paste, anchorage device and steering device. The advantages of this reinforcement method are that it is suitable for long-span heavy bridge structures, can actively adjust the stress state of the original structure, effectively limit deflection, cracks and significantly improve the bearing capacity of the original bridge, and has the dual effect of unloading and reinforcement. In addition, the reinforcement method can replace the prestressing tendons at any time to facilitate maintenance, increase the weight of the bridge is small, and has little impact on the stress condition of the lower structure of the original bridge and normal traffic. The shortcomings are: the external prestressing tendons are easy to corrode under the adverse external environment, and if the original structure concrete has a large shrinkage, creep, relaxation of external prestressing tendons, broken tendons and other phenomena will affect the reinforcement effect of the bridge.

3.2 Key Construction Points of External Prestressing Reinforcement Technology

The following construction points should be followed in the construction of this reinforcement technology:

1) External prestressing tendons are not only the main parts of the whole reinforcement system, but also the main bearing components of the original bridge through anchorage. Their durability also affects the durability of the original bridge. Therefore, epoxy resin or galvanized tendons should be coated or casing should be installed to prevent corrosion in the reinforcement process. In addition, anti-corrosion measures should be taken for anchorage devices and steering devices.

2) External prestressing tendons should be tensioned according to the following procedures: 0 10% design tension 105% design tension back to 100% design tension top plug.

3) Steering device is an important component to reinforce external prestressing tendons, which also affects the effect of prestressing. If the design and construction measures are not appropriate, the prestressing tendons are prone to large friction losses. Therefore, in the design of steering gear, it is necessary to ensure that the position of prestressing tendons at the corner is accurate in order to prevent additional stress. The minimum bending radius of the embedded pipeline in the steering device should also meet the requirements of the code.

4 CFRP Reinforcement Method and Construction Points

4.1 CFRP Reinforcement Method

Carbon fiber cloth is a kind of soft sheet made of carbon fiber filament and has good physical and mechanical properties such as light weight, high strength, fatigue resistance, corrosion resistance and so on. The reinforcement principle of CFRP bonding method is similar to that of steel plate bonding method. It uses epoxy resin bonding material to bond CFRP on the surface of the main stressed components of bridge, so that it can form an overall force with the original structure, thereby improving the stiffness and bearing capacity of the structure, restricting the development of concrete cracks, and achieving the purpose of strengthening and reinforcing the original structure. The advantages of this reinforcement method are that the durability and fatigue resistance of the strengthened bridge structure become better, and the later maintenance cost is lower. It has little effect on the integrity of the original bridge, the weight of the structure, the appearance size and the clearance under the bridge. It is easy to form, reliable in quality, fast in construction, no need of large-scale construction equipment, strong in operability and adaptability, and can be used to reinforce various types and shapes of bridge components. The disadvantage is that the epoxy resin bonding material will soften when the temperature exceeds 60 C, while the bridge structure will usually exceed 60 C under the direct sunlight in summer.

4.2  Key Construction Points of CFRP Reinforcement Technology

The following construction points should be followed in the construction of this reinforcement technology:

1) In order to ensure the smoothness and compactness of the bonded carbon fiber cloth, before bonding the carbon fiber cloth, the dirt and dregs on the concrete surface of the reinforced members should be cleaned and kept dry. Before grinding the components, the deteriorated concrete should be removed and repaired smoothly. The cracks in the structure should be sealed with epoxy resin.

2) When bonding carbon fiber cloth, try not to bend, in order to prevent wire breakage. During the bonding process, the carbon fibre cloth should be rolled repeatedly to remove the bubbles as far as possible.

3) In order to ensure the bonding quality of carbon fiber cloth, when the temperature is higher than 60 C, thermal insulation measures should be taken to protect it. Construction should be suspended when the temperature is lower than 5 C and the relative humidity is higher than 85%.

5. Selection Principles of Bridge Reinforcement Methods

In summary, there are many commonly used reinforcement methods for bridges. In engineering practice, reasonable selection should be made according to the following principles. Firstly, the reinforcement methods with less consumables, simple construction equipment, less damage to the original structure, no impact on traffic, economic and effective, and less maintenance workload are usually considered first. Secondly, when strengthening bridges, the form of the original bearing structure of the main body is generally not changed, and only in complex circumstances, taking into account the principle of economy, the reinforcement method of changing the form of the structure can be considered. Thirdly, the selected reinforcement method should ensure that the strength, stiffness and durability of the strengthened bridge structure can meet the requirements of design specifications and use.

6 Conclusion

In short, all kinds of reinforcement methods of bridges have their own advantages and disadvantages. Only by considering the status quo, characteristics, reinforcement requirements, the degree of bearing capacity weakening and future traffic volume of the original bridge, can we make reasonable selection according to local conditions, and focus on grasping the construction points of various reinforcement technologies. Only in this way can the ideal reinforcement effect be achieved, and the hidden dangers of traffic safety can be eliminated in an all-round way, and the traffic capacity of bridges can be improved.