Reinforcing and repairing bridges is to restore the ability of existing bridges to bear the load and extend their service life. In continuous rigid frame bridges, it is necessary to adopt different reinforcement methods depending on different disease conditions and actual analysis of actual problems. According to the characteristics of the disease, when corresponding reinforcement measures are taken, the concrete continuous rigid frame bridge should be repaired according to the actual situation. This chapter introduces several reinforcement methods commonly used in continuous rigid frame bridges.

1 Method of strengthening by enlarging the main beam section

When the continuous rigid frame bridge girder body is damaged, the rigidity of the structure is reduced, the strength is insufficient, and the structure's resistance to cracking and its load-bearing stability are affected, the main girder can be strengthened by increasing the main girder section, Reinforcement method of main reinforcement by welding. Within the range of suitable reinforcement for the beam, the increase in the load-bearing capacity of the front section of the concrete member subjected to the bending moment is proportional to the area of the main reinforcement in the section and its strength. When the reinforcement ratio of the original section of the beam is too low, increasing the area of its main reinforcement can significantly improve the ability of the front section of the beam to resist bending moments, thereby increasing the bearing capacity of the structure. When the clearance under the bridge is not limited, the original bridge deck paving can be chiseled off, and then a reinforced concrete reinforcement layer can be laid. This method can improve the flexural bearing capacity of the structure.

The main means to increase the cross section of the main girder are: ① increase the cross-sectional area of the main girder; ② increase the number of reinforced steel bars; ③ thicken the original bridge deck; ④ anchor shotcrete to the original structure.

The main principle of the strengthening method of increasing the main beam section is to increase the moment of inertia of the main beam section to improve the load-bearing capacity of the main beam section. This kind of reinforcement method is mainly to increase the compressive section of the structure, which is limited in improving the bending and shear resistance of the section. It is mainly used for small and medium-span bridges.

2 Thicken bridge deck reinforcement method

When the original structure has a good geological foundation and sufficient bearing capacity of the pier and abutment, but the cross-sectional area of the main girder of the bridge is small and insufficient to bear the effect of live load, considering the ease of reinforcement, the “thickened bridge deck Method of maintenance and reinforcement. This kind of reinforcement method is to pass certain measures to chip away the pavement of the old bridge's driveway, exposing the bridge deck. On top of it, a reinforcement layer is added. The reinforcement layer is mainly composed of reinforced concrete. Its function is to increase the ability of the bridge to resist bending moments. Before reinforcement, it will be chiseled. In order to ensure that the concrete surface of the original bridge deck is clean enough, prepare for the pouring of the reinforcement structure, so that the two can be better combined together and bear the force together. After the chiseling is completed, a number of tooth-shaped shear grooves will be set on the original bridge deck, and the shear grooves are separated by an appropriate length to ensure the overall force of the new and old structures. In addition, you can also choose to apply epoxy resin on the original bridge deck to make the two tightly bonded. At the same time, in the reinforcement process, steel meshes are generally arranged on the bridge deck. In the reinforced mesh, the specific diameter of the steel bars and the distance between them are determined by specific calculations. This is to strengthen the ability of the new and old structures to resist pressure collaboratively, and to avoid damage to the newly poured concrete reinforcement structure due to insufficient integrity.

3 Adding longitudinal and longitudinal beam reinforcement method

When the geological conditions of the concrete continuous rigid frame bridge are good, it can provide sufficient bearing capacity for the structure. And under the condition that the main girder of the continuous-cut bridge is basically not corroded by disease, new longitudinal and cross girder with sufficient rigidity to bear the load can be added between the main girder. In order to bear the role of live load together with the old structure. The newly added beam can reduce the lateral distribution coefficient of the old beam, reduce the load on the original beam, and reduce the burden of the original structural system. Play the role of unloading, the use of this method to strengthen the bridge, can increase its load bearing capacity and the overall rigidity of the structure. When the reinforced longitudinal beam structure is located on one or both sides of the main beam, it not only increases the load-bearing capacity of the system, but also increases the width of the bridge deck.

4 FRP Composite Strengthening System

HM FRP composite strengthening system provide efficient fiber reinforced polymer (FRP) materials for the structural reinforcement and strengthening of concrete, masonry and timber structures in need of repair or upgrade. The performance characteristics of FRP compsite strengthening have become increasingly popular in construction and retrofit applications, specifically in aging, damaged or overloaded concrete structures. HM fiber reinforced polymer(FRP) composite strengthening system can be used to increase the strength of slabs, slab openings, beams, walls, new wall openings, columns, silos, chimneys, piles and pier caps. FRP can be applied to strengthen reinforced concrete and pre-stressed/post-tensioned concrete, steel, masonry and timber.

Common reasons for FRP structural strengthening:

• Durability problems due to poor or inappropriate construction materials

• Inadequate design or construction

• Aggressive environments not properly understood during the design stages

• Increased loading requirements due to changes of policy or use of structures

• Increased life-span requirements made on ageing infrastructure

• Exceptional or accidental loading

Advantages of Fiber Reinforced Polymers(FRP) composite strengthening

• Economical and durable

• Easy application

• Extremely high tensile strength

• Outstanding fatigue behavior

• Absolute corrosion resistance

• Ability to upgrade structure while in use

• Upgrades possible even with limited access

5 External prestress reinforcement method

In a continuous rigid frame system, the main method of using external prestressed steel reinforcement is to arrange the prestressed steel outside the concrete section of the tension zone. After adding prestressed steel bars, the external prestress mainly generates eccentric bending moments to the beam to improve the stress of the structure and increase the concrete compressive stress reserve in the bending zone. For bridges with excessive deflection, it also plays a role in alleviating the deflection, so as to enhance the load bearing capacity of the structure.

6 Sticking steel plate reinforcement method

The method of sticking steel plate reinforcement refers to sticking the steel plate to the tension area or weak area of the reinforced concrete beam with an adhesive, and the load is transmitted from the bridge structure to the reinforced steel plate through the adhesive. It is a reinforcement method for the original structure and steel plate to bear the load. Its function is to increase the rigidity of the original structure, improve the stress state of the structure, and limit the development of cracks. Compared with other reinforcement methods, its main advantage is that the secondary stress characteristics of bonded steel reinforcement members are not obvious, and it is widely used in reinforcement.