What is External Prestressed Reinforcement of Bridges?

Reinforced concrete beam bridges usually include simply supported beams (T-shaped beams, composite beams with less reinforced micro-bent plates, π-shaped beams and plate beams, etc.), cantilever beams, and continuous beams. When there are structural defects, especially insufficient bearing capacity or the need to increase the load level, the main stress-bearing structure of the bridge needs to be reinforced. At this time, steel bars or steel wire bundles can be set outside the beam body (beam bottom and both sides of the beam), and prestressing can be applied to improve the stress condition of the bridge and achieve the purpose of improving the bearing capacity of the bridge. This is external prestressing.

External prestressing is the arrangement of prestressed tendons outside the main structure, relative to internal prestressing. When external prestressed cables are applied to concrete structures, they are called external prestressed concrete structures. External prestressing technology used for bridge reinforcement is called external prestressing reinforcement. In terms of mechanical characteristics, the deformation of the external prestressed cable and the main body of the surrounding structure in the same section is not coordinated.

External prestressing cables use materials such as fully threaded steel bars, steel strands, or high-strength steel wires as force tools to apply external prestressing to the upper structure of the bridge. The anti-bending moment generated by the prestress partially offsets the internal force generated by the external load, thereby improving the service performance of the old bridge and increasing its ultimate bearing capacity.

The external prestressing reinforcement method has the triple effect of reinforcement, unloading, and changing the internal force of the structure, and is suitable for beam bridges with medium and small spans. For bridges with larger spans, when this method is used for reinforcement, it should be combined with other reinforcement methods to achieve a better reinforcement effect.

Engineering practice shows that external prestressed reinforcement of bridges has the following advantages:

1. It can greatly improve the bearing capacity of the old bridge. After reinforcement, the bearing capacity of the original bridge can generally be increased by 30%-40%.
2. The external prestressed cable reinforcement technology requires simple equipment, less manpower input, a short construction period, and obvious economic benefits.
3. During the reinforcement process, the traffic can be not interrupted or limited for a short time.
4. Less damage to the original bridge. It can be done without affecting the clearance under the bridge and without increasing the elevation of the road surface. Commonly used external prestressed reinforcement techniques include external prestressed wire bundle reinforcement and under-supported prestressed tie rods (fully threaded steel bars) reinforcement.
5. The external prestressed reinforcement method does not need to clear the concrete protective layer, and the damage to the beam body is small.

However, for the prestressed tendons and related components outside the beam, effective protective measures should be taken, otherwise, under the action of external conditions such as temperature and corrosion, the prestressed tendons will easily break, resulting in failure of the reinforcement work.