The effectiveness of using carbon plates to strengthen bridges can be understood from two perspectives: material properties and mechanical principles. We already know a lot about material properties, but today we'll discuss carbon plate reinforcement of bridges from the perspective of mechanical principles.

Bridges, especially girder bridges, are primarily subjected to bending. Think of it like a slightly bent ruler:

The upper portion experiences compression (shortening), and the lower portion experiences tension (stretching). Concrete has high compressive strength but very low tensile strength, relying entirely on the internal steel reinforcement to resist tensile forces.

When a bridge ages, is overloaded, or suffers from steel corrosion, the tension at the bottom can no longer be fully resisted, resulting in:

• Excessive deflection of the beam

• Cracking of the concrete at the bottom

• Decrease in bearing capacity

The purpose of the carbon plate is to help the bottom reinforcement resist tensile forces.

Attaching carbon plates (passive reinforcement - most commonly used):

How it works: 

The carbon plate is firmly attached to the tensile side of the bridge beam (usually the bottom) using high-strength epoxy resin adhesive. The carbon plate forms an integral part of the concrete.

Why it works:

• Increases tensile strength: When a bridge bends downward under load, tensile forces are generated at the bottom. This force is now shared between the original reinforcement and the newly attached carbon plate. This acts as an additional "insurance" against tensile forces.

• Restricts crack development: The carbon plate acts like a "Band-Aid," firmly adhering to the concrete surface and effectively limiting the opening and propagation of cracks.

• Increases stiffness: The addition of the carbon plate enhances the overall beam's ability to resist deformation (i.e., stiffness). This means that under the same load, the bridge's downward deflection will be reduced.

Prestressed Carbon Plates (Active Reinforcement - More Efficient):

This is a more advanced method, in which a tensile force (prestressing) is applied to the carbon plates before they are attached.

Why is it more effective:

• Active stressing: Not only does it share future loads, it also actively offsets some of the tensile forces generated by the bridge's existing loads (such as its own weight).

• More significant results: It can more effectively reduce deflection, even close cracks, and significantly increase load-bearing capacity. The material strength utilization rate is also far higher than with conventional adhesive bonding.

In summary, the effectiveness of carbon plate reinforcement for bridges lies in its use of a "stronger, lighter, and more durable" material. Through scientific mechanical principles, it precisely compensates for the weak links in the original structure, thereby efficiently restoring or even enhancing the bearing capacity and performance of the bridge.