Repair the Cracks in the Box Girder Bridges

The bridge is a crucial transportation artery in the region. Its structure is a fully prestressed concrete variable - section continuous box girder bridge. Since its construction, it has been playing an important role in facilitating the flow of people and goods. However, over the years, as the traffic volume has been increasing continuously, and under the long - term influence of vehicle loads and natural factors such as rain, sun, and temperature changes, the bridge structure has gradually shown signs of aging and disease.

Cracks

Before the repair work, a comprehensive assessment of the bridge was carried out. Professional engineers used various inspection techniques, including visual inspection, non - destructive testing (such as ultrasonic testing and ground - penetrating radar), and load - bearing capacity testing. The results showed that there were numerous cracks at the bottom of the bridge, and some of the box beams had also suffered different degrees of damage. These cracks and damages not only affected the appearance of the bridge but also posed a potential threat to its structural safety and service life.

 1) Vehicle Loads

The continuous passage of heavy - duty vehicles, especially over - loaded trucks, has imposed excessive stress on the bridge structure. The repetitive dynamic loads from vehicles have gradually caused the concrete to crack and the prestressed tendons to lose some of their effectiveness. The long - term accumulation of these loads has led to the continuous development and expansion of cracks.

2) Natural Factors

Weathering: Exposure to the sun, rain, and wind for a long time has caused the surface of the concrete to weather. The continuous wet - dry cycles and freeze - thaw cycles have weakened the concrete structure, making it more vulnerable to cracking.

Temperature Changes: The large temperature differences between day and night and between seasons can cause the concrete to expand and contract. Since different parts of the box - girder bridge may have different responses to temperature changes due to its complex structure, internal stress is generated, which is one of the main reasons for the generation of cracks.

Reinforcement Methods

1 Sealing Glue for Concrete Cracks at the Bottom of the Bridge

Surface Preparation: First, the cracks at the bottom of the bridge were carefully cleaned. All the dirt, dust, and loose concrete around the cracks were removed using high - pressure air blowers and wire brushes. Then, the cracks were widened slightly to ensure better penetration of the sealing glue.

Sealing Glue Application: Using HM-9 crack seal adhesive to repair the surface of cracks, then using HM-120L to inject the cracks. The glue was injected into the cracks using a specialized injection device. The injection pressure was carefully controlled to ensure that the glue could fill the entire crack. After injection, the surface of the crack was smoothed to make it flush with the surrounding concrete.

2 Carbon Fiber and Steel Plate for Box Beam Reinforcement

CFRP Reinforcement:

Surface Treatment: The surface of the box beam to be reinforced with carbon cloth was roughened to increase the adhesion. Any oil stains, rust, and loose materials were removed.

Carbon Cloth Laying: The carbon cloth was cut to the appropriate size according to the design requirements. An epoxy - resin - based adhesive was evenly applied to the surface of the box beam, and then the carbon cloth was carefully laid on it. The carbon cloth was pressed firmly to ensure full contact with the adhesive and to expel any air bubbles. Multiple layers of carbon cloth were laid as required, and each layer was bonded firmly.

Steel Plate Reinforcement:

Steel Plate Preparation: The steel plates were cut to the required shape and size. Their surfaces were sand - blasted to remove rust and increase roughness for better adhesion.

Installation: The steel plates were fixed to the box beam using high - strength bolts. Before installation, an adhesive was also applied between the steel plate and the box beam to enhance the bonding strength. The bolts were tightened to the specified torque to ensure the stability of the steel plate.

After the reinforcement work was completed, a series of tests and inspections were carried out again. The results showed that the bearing capacity of the bridge structure had been greatly improved. The cracks that had been treated were effectively sealed, and the box beams were much stronger and more stable. The potential safety hazards that had worried the local transportation department and the public were completely eliminated.