Study on Bonding Performance of Double-Layer CFRP-Steel Interface under Salt Erosion and Freeze-Thaw Environments

Steel structures form the silent backbone of modern life, holding up bridges that span wide rivers and towers that pierce the sky. They are chosen for their strength and lightness, but like all materials, they face a slow, relentless battle against the elements. In many parts of the world, these structures endure a double assault: the corrosive bite of salt from sea spray or winter road treatments, and the physical pounding of freeze-thaw cycles, where water seeps in, freezes, expands, and cracks the material from within. To keep these structures safe, engineers often apply a protective skin made of carbon fiber reinforced polymer, a material known for being incredibly strong and resistant to rust. This skin is glued to the steel with a special adhesive. The success of this repair depends entirely on the bond between the steel and the glue; if that connection fails, the reinforcement becomes useless. The challenge lies in understanding how this delicate bond holds up when salt and freezing temperatures work together to weaken it over time.

A team of researchers at Xi'an University of Science and Technology set out to solve this puzzle by looking at a specific, advanced repair method: a double-layer system. Instead of just one layer of carbon fiber, they tested a combination of a stiff carbon fiber plate on the inside and a flexible carbon fiber fabric on the outside. To understand how this system behaves under harsh conditions, they did not just rely on physical tests, which can take years to show results. Instead, they built a sophisticated digital twin of the structure using powerful computer simulations. This virtual model allowed them to watch, in slow motion, how salt ions and freezing water move through the glue, how tiny cracks form and spread, and how the bond between the steel and the carbon fiber degrades. They created a model that linked the movement of salt, the changing temperature, and the physical stress on the material all at once, mimicking the complex reality of a bridge in a cold, salty environment.

The simulations revealed a vicious cycle that accelerates damage. When the glue layer absorbs water and salt, the freezing process causes the water to expand, creating microscopic cracks within the adhesive. These tiny fissures act as highways, allowing salt ions to rush deeper into the material much faster than they would through solid glue. Once inside, the salt weakens the chemical structure of the glue, making it softer and more prone to cracking. This softening, in turn, makes it easier for the freezing water to cause even more damage. The researchers found that this positive feedback loop—where damage invites more intrusion, and intrusion causes more damage—is the primary reason the bond fails so quickly in these environments. After simulating 120 cycles of freezing and thawing, which represents a significant period of exposure, the single-layer systems showed a sharp decline in their ability to carry weight.

However, the double-layer approach told a different story. The combination of the stiff inner plate and the flexible outer fabric created a system that was far more resilient. The inner plate provided the necessary rigidity to transfer loads efficiently, while the outer fabric acted as a flexible shield, absorbing stress and delaying the entry of corrosive elements. In the computer simulations, this double-layer arrangement retained 88.1 percent of its original strength after 120 freeze-thaw cycles, a significant improvement over single-layer repairs. Furthermore, the researchers discovered that the arrangement mattered; placing the plate closest to the steel and the fabric on the outside was the most effective configuration. This setup allowed the structure to maintain a high load-bearing capacity of 116.24 kilonewtons at room temperature, which was 16.2 percent stronger than a single-layer plate, and it held up remarkably well even after the simulated harsh weather.

The study also pinpointed the exact dimensions needed for this repair to work best. The researchers tested different lengths for the carbon fiber plate and found that there is a critical threshold for how long the bond needs to be to work effectively. If the plate is too short, the stress concentrates at the ends, causing the bond to peel off prematurely. The simulations indicated that the plate needs to be at least 200 to 250 millimeters long to ensure the strength is fully utilized without failing at the edges. Similarly, the thickness of the glue layer proved to be a balancing act. A layer that was too thin created high stress points that were vulnerable to cracking, while a layer that was too thick trapped moisture and created its own internal weaknesses. The sweet spot was found to be a glue thickness of 2.0 millimeters, which offered the best balance between transferring the load and distributing stress evenly.

These findings provide a clear roadmap for engineers designing repairs for steel structures in harsh climates. By using a double-layer system with the specific arrangement of an inner plate and outer fabric, and by adhering to the recommended bond lengths and glue thickness, it is possible to create a reinforcement that resists the combined assault of salt and ice. The research confirms that while the environment is aggressive, the right combination of materials and design can significantly extend the life of these critical structures. The study does not claim to have solved all durability issues, but it offers a proven, optimized strategy that moves beyond simple patching to a more robust, long-term solution for keeping our steel infrastructure safe and sound.