Experimental and Analytical Evaluation of Reinforced Concrete Beams Strengthened Using a Near-Surface Mounted CFRP U-Wrap Technique

Concrete is the silent workhorse of the modern world, forming the skeleton of our bridges, buildings, and homes. It is strong enough to hold immense weight, but like any material, it has limits. Over time, the demands placed on these structures can grow, or the materials themselves can age, leaving them vulnerable to bending or breaking under stress. When a concrete beam begins to show signs of weakness, engineers must find a way to make it stronger without tearing the whole structure down. For decades, the standard solution has been to glue thin, high-strength sheets of carbon fiber onto the outside of the beam. While effective, this method has a flaw: the glue can sometimes fail, causing the sheet to peel off before the beam reaches its full potential. To solve this, researchers have developed a technique that tucks the strengthening material inside the concrete itself, hiding it within a groove cut into the surface. This approach, known as near-surface mounting, creates a much tighter bond between the beam and the reinforcement, protecting the material and allowing it to work harder.

A team of researchers recently put this hidden-strengthening method to the test, specifically looking at how it performs when combined with a wrapping technique that squeezes the beam from the sides. They built six concrete beams, each measuring one hundred millimeters wide, two hundred millimeters tall, and one thousand five hundred millimeters long. Half of these beams were left plain to serve as a baseline, while the other half were strengthened using a specific process. First, the researchers cut a groove into the bottom of the beam and ran a strip of carbon fiber through it, securing it with a two-part epoxy resin. Then, they wrapped additional carbon fiber sheets around the sides and bottom of the beam in a U-shape, effectively creating a tight embrace around the concrete. Finally, they covered the entire strengthened area with a fresh layer of concrete to restore the beam's original shape. Once the new concrete had cured, they subjected all six beams to a rigorous four-point bending test, slowly applying weight until the beams failed.

The results of the experiment were striking. The plain beams, which had no extra reinforcement, began to bend noticeably under a load of about fifty-five kilonewtons and ultimately failed at an average load of sixty-six kilonewtons. At the moment they broke, they had sagged by less than five millimeters. The strengthened beams, however, told a different story. They held their shape much longer and withstood a maximum load of one hundred kilonewtons before failing. This represents a fifty-one percent increase in the amount of weight the beams could carry. Perhaps even more impressive was how much they could bend before breaking. The strengthened beams sagged by sixteen point seven millimeters at their limit, more than three times the deformation of the plain beams. This extra flexibility is crucial in real-world scenarios, as it allows a structure to absorb energy and give warning before a catastrophic collapse, rather than snapping suddenly.

The researchers also observed how the beams cracked. The plain beams developed a few large, deep cracks that quickly traveled upward through the concrete, reaching depths of about one hundred fifty-five millimeters. In contrast, the strengthened beams developed many smaller, shallower cracks that stayed closer to the surface, never exceeding a depth of one hundred thirty millimeters. The carbon fiber wrapping acted like a tight belt, holding the concrete together and preventing the cracks from spreading too far. This confinement meant the stress was distributed more evenly across the beam, allowing the material to work more efficiently. To ensure these findings were not just a fluke of the specific beams they built, the team also created a detailed computer simulation of the experiment. The digital model predicted the behavior of the beams with high accuracy, matching the real-world test results almost perfectly in terms of how much they bent and how they eventually failed.

The study confirms that embedding carbon fiber into the concrete and wrapping it with a U-shaped layer is a highly effective way to restore and strengthen aging structures. It significantly boosts the load-carrying capacity and allows the beams to bend further without breaking, all while keeping the cracks small and manageable. The researchers noted that while the strengthened beams were slightly more flexible than the plain ones, they did not lose their essential ability to deform safely, a quality known as ductility. This suggests that the method improves safety without making the structure brittle. While the current work focused on beams of a specific size and under controlled conditions, the findings offer a clear path forward for engineers looking to extend the life of concrete infrastructure. The technique proved to be a reliable tool for enhancing performance, offering a way to make old structures stronger and safer for the future.