Interface Bond Performance of Concrete-Filled Galvanized Steel Tubes Strengthened with Internal Circular Rings: A Taguchi–Grey Relational Study

In the world of heavy construction, engineers often rely on a simple but powerful idea: combining two different materials to create something stronger than either could be alone. A common example is filling a hollow steel tube with concrete. The steel provides a tough shell that prevents the concrete from crushing under pressure, while the concrete inside stops the thin steel walls from buckling or collapsing inward. This partnership works beautifully for building columns and bridge supports, but it relies entirely on a single, critical condition: the two materials must move together as one unit. If the steel tube and the concrete core slide past each other, the structure loses its strength and can fail. This sliding happens at the invisible boundary where the metal meets the rock-hard cement, a place where the bond is often weak, especially if the metal surface is smooth.

To fix this weakness, researchers have tried adding mechanical locks, such as welding small metal rings inside the tube to stop the concrete from slipping. However, a new study has uncovered a surprising detail about how these locks work: it is not just about having a ring, but exactly where you place it. The team behind this research focused on a specific type of construction material: galvanized steel tubes filled with a standard, high-strength concrete mix. They wanted to understand how the bond between the metal and the concrete changes when a single, welded circular ring is placed at different heights inside the tube. By testing dozens of samples with varying sizes and shapes, they discovered that the position of the ring is the most important factor in determining how well the two materials hold together.

The researchers began by testing plain tubes with no internal rings to establish a baseline. They found that without any extra help, the bond between the concrete and the galvanized steel was relatively weak, with the average strength ranging from less than one to nearly two units of pressure per square millimeter. This confirmed that for smooth, galvanized surfaces, the natural grip between the materials is often not enough for heavy-duty applications. The team then moved on to their main experiment, where they welded a single steel ring inside the tube at one of three specific locations: near the top, in the exact middle, or near the bottom. They tested these rings across nine different tube sizes, changing the width of the tube, the thickness of the steel walls, and the overall length of the specimen to see how these factors interacted with the ring's position.

The results were clear and consistent. Every tube with a ring performed better than a plain tube, but the location of the ring made a massive difference. When the ring was placed near the top of the specimen, the bond strength increased by about sixty percent compared to the plain tubes. Moving the ring to the middle raised the strength by roughly eighty-six percent. But the most dramatic improvement came when the ring was placed near the bottom, where the bond strength more than doubled, increasing by over one hundred and twenty percent. On average, the bottom-positioned ring was nearly forty percent stronger than the top-positioned ring and about twenty percent stronger than the middle one. This pattern held true regardless of the tube's size or the thickness of its walls.

The researchers explained this phenomenon by looking at how the force travels through the material during a test. In their setup, the concrete core was pushed down while the steel tube was held in place. A ring located near the bottom, which is close to where the force is applied, engages almost immediately. It acts like a physical stop, forcing the concrete to bear against the steel plate before any significant sliding can occur. This creates a strong mechanical lock right at the source of the stress. In contrast, a ring placed higher up has to wait for the concrete to slide down the length of the tube before it can engage, by which time the bond has already weakened. The study suggests that for engineers designing these structures, placing a connector near the loaded end is far more effective than placing it in the middle or at the far end.

Beyond just the position of the ring, the study also looked at how the size and shape of the tube influenced the results. For tubes with rings at the top or middle, the length of the tube relative to its width was the most significant factor; shorter, stouter tubes held together better than long, slender ones. However, for the tubes with rings at the bottom, the width of the tube became the dominant factor, with wider tubes showing the strongest performance. The team used a statistical method to find the single best combination of all these variables. They identified that a tube with a specific medium width, the thickest steel walls, and the shortest length-to-width ratio performed the best in their tests. While this specific combination was not one of the exact samples they built, the data strongly points to it as the ideal design.

The researchers were careful to note that their findings are based on a specific set of conditions and that more testing would be needed to confirm these results for every possible construction scenario. They did not claim to have solved the problem of concrete bonding forever, but they did provide a clear, measured answer to a specific question: where should you put the lock? The study concludes that for galvanized steel tubes filled with concrete, the most effective way to prevent the materials from slipping apart is to place a welded ring near the bottom, close to where the load is applied. This simple adjustment transforms a weak connection into a robust, interlocked system, offering a practical and reliable way to build stronger, safer structures.