Changing the Cross-Sectional Shapes of the Prestressed Rovings in Warp-Knitted Glass and Carbon Reinforcing Fabrics

In the world of construction, concrete is a marvel of compression; it is incredibly strong when squeezed, but it cracks easily when pulled apart. To fix this weakness, engineers embed steel bars inside the concrete, a technique known as reinforcement. However, steel has a flaw: it rusts. In recent decades, builders have turned to a different kind of reinforcement made from thousands of tiny glass or carbon fibers bundled together. These bundles, called rovings, are woven into fabrics that are then stretched tight and embedded in concrete. This process, called prestressing, puts the concrete under constant pressure, allowing structures to be thinner, lighter, and far more resistant to cracking. The key to making this work lies in the fabric itself. When these fiber bundles are pulled tight, they do not just stretch like a rubber band; their internal shape changes. The thousands of individual threads inside the bundle shift, slide, and rearrange themselves. If the bundle squashes too much or changes shape in the wrong way, it can lose its grip on the surrounding concrete, weakening the entire structure. Understanding exactly how these bundles deform under tension is essential for predicting how long a building will last and how much weight it can safely carry.

Researchers at Peter the Great St. Petersburg Polytechnic University set out to watch this invisible rearrangement happen in real time. They focused on two specific types of warp-knitted fabrics, a method of manufacturing that creates open, mesh-like structures. One type of fabric uses a stitch pattern called "tricot," which creates a diagonal, interlocking structure, while the other uses a "pillar" stitch, which forms a more vertical, chain-like pattern. The team tested fabrics made from both glass and carbon fibers, subjecting them to increasing levels of tension. They did not simply pull the fabric until it broke; instead, they stopped at specific points—25%, 50%, and 75% of the fabric's maximum strength—to examine what was happening inside. To do this, they froze the fabric in its stretched state using a fast-hardening resin, cut tiny slices, and looked at them under a powerful microscope. They then used computer software to map the position of every single fiber within the cross-section of the bundle, creating a detailed statistical picture of how the threads moved as the load increased.

The results revealed a dramatic difference between the two stitch patterns. In the fabrics made with the tricot stitch, the cross-section of the fiber bundle changed significantly as the load increased. Initially, these bundles had a flattened, oval shape, with the fibers spread out over a wide area. As the tension rose, the bundle began to squeeze down, losing its flatness and becoming more circular. The fibers, which were once spread out, migrated inward toward the center of the bundle. This rearrangement happened most rapidly in the early stages of stretching, specifically between the start and 25% of the maximum load. By the time the fabric reached 75% of its strength, the bundle had transformed into a shape much closer to a circle, and the fibers were tightly packed near the middle. This shift meant the bundle was losing contact with the surrounding material, a critical factor because the bond between the fiber and the concrete relies on the surface area of the bundle touching the cement.

In stark contrast, the fabrics made with the pillar stitch behaved very differently. These bundles started with a nearly circular shape and remained that way throughout the entire stretching process. Even as the tension increased to 75% of the maximum strength, the cross-section of the pillar-stitched bundles only shrank slightly in diameter. The fibers did not migrate dramatically toward the center; instead, they maintained their original, uniform distribution. The researchers found that the pillar stitch provided a much more stable geometry, resisting the flattening and reshaping that occurred in the tricot samples. This stability suggests that the pillar stitch might offer a more consistent grip on the concrete, as the shape of the reinforcement does not undergo a radical transformation under load.

The study also highlighted that this reshaping is not just a matter of the bundle getting smaller; it is a complex internal migration of thousands of individual filaments. In the tricot stitch, the fibers move from the outer edges to the core, effectively reducing the "contact length"—the perimeter of the bundle that touches the concrete. The researchers calculated that this contact length drops sharply in the first quarter of the stretching process, after which it stabilizes. This finding suggests that the most critical changes happen early, and that stretching the fabric beyond a certain point does not significantly alter the internal arrangement further. The difference between glass and carbon fibers was also noted, with carbon bundles containing far more individual filaments than the glass ones, yet both followed the same general rules dictated by their stitch pattern.

Ultimately, the work provides a clear map of how these high-tech fabrics behave under stress. It shows that the choice of stitch pattern is not merely a manufacturing detail but a fundamental factor in how the reinforcement interacts with the concrete. For engineers designing prestressed concrete structures, this means that the tricot stitch, while common, undergoes a significant geometric transformation that could affect the long-term performance of the material. The pillar stitch, by maintaining its shape, offers a different mechanical profile. The research confirms that to predict the strength and durability of these modern structures, one must look beyond the simple strength of the fibers and understand the complex, shifting geometry of the bundles that hold them together.