Influence of Recycled Waste Pet Fiber Geometry on the Destructive, Non-destructive, and Post-failure Characteristics of Structural Concrete

Concrete is the most ubiquitous material in the built world, forming the skeleton of our cities and the foundation of our homes. It is a composite of crushed stone and sand held together by a paste of cement and water. While this mixture excels at withstanding heavy squeezing forces, it has a notorious weakness: it is brittle. Like a dry twig, plain concrete resists compression well but shatters suddenly when pulled or bent, offering little warning before it fails. To fix this, engineers have long turned to fiber-reinforced concrete, a variation that embeds tiny, discrete strands throughout the mix. These fibers act as internal safety nets, bridging cracks as they form and holding the material together even after it has started to break, transforming a brittle snap into a more gradual, manageable failure.

For decades, the search for these reinforcing strands has focused on steel or synthetic polymers, but a growing body of research asks a different question: can we turn the world's plastic waste into a structural asset? Polyethylene terephthalate, or PET, is the clear plastic used for countless beverage bottles. While recycling programs exist, much of this material still ends up in landfills. The challenge lies in the shape of the plastic; standard plastic strips cut from bottles are smooth and straight. In the rough, rocky environment of concrete, these smooth fibers tend to slip out rather than grip, failing to provide the necessary reinforcement. This study, conducted by researchers at Lovely Professional University in India and Hajee Mohammad Danesh Science and Technology University in Bangladesh, explores whether reshaping these waste fibers into a corrugated, zigzag form can solve the grip problem, and how this modification affects the strength and testing of the final concrete.

The researchers began by collecting used soda and water bottles, stripping away the necks and bottoms, and slicing the cylindrical bodies into uniform strips. They created two distinct types of fibers from this waste: one set remained straight and smooth, while the other was passed through a mechanical device to press a continuous zigzag pattern into the plastic. Both sets were cut to a length of 40 millimeters. These fibers were then mixed into standard M20 grade concrete, a common structural mix, at a rate where the fibers replaced one percent of the volume of the coarse stone aggregate. The team cast dozens of cylindrical samples and cured them for 28 days in an open-air environment, exposing them to natural sunlight, rain, and temperature fluctuations to mimic real-world construction conditions rather than the controlled climate of a laboratory.

When the fresh concrete was tested for workability, the addition of the plastic fibers immediately showed a physical cost. The smooth, straight fibers reduced the mixture's flow by roughly ten percent, but the zigzag fibers caused a much sharper drop, reducing flow by nearly thirty-three percent. The corrugated shape of the zigzag fibers created a physical entanglement, causing the plastic strands to hook onto each other and the surrounding sand and stone, making the mixture stiffer and harder to pour. Despite this loss in ease of handling, the hardened concrete proved surprisingly resilient. When crushed in a testing machine, the plain concrete control sample reached a strength of 20.20 megapascals. The mix with straight fibers held up at 18.90 megapascals, a minor decrease, while the zigzag mix achieved 17.65 megapascals. Although the strength dipped slightly, the researchers confirmed that using this small amount of recycled plastic did not compromise the structural load-bearing capacity of the concrete.

A more surprising discovery emerged when the team tested the concrete using a non-destructive method known as the Schmidt rebound hammer. This device works by firing a small spring-loaded mass against the concrete surface and measuring how far it bounces back; a higher bounce usually indicates a harder, stronger surface. In standard concrete, this method provides a reasonably accurate estimate of strength. However, in the fiber-reinforced samples, the hammer consistently underestimated the true strength. The smooth plastic fibers near the surface acted as a shock absorber, dampening the impact of the hammer and causing the bounce to be lower than it should be. For the zigzag mix, the hammer suggested the concrete was only about half as strong as it actually was. To correct this, the researchers developed specific adjustment factors, essentially creating a new rulebook for how to interpret hammer readings when plastic fibers are present, ensuring that inspectors do not mistakenly reject a sound structure.

The most dramatic difference appeared when the cylinders were finally crushed to failure. The plain concrete shattered instantly, breaking into a pile of fragments with a sudden, violent shear fracture. The mix with straight fibers showed a slight improvement, cracking along the length of the cylinder, but the smooth fibers eventually pulled out of the cement paste, leaving the outer shell to crumble. The zigzag fiber mix, however, behaved differently. As the cylinder cracked, the corrugated plastic strands locked firmly into the surrounding cement, much like the ridges on a steel rebar. This mechanical interlock prevented the fibers from slipping out, allowing the concrete to hold together even after the peak load was passed. The specimen did not disintegrate; instead, it maintained its cylindrical shape while developing a network of fine cracks, demonstrating a ductile, or flexible, failure mode that offers far more safety in a real-world collapse.

The study concludes that while turning waste plastic bottles into concrete reinforcement requires careful handling to manage the mix's stiffness, the results are promising. The zigzag geometry is the key, transforming a slippery plastic strip into a robust anchor that significantly improves the concrete's ability to withstand cracking and hold together after damage. By establishing new calibration methods for non-destructive testing, the researchers have also provided a practical tool for engineers to accurately assess these new materials in the field. The work suggests that with the right shape, discarded plastic bottles can be more than just waste; they can become a vital component in making our infrastructure tougher and more resilient.