Reusing damaged tennis strings in asphalt pavements: experimental investigation on the anti-rutting potential

In today’s highly industrialized world, the production of various plastic materials has reached its peak, leading to the widespread issue known as plastic pollution. It is a long-lasting, large-scale concern, with plastic debris and particles found in numerous ecosystems. This pollution disrupts habitats and natural processes, decreasing ecosystems' ability to handle with climate change, and negatively impacts the food security and production, and well-being of millions of people. Additionally, the production, use, and disposal of plastics contribute to about 4% of global greenhouse gas emissions. Plastic waste contaminates food, water, and oceans, with 85% of marine debris being plastic. Since plastic does not biodegrade, 46% of it ends up in landfills, while 22% is improperly managed and becomes litter. Thanks to modern technology, waste plastic can be utilized in various engineering applications, especially in the civil construction fields. In particular, plastic waste can be incorporated into road construction. One of the typical plastic wastes derived from the sport industry is discarded tennis strings, which constitutes a significant portion of plastic waste. With around 87 million players worldwide, the frequent replacement of strings due to their loss of tension and flexibility over time contributes to this accumulation. As mentioned before these wasted tennis strings in road pavements can contribute for reducing plastic waste globally. Additionally, it may help to improve pavement performance. Tennis strings are typically made from materials such as nylon, polyester, natural gut, Kevlar, or combination of these. Tennis strings are added to asphalt mixture as fibers and the dry method is used, as their melting point exceeds the binder's mixing temperature. Based on data from the literature, fibers around 5 mm in length are recommended. The experimental study begins with the initial phase of mix design, aimed at determining the optimal dosage of fibers derived from discarded tennis strings and bitumen content. Three fiber quantities were evaluated: 1.0%, 0.5%, and 0.3% by the weight of the aggregate. Due to volumetric results with 1% fiber is unable to attain the required binder content, while the other evaluated dosages demonstrated appropriate binder content during the mix design. Based on the mix design results and the related studies, 0.3% is selected for the mechanical tests as it provided the most suitable performance. The specimens of two different type of reference mixture and mixture with fiber are subjected to experimental tests to determine the material’s relaxation characteristics and overall dynamic properties using master curve and permanent deformation behaviour of asphalt mixes under repeated loading through the flow number test, above tests are conducted according to AASHTO R 84 (Master Curve by AMPT) and AASHTO T378 (E and Flow Number by AMPT) consequently. The master curve tests are conducted at 4°C, 20°C, and 40°C. While flow number test is done at 58°C. The results, along with the comparison of the two mixtures, indicate that incorporating tennis strings as fibers can enhance the rutting resistance and viscoelastic properties of asphalt mixtures.