This study investigates the feasibility of using waste Polyethylene Terephthalate (PET) bottle fibers as a partial substitute for fine aggregate in concrete and evaluates their influence on the mechanical properties of concrete. The improper disposal of plastic waste has become a serious environmental concern due to its non-biodegradable nature. PET bottles, which are extensively used for packaging drinking water and carbonated beverages, contribute significantly to plastic pollution. To address this issue and reduce the consumption of natural sand, the construction industry is exploring sustainable and economical alternative materials.

In this research, waste PET bottle fibers were incorporated into Ordinary Portland Cement (OPC) concrete as a partial replacement for fine aggregate. Since complete replacement of natural sand with PET fibers is impractical, replacement levels of 1%, 2%, 2.5%, 3%, and 5% were investigated. Prior to concrete preparation, the physical and mechanical properties of the constituent materials were determined. The cement exhibited a normal consistency of 31%, soundness of 3 mm, initial and final setting times of 65 minutes and 280 minutes, compressive strength of 59.38 N/mm², and a specific gravity of 3.15. The water absorption and specific gravity values were 1.40% and 2.63 for fine aggregate, and 1.19% and 2.69 for coarse aggregate, respectively. Sieve analysis was carried out to determine the particle size distribution and fineness modulus of the aggregates.

An M20 grade concrete mix with a water-cement ratio of 0.50 was designed for the investigation. Concrete cubes measuring 150 × 150 × 150 mm and beams of 75 × 75 × 300 mm were cast, cured, and tested after 7 and 28 days. The performance of concrete containing varying percentages of PET fibers was compared with a conventional control mix without PET replacement. Compressive strength and flexural strength tests were conducted to evaluate the structural performance of the specimens.

The experimental findings indicated that replacing fine aggregate with 2%–2.5% PET bottle fibers resulted in improved compressive and flexural strengths compared to conventional concrete. However, higher replacement levels led to a reduction in strength. The results suggest that a limited percentage of PET bottle fibers can effectively enhance concrete performance while promoting the recycling of plastic waste and reducing dependence on natural fine aggregate. Therefore, the use of PET bottle fibers as a partial replacement for sand represents an environmentally friendly and sustainable approach for concrete production.