Advancing Sustainable Construction with Recycled PET Fiber–Reinforced Concrete Optimization

The growing accumulation of plastic waste, particularly Polyethylene Terephthalate (PET), creates a critical environmental challenge, demanding innovative recycling strategies. Recycled PET fibers have exhibited potential in controlling crack propagation and improving tensile and flexural strength in concrete, while reducing plastic waste. However, existing research includes only limited fiber lengths and volume fractions, lacking a systematic approach. This study investigates the influence of a wide range of recycled PET fiber lengths (6–50 mm) and volume fractions (0.5%–3.0%) on the mechanical properties of concrete including compressive, split tensile, and flexural strengths. The research employs the Grey Relational Analysis (GRA) method to identify optimum fiber length and volume fraction for performance enhancement. Additionally, the pore structure, microstructure, and Interfacial Transition Zone (ITZ) were analyzed to evaluate the influence of fiber incorporation. GRA analysis revealed that 40 mm fiber length and 2.5% volume fraction yielded the optimal performance, with compressive, split tensile, and flexural strengths increasing by 20.7%, 51.6%, and 38.2%, respectively, compared to the control. Scanning electron microscopy (SEM), X-ray microcomputed tomography (micro-CT), and nanoindentation demonstrated that fiber inclusion enhanced matrix densification, fiber bridging, reduced porosity, and refined ITZ. Notably, recycled PET fibers reduced the open porosity by 57.9% while presenting an ITZ thickness of 15 μm, indicating enhanced bonding and denser interfacial zones. These findings support the ability of optimized recycled PET fibers to significantly improve both macro and microlevel concrete performance, facilitating as a sustainable concrete solution and aligning with global waste management and circular economic goals in construction.