Plastic waste poses a significant environmental challenge due to its non-biodegradable nature, while concrete production continues to deplete natural sand resources. Incorporating recycled plastic as a fine aggregate replacement offers a potential solution to both problems. This study experimentally investigates crack formation and deflection behavior in reinforced concrete beams with recycled plastic (RPET and PP) partially replacing fine aggregate. Six beam series ( \(750 \times 150 \times 150\) mm) were tested under two-point loading, with plastic contents of 0%, 2%, 4%, 6%, 8%, and 10% by weight of fine aggregate. Each beam was reinforced longitudinally with two 12 mm diameter steel bars and lacked transverse reinforcement to isolate shear behavior. Beams with 2–4% plastic replacement outperformed the control, reaching peak loads of 100–110 kN and mid-span deflections of 2.46–2.78 mm, indicating enhanced ductility and energy absorption. At higher replacement levels (6–10%), load capacity and deflection decreased sharply, with premature failure at loads as low as 40 kN. Crack analysis revealed that the 6% specimen produced the longest average crack (25.75 cm), reflecting a critical bonding threshold. All specimens failed in shear, either by diagonal tension or shear compression, with crack severity increasing with plastic content. A practical upper limit of 4% recycled plastic replacement is recommended to maintain adequate structural performance while supporting sustainable construction objectives.
