Evaluation of Mechanical and Durability Performance of Concrete with and Without Surface-Treated Plastic Fine Aggregates

Global plastic waste generation and excessive sand extraction are major environmental challenges, but replacing fine aggregate with plastic waste often degrades concrete performance. This work characterizes concrete incorporating recycled HDPE and PET fine aggregates at a 10% volumetric replacement level, with and without polymer-specific surface treatment, across fresh, mechanical, and durability properties. Untreated plastic aggregate generally lowered mechanical performance due to low polymer stiffness, weak plastic–paste bonding, and greater interfacial void formation. Surface treatment partially offsets these effects by strengthening the plastic–paste bond. H2O2-treated HDPE granules recovered the 28-day elastic modulus to within 3% of the control while also improving compressive strength, ultrasonic pulse velocity, and freeze–thaw resistance. H2O2-treated HDPE chips showed the highest electrical resistivity and the lowest permeable void content. NaOH-treated PET chips gave the lowest chloride penetrability and the greatest drying shrinkage reduction, approximately 25% relative to the control, though NaOH produced no resistivity gain for PET-C. Freeze–thaw durability factor increased with surface treatment for HDPE-G and PET-C, with HDPE-G-T exhibiting the highest durability factor among the recycled plastic mixtures at 94.20%. These results show that surface-treated recycled HDPE and PET fine aggregate can be incorporated at 10% replacement while maintaining acceptable mechanical and durability performance, supporting recycled plastics as a viable partial fine-aggregate replacement.