Model and optimum mix designs for sustainable structural lightweight concrete with aggregates from pet plastic waste

Abstract

This study addresses the growing environmental burden of polyethylene terephthalate (PET) waste by developing and optimizing structural lightweight concrete using fully recycled PET aggregates. A two-level factorial design generated eight mix combinations varying water–cement ratios (0.40–0.45) and PET fine and coarse aggregate contents. Fresh densities ranged from 1455 to 1515 kg/m³, confirming lightweight classification, while Vebe times of 13–40 s indicated medium-to-low workability. Dry densities were consistently within 1490–1537 kg/m³. Compressive strength ranged from 14.1 to 16.5 MPa, meeting LC 12/13 structural lightweight concrete requirements. Splitting tensile strength varied between 0.84 and 1.40 MPa, and water absorption levels of 4.66–10.2% reflected PET’s porous–hydrophobic influence on pore structure. ANOVA-based regression models demonstrated high predictive accuracy for compressive and tensile strength (R² ≥ 0.95), with moderate accuracy for workability (R² = 0.69) and water absorption (R² = 0.58). Numerical optimisation identified an ideal mix comprising a water–cement ratio of 0.45, fine aggregates of 372 kg/m³, and coarse aggregates of 532 kg/m³. Validation confirmed all predicted responses within a 95% confidence interval. The results demonstrate the technical feasibility of producing fully PET-based structural lightweight concrete, contributing to sustainable construction, plastic waste valorisation, and circular-economy objectives under SDG 11.