Mechanical, thermal, and non-destructive evaluation of concrete incorporating recycled PET bottle aggregates

Abstract The valorization of post-consumer polyethylene terephthalate (PET) bottle waste in concrete represents a promising approach toward more sustainable construction materials. This study investigates the influence of recycled PET aggregates, used as partial volumetric replacements for natural sand and gravel, on the fresh, physical, mechanical, thermal, ultrasonic pulse velocity (UPV), and microstructural properties of concrete. A sustainability assessment was also conducted based on CO 2 emissions, energy demand, and natural aggregate consumption. Recycled PET bottles were processed into fine (FPET) and coarse (CPET) aggregates and incorporated at replacement levels of 10%–30%. The results show that PET incorporation reduces workability and density while increasing entrapped air content. Mechanical performance strongly depends on PET size and replacement level: a slight increase in 28-day compressive strength was observed at 10% FPET replacement, whereas higher substitution levels led to strength reductions of up to 31% and elastic modulus losses ranging from 4.5% to 34%, with more pronounced effects for CPET aggregates. In contrast, bulk density and thermal conductivity were significantly reduced by up to 15% and 42%, respectively, highlighting the potential of PET-based concretes for lightweight and thermally insulating applications. Microstructural analyses revealed a more porous matrix and a wider and weaker interfacial transition zone between PET aggregates and the cementitious matrix, which explains the observed strength degradation at higher replacement levels. In addition, strong correlations were established between physical and mechanical properties and UPV measurements, with R 2 ≈ 0.8–0.92 and low root mean square errors, enabling reliable non-destructive prediction of PET-based concrete performance. The sustainability results showed that CO 2 emissions exhibited only a marginal reduction, while energy consumption slightly decreased, and natural aggregate use was reduced by up to 18%, leading to an overall improvement in environmental performance. Overall, recycled PET aggregates provide a viable solution for producing lightweight concrete with improved thermal performance while contributing to plastic waste valorization. PET aggregate size is identified as a governing parameter that balances mechanical performance and functional properties.