Synergistic impact of recycled PET microparticles on the mechanical performance of high-strength and conventional concretes

The global plastic waste crisis demands innovative recycling, such as using recycled polyethylene terephthalate (PET) microparticles as a replacement for natural fine aggregate in concrete. In this study, the volumetric substitution of PET particles (0%, 5%, 10% and 15% by sand weight) on the mechanical properties of conventional concrete (CC) (target compressive strength fc′ = 33 MPa) and high-strength concrete (HSC with 5% microsilica gel + 1% superplasticiser (target fc′ = 45 MPa) was examined, using 144 specimens tested at 7 and 28 days. PET incorporation consistently reduced the compressive strength, splitting tensile strength and modulus of elasticity (MoE). At 10% substitution, fc′ decreased by 24% in CC (from 33 MPa to 25 MPa) and by 22% in HSC (from 45 MPa to 35 MPa). At 15% PET, the reductions were 45% and 31%, respectively. The HSC showed better compressive strength retention but a greater loss in MoE (up to 34% at 15% PET). Unlike studies on PET fibres or macro-recycled aggregates, this work provides the first simultaneous multi-property quantification (here termed ‘synergistic’) of PET microparticle effects across both CC and admixture-optimised HSC, revealing matrix-dependent divergent degradation patterns through volumetric substitution. A practical 5% upper limit is recommended for non- and semi-structural applications, balancing waste utilisation with performance.