Recycled PET aggregates as engineered flaws in strain-hardening cementitious composites: Synergistic control of matrix strength, crack activation and tensile ductility

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Recycled polyethylene terephthalate (PET) aggregates offer a promising route for developing sustainable strain-hardening cementitious composites (SHCC). However, their function should not be regarded merely as aggregate replacement, but as engineered flaws that can regulate crack activation and tensile ductility. In this study, 100% quartz powder was replaced by recycled PET aggregates to prepare PET-based SHCC. The coupled effects of PET particle size and matrix strength, controlled by water-to-binder ratio, were investigated through flowability, compressive, uniaxial tensile, crack-pattern, microstructural, and sustainability analyses.The results show that PET-SHCC achieved stable strain-hardening behavior despite reductions in compressive strength and elastic modulus. Compared with Q-SHCC, PET-SHCC exhibited significantly higher tensile deformability, especially in dense matrices. At a water-to-binder ratio of 0.21, 0.21-P-M exhibited a 124.2% increase in ultimate tensile strain while maintaining an ultimate tensile strength above 9 MPa. The influence of PET particle size was strongly matrix-dependent. In high-strength matrices, larger PET particles increased tensile strain capacity, whereas in weaker matrices, excessive particle size caused crack localization and reduced tensile deformation. SEM/EDS results confirmed that the wider PET–matrix interfacial transition zone was the key microstructural origin of early crack activation.This study demonstrates that recycled PET aggregates can serve as matrix-dependent engineered flaws for designing sustainable, high-ductility SHCC with enhanced crack-control capability.

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