The utilization of polyethylene terephthalate (PET) powder as aggregate in the development of environmentally friendly high-ductility composites (P-EHDC) offers a promising pathway for advancing sustainable and high-performance concrete materials. Despite its potential, the fracture behavior of P-EHDC—particularly under the influence of alkali-activated precursors—remains insufficiently explored. In this study, the fracture performance of P-EHDC was evaluated by varying the precursor composition ratios (GGBS:FA = 4:6, 3:7, and 2:8) and PET powder replacement ratios (0%, 15%, 30%, and 45% by volume). Fracture modes, Mode I fracture energy (G_F), and crack propagation behavior were analyzed using the J-integral method. All specimens exhibited ductile fracture characteristics, a clear contrast to the brittle failure observed in conventional concrete. The replacement of 15 vol% PET powder significantly increased G_F in precursor systems with higher GGBS content (4:6 and 3:7), and 30 vol% was more effective in fly ash-rich systems (2:8). The J-integral method, which offers broader applicability compared to conventional methods such as the double-K fracture model, provided a more comprehensive understanding of the fracture behavior. The results showed that PET powder reduced the matrix fracture toughness, promoted matrix cracking, and weakened the fiber-bridging effect, leading to enhanced energy absorption via fiber pull-out. At low PET powder replacement ratios (e.g., 15 vol%), the cracking threshold of the matrix was not significantly reduced, while more fibers engaged during the crack instability stage to absorb fracture energy through pull-out. This behavior highlights the synergistic toughening effect between PET powder and fibers in the P-EHDC system. The effect became more pronounced when the PET content was below 45 vol% and the precursor matrix contained a higher proportion of GGBS, leading to enhanced ductility. This study introduces a novel approach to fracture behavior analysis in PET-modified alkali-activated composites and provides theoretical support for the toughening design of high-performance, low-carbon concrete materials.

Strain-hardening cementitious composites (SHCCs) achieve tensile strain hardening and distributed multiple cracking through coordinated matrix cracking, fiber bridging, and interfacial stress transfer. This structured narrative review synthesizes evidence on single- and hybrid-fiber systems using a multiscale framework linking fiber properties and geometry, dispersion and orientation, fiber–matrix interfaces, crack-bridging response, strain-hardening criteria, and engineering performance. The evidence shows that no fiber is universally optimal: performance depends on matching fiber strength and geometry with matrix fracture properties, controlled debonding and frictional sliding, and reliable processing. Excessively weak interfaces provide insufficient bridging stress, whereas excessively strong interfaces promote fiber rupture and crack localization. Nominal fiber volume fraction does not equal the effective bridging population because dispersion and orientation vary with mixing, casting, pumping, spraying, and extrusion. Hybrid systems may broaden the crack-bridging range, but a positive hybrid effect requires comparison with single-fiber controls using the same matrix and total fiber volume fraction; poor rheology and competition among fibers can instead reduce performance. Engineering selection should therefore be application-specific and evidence-limited, with direct tensile behavior, crack control, durability, constructability, and life-cycle performance evaluated together.