Mechanical Performance, Crack Resistance and Microstructural Evolution of Engineered Cementitious Composites Reinforced with Multiscale Hybrid Fibers

To further enhance the mechanical performance and early-age crack resistance of engineered cementitious composites (ECC), an ECC-based multiscale hybrid-fiber system was investigated. The system used carbon nanotube-modified polyethylene (M-PE) fibers as the primary reinforcement together with polypropylene (PP) and basalt (BF) fibers. The effects of fiber hybridization on compressive strength, uniaxial tensile behavior, flexural performance, early-age crack resistance, and microstructure were systematically evaluated. Mixtures retaining at least 60% M-PE exhibited a clear post-cracking strain-hardening response, whereas lower M-PE fractions led to crack localization and loss of strain hardening. At 28 days, BF-0 (1.5 vol.% M-PE + 0.3 vol.% BF) reached compressive, tensile, and flexural strengths of 85.3, 7.35, and 36.38 MPa, respectively. A six-indicator entropy-weighted TOPSIS evaluation identified BF-0 as the best-balanced mixture among the investigated groups. Increasing PP or BF content improved early-age plate crack resistance; BF-5 (1.5 vol.% BF) achieved the highest crack reduction coefficient of 67.98%, with a nominal total crack area of 27.6 mm2. Scanning electron microscopy (SEM) observations were used only as qualitative morphological evidence, whereas mercury intrusion porosimetry (MIP) revealed quantitative pore-structure trends and X-ray diffraction (XRD) indicated that fiber hybridization did not generate new detectable crystalline phases. The results reveal the performance trade-offs among strength, ductility, and early-age crack control in multiscale hybrid-fiber cementitious composites.